WO2003003611A2 - Cellular communication system which uses multicarrier spread-spectrum transmission - Google Patents

Abstract

A frequency band, to be used by each cell within a group of cells, is obtained and the number, m=2n (n=1, 2...), of orthogonal sub-carriers, f1, f2, .., fm, over which individual data are transmitted in each communication channel, is determined for all cells within the group. A phase, φi, is allocated to each sub-carrier fi(I=1,2,..,m) within a cell, such that the absolute value φi is determined by the data that is transmitted, and by the digital modulation scheme, and is identical for all fi within the cell, and the sign of each φi within the cell is identical to the sign of a corresponding element taken from a row in an mxm Walsh-Hadamard matrix that corresponds to that cell, such that neighbouring/overlapping cells use different rows. The data in each cell are transmitted over the sub-carriers with their corresponding phases. All transmissions carried by all sub-carriers are received and the total phase Φi of all the received signals at frequency fi is obtained. The sign of total phase Φi is changed for each cell, for all values of i that correspond to negative elements in the corresponding row of that cell's matrix. The phase, φi, of any transmitted sub-carrier, fi, is obtained by summing the values of the total phase, Φi, after changing phase signs, and dividing the result of the summation by a number determined according to m and/or the ratio between the magnitudes of the sub-carriers within a cell, and the magnitudes of the sub-carriers within its neighbouring/overlapping cells.

Description

A METHOD FOR IMPROVING THE UTILIZATION

OF RADIO COMMUNICATION BANDS.

AND A COMMUNICATION SYSTEM

USING THE METHOD

Field of the Invention

The present invention relates to the field of wireless communication systems. More particularly, the invention relates to a method and system for increasing the number of communication channels that can be simultaneously established within a cell, by using essentially the same set of radio frequency (RF) sub-carriers for transmitting the data that corresponds to each communication channel. ^'

Background of the Invention

With the growing amount of wireless communications, and the penetration of the Wireless Apphcation Protocol (WAP),, the need to sufficiently utilize the existing frequency range is immense. Meanwhile the frequency spectrum is a limited and precious natural resource. Thus, there is an economic need of telecommunication companies to be able to transmit the largest possible communication capacity on the existing/allocated frequency range.

Almost every wireless RF system has an area, where the RF signals become a disturbing signal. The disturbance is caused by two factors, one of which is the difference in the signal power density between the main signal to the disturbing signal (signal to interference ratio - S T), and the second factor is the proximity between the disturbing frequency and the operating frequency. This disturbance radically affects the frequency reuse in such a way, that it limits the communication capacity in the critical area. For example, in cellular system like Global System for Mobile communication (GSM) system, the critical area is in the closest cell with the same frequency.

Review of critical areas in existing communication systems:

Code-Division Multiple Access (CDMA) cellular systems

Each cell in this system (Fig. 1A) uses the same frequency. According to this Fig. 1, it seems that in CDMA systems a maximum efficiency of the available frequency band is achieved, since the maximum number of communication signals in the cell is N. However, there is more than one cell in the system. Therefore, as is illustrated in Fig. IB, having three adjacent cells, for example, results in splitting the channel between these cells. Consequently, the communication signals that can be exploited by each cell are reduced to (N/3). In this type of systems, the critical area is inside the neighboring cells.

Time Division Multiple Access (TDMA) cellular systems

In these systems (GSM, Advanced Mobile Phone Service - AMPS and Digital AMPS - DAMPS), different frequencies are used to separate between the cells or between sectors in a cell. There are different methods for allocation of these frequencies between different cells (Figs. 2A, 2B, 2C). From the aforementioned figures, it is clear that the same frequency can be reused only in the third cell, after the main working cell (Fig. 3A). The critical area in all these systems is the second (subsequent) cell. It should be noted that the basis of every current cellular system usually comprises a cluster of six, or more, different Radio Frequency (RF) channels, of which pattern repeats itself, thereby wasting Radio Frequency (RF) channels.

Another problem exists in cellular communication systems. If more and more people start using their cellular phones in crowded places (e.g. stadiums, theater and parking lots), the communication band becomes congested until at a certain time no more people will be able to use their cellular phone. In conventional systems such as GSM, the number of people that may simultaneously use their cellular phones in a crowded place is rather small. In GSM, for example, the number of users may be as high as 160. This drawback is due to the limited maximum bandwidth that is allowed for each sector in each cell. The bandwidth allowed for each sector is limited in order to minimize the interference between each two adjacent sectors. Consequently, the number of channels that might be utilized by a sector in a cell is rather low. In several conventional systems communication, channels are allocated to sectors in a cell according to the number of users in the sectors, namely channels are allocated to more 'busy' sectors on the expanse of less busy sectors. However, there is a maximum number of channels that can be utilized by a specific sector, which is rather low (e.g. 15-20 channels per sector). If each channel can serve (i.e. modulated by) 8 data sources, then one sector's communication band, which comprises e.g. 20 channels, can 'serve' a maximum of 20*8=160 data sources (i.e. users), which is a low number when thinking e.g. of thousands potential cellular phone users in a football stadium. Other RF systems

There are communication systems that use many communication channels in essentially the same frequency range. For instance, military and police uses, airline companies, taxi stations etc.

In these systems the critical areas are near each transmitter. A receiver is often located nearby transmitting antennas of other communication systems, resulting in RF disturbances sensed by the receiver (Fig. 4). The conventional solution to this problem is to allocate a unique frequency range to each user/system. The more communication systems are in a given area, the narrower the frequency band must be, thus decreasing the communication capacity and wasting frequency bands.

In order to have a better use of a communication channel, several communication systems use multiplexing, whether in the time domain (Time-Division Multiplexed-TDM) or in the frequency domain (Frequency-Division Multiplexed- FDM). Even though in FDM several carriers are transmitted simultaneously, each carrier is modulated independently regardless of other carriers, thus conveying a different source of information. The final transmitted signal is a summation of all the modulated carriers. However, several RF carriers are required, and a nearby base station can not utilize the same set of RF carriers due to mutual interference. Additionally, there is a need to demultiplex the received signal.

Coded Orthogonal Frequency Division Multiplexing (COFDM)

Currently, there are communication systems based on COFDM method, which has been specified for digital broadcasting systems for both audio and television signals. According to this technique, a communication channel comprises m orthogonal carriers, and the data to be transmitted is distributed between these carriers. Using multi-carriers according to this technique allows reducing the symbol (i.e. a set of logic '0' and ' 1') transmission rate at each carrier, and using the orthogonality property significantly reduces inter-symbol interference. However, the essence of this technique is keeping high symbol/data rate transmission. It does not solve the problem of efficiently allocating Radio Frequency bands.

All of the methods described above have not yet provided satisfactory solutions to the problem of efficiently allocating radio frequency bands in communication systems.

It is an object of the present invention to provide a method for allowing multiple base stations in cellular systems to utilize essentially the same set of RF frequencies without mutual interference.

It is another object of the present invention to provide a method for increasing the number of communication channels that can be simultaneously established within a cell.

Other objects and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention

In order to facilitate the reading of the description to follow, a number of

terms and acronyms are defined below: 'Multi-carrier' is a Radio Frequency (RF) signal comprising a set of m

sub-carriers (fι,f₂,~,f_m ).

φ_t is a phase (e.g. ^ = +10° ) representing a unique digital symbol (e.g.

'01'), by which a sub-carrier is modulated. The sub-carrier phase is shifted by φ_t whenever modulated by said digital symbol.

+ Ψ is a group of φ_t , wherein each ^ represents a different symbol; e.g.

+ Ψj = {+10°,+100^o,-170^o,-80°} , wherein + 10° may represent a symbol '00',

+ 100° may represent symbol '01' etc. This group may be also called a 'positive symbol scheme'. A sub-carrier may be modulated by such positive symbol scheme in which case it is said that the sub-carrier is assigned a positive symbol scheme.

- Ψ is a group of φ_t , wherein each symbol is represented by the respective negative phase as appears in group + Ψ,- . Considering the above example of group + Ψι , the inverted version of + Ψj is: - Ψj = {-10°,-100°,+170\+80⁰} . As opposed to group + Ψ_t , the symbol '00', for example, is represented in group -Ψ_j by -10° . The same rule applies to other symbols representations. This group (i.e. -Ψ_j) may be also called a 'negative symbol scheme'. A sub-carrier may be modulated by a negative symbol scheme in which case it is said that the sub-carrier is assigned a negative symbol scheme. 'Inversion rule' is a rule applied by a receiver on the received signal, determining which of the sub-carriers' schemes should be inverted from negative scheme (-Ψj) to positive scheme ( + Ψ_j).

'Composite-carrier' is a multi-carrier signal, wherein every sub-carrier

in the multi-carrier signal is phase-modulated by either a positive or a

negative symbol scheme.

'Schemes combination' (S ) is a set comprising positive and negative symbols schemes, each said scheme modulates a different sub-carrier in a specific multi-carrier signal. For example, such a schemes combination may be (SQ)

wherein ft through / are the sub-carriers of a specific composite carrier, which are modulated by positive and negative symbols schemes as specified (i.e. ft by +Ψj etc.). Another example for schemes combination is SC₂={/₁ : +Ψ₂ , ₂ : -Ψ₂ , ₃ : -Ψ_{2 > 4} : +Ψ₂}. According to the present invention, each cell has a unique schemes combination SC_t , since it is assigned a unique | Ψ. | , as well as a combination of positive and negative symbol schemes (SC_t).

'Communication channel' is a wireless connection established between a (base) station and an end-user using a mobile, or stationary, transceiver.

The new system, which comprises a transmitter, wherein, and from which, the above-mentioned composite carrier is generated and transmitted, and a receiver that applies the inversion rule as specified above, is hereinafter referred to as the Frequency Bank System (FBS). The present invention is directed to a method for increasing the number of communication channels that can be simultaneously established within a cell being part of a group of cells. A frequency band to be used by each cell within the group is obtained and the number of m=2ⁿ (n=l, 2,..,) identical orthogonal sub-carriers ft, fa,.., f_m, over which individual data is transmitted in each communication channel is determined for all cells within the group. A corresponding phase φ_t is allocated for each sub-carrier ft (i=l, 2,.., ) within a cell, such that the absolute value | φ_i \ is determined by the data that is transmitted and the digital modulation scheme, and is identical for all f_t within the cell, and the sign of each φi within the cell is identical to the sign of a corresponding element taken from a row in a mxm Walsh-Hadamard matrix that corresponds to the cell, such that neighboring/overlapping cells use different rows. The data in each cell is transmitted over the sub-carriers and their corresponding phases. All transmissions carried by all sub-carriers are received and the total phase Φi of all the received signals at frequency ft is obtained. The sign of total phase Φi is changed for each cell for all i values that correspond to negative elements in the corresponding row of the cell. The phase φi, of any transmitted sub-carrier ft, is obtained by summing the values of the total phase Φi, after changing phase signs, and dividing the result of the summation by a number determined according to m and/or the ratio between the magnitudes of the sub-carriers within a cell, and the magnitudes of the sub -carriers within its neighboring/overlapping cells.

The transmitter and receiver may be a base station and a mobile, or a stationary, receiver of a cellular system, respectively. The transmitter may be a local TV or Radio station and the receiver is a mobile, or a stationary, TV receiver or Radio receiver, respectively. The digital modulation may be carried out by phase-shifting the sub-carriers' RF frequencies, such that the phases being uniformly spaced on a O ÷ 360° circle and representing digital symbols. The modulation method may be selected from the group of: BPSK, DBPSK, QPSK, DQPSK, MSK, MPSK, DMPSK. The data may consist of digital symbols.

A guard interval may be added to each symbol before transmission of the symbol, for reducing the effect of transient interference in the receiver, that can be later identifying and removing.

The present invention is directed to a system for increasing the number of communication channels that can be simultaneously established within a cell being part of a group of cells, that comprises: a) means for generating the number of m^:=2ⁿ (n=l, 2,..,) identical orthogonal sub-carriers i, fz,.., f_m, over which individual data is transmitted in each communication channel, for all cells within the group; b) means for allocating a corresponding phase φ for each sub-carrier fi. (i=l, 2,..,m) within a cell, wherein the absolute value I φ I is determined by the data that is transmitted and the digital modulation scheme, and is identical for all φi within the cell, and the sign of each φi within the cell is identical to the sign of a corresponding element taken from a row in a mxm Walsh - matrix that corresponds to the cell, such that neighboring/overlapping cells use different rows; c) means for transmitting its data over the sub-carriers and their corresponding phases in each cell; d) means for receiving all transmissions carried by all sub-carriers and obtaining the total phase Φi of all the received signals at frequency fϊ, e) , means for changing, for each cell, the sign of total phase Φi for all i values that correspond to negative elements in the corresponding row of the cell; and f) means for obtaining the phase φi, of any transmitted sub-carrier fi, by summing the values of the total phase Φi, after changing phase signs, and dividing the. result of the summation by a number determined according to m and/or the ratio between the magnitudes of the sub-carriers within a cell, and the magnitudes of the sub-carriers within its neighboring/overlapping cells.

The system may further comprise: a) means for simultaneously modulating each of the m sub -carriers by the same digital symbols; b) means for generating a digital composite carrier by the summation of the modulated sub -carriers; and c) circuitry for converting the digital composite carrier to an analog composite carrier, for transmitting the analog composite carrier; d) means for digitizing the received transmissions carried by all sub-carriers; e) means for calculating the magnitudes and phases of the spectral components of the received signal; f) means for calculating the phases representation of the symbols of the selected transmission point, by summing up the calculated phases; g) means for receiving training signal for measuring its magnitude, propagation delay and multi-path propagation interference; h) means for generating a phase correction signal being capable of compensating undesired phase shifts in the received signal; i) means for generating a correction signal being capable of compensating undesired errors in the calculated phases representing the symbols of the selected transmission point; j) means for reconstructing the symbols according to the corrected phases and the measured magnitudes; and k) means for converting the reconstructed symbols into analog signal.

Brief Description of the Drawings

The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein:

Fig. 1A Schematically illustrates one CDMA cell (prior art);

Fig. IB Schematically illustrates three .CDMA cells (prior art);

Fig. 2A Schematically illustrates a TDMA cellular system (GSM, AMPS, DAMPS), in which seven adjacent cells have different frequencies to eliminate mutual RF interference (prior art);

Fig. 2B Schematically illustrates a TDMA cellular system (GSM, AMPS, DAMPS), using the 'Tree Sector Cell' technique (prior art); Fig. 2C Schematically illustrates a TDMA cellular system (GSM, AMPS, DAMPS), using the 'Six Sector Cells' technique (prior art);

Fig. 3A Schematically illustrates the minimal critical area(s) in conventional systems;

Fig. 3B Schematically illustrates the absence of critical area, according to a preferred embodiment of the invention;

Fig. 4 Schematically illustrates mutual interference between two communication systems using essentially the same RF channels (prior art);

Fig. 5A Schematically illustrates applying an inversion rule in a simplified case (i.e. two cells), according to a preferred embodiment of the invention;

Fig. 5B Schematically illustrates applying an inversion rule in a more practical case (i.e. three cells), according to a preferred embodiment of the invention;

Fig. 6 Schematically illustrates transmitting exemplary training signals from cells using essentially the same four sub-carriers, according to a preferred embodiment of the invention;

Fig. 7A Schematically illustrates a block diagram of a transmitter, according to a preferred embodiment of the invention; Fig. 7B Schematically illustrates a block diagram of a receiver, according to a preferred embodiment of the invention;

Fig. 7C Schematically illustrates a two-cell model having exemplary schemes combinations, according to a preferred embodiment of the invention;

Fig. 8 is a flowchart of a process by which the data is interpreted by a receiver, according to a preferred embodiment of the invention; and

Fig. 9 Schematically illustrates exemplary FBS systems having four sub-carriers (n=2) and eight sub-carriers (n=3), according to a preferred embodiment of the invention.

Detailed Description of Preferred Embodiments

In one aspect the present invention is directed to a method and system for increasing the number of communication channels that can be simultaneously established within a cell. A signal, which is transmitted from a cell in cellular systems, or from local radio and television stations, comprises m sub-carriers spaced uniformly throughout allocated frequency band. The preferred number of sub-carriers in the multi-carrier signal is m-4. However, m can be any value which complies with the rule m = 2" (Λ=1, 2, 3 etc.).

In order to allow better understanding of the principles present invention, a reference is made hereinafter to a simple case wherein a data, which is to be transmitted from a cell, is represented by a phase φ_t . Since the same data is transmitted simultaneously by every sub -carrier within a cell, the same absolute value of | φ_t | is allocated to all m sub-carriers in e.g. cell 1, meaning that whenever said data is to be transmitted from said cell, every sub-carriers is modulated by the same \ φ_t \ . The absolute value of | φ_t \ is determined by the data and the digital modulation scheme (e.g. QPSK), and the sign of each^, within the cell is selected as to be identical to the sign of a corresponding element taken from a row in a m * m Walsh matrix that corresponds to said cell. Neighboring (to said cell, or when cells are partially or completely overlapping) cells use different rows taken from the same matrix, as to guarantee that transmissions from neighboring/overlapping cells are orthogonal to each other.

A receiver receives all transmissions carried by all sub-carriers and obtain, by summing the corresponding phases, the total phase Φ_; of all the received signals at frequency ft . The receiver, then, changes the sign of the total phase Φ_;- for all i values that correspond to negative elements in the corresponding row of the cell. Finally, the phase φ_t is obtained in the receiver, by summing the values of the total phase Φ, , after changing phase sign, and dividing the result of the summation by a number determined according to m and/or the ratio between the magnitudes of the sub -carriers within a cell, and the sub -carriers within its neighboring/overlapping cells.

Choosing sub -carriers

According to the invention, each of the m sub-carriers carries simultaneously (i.e. modulated by) the same data (i.e. digital symbols). In order to guarantee the absence of inter-symbol interference at the sampling time, and to allow clear separation of the modulation signals of the m sub-carriers in the receiver, the multi-carrier signal satisfies the orthogonality condition ft = k * (l/T_s) , wherein T_s is the duration of the symbol, ft is the frequency spacing between the sub-carriers in the multi-carrier signal and k is an integer. In other words, the sub-carriers in a multi-carrier signal are uniformly spaced throughout a selected frequency band, and the spacing is inversly related to the symbol duration T_s . In order not to waste channel bandwidth, '/V is chosen to be unity (i.e. k=l).

Choosing schemes combinations (SCi)

According to the invention, the same m sub-carriers are allowed to be used for transmitting data from several adjacent cells. However, the present invention is also characterized by another unique feature, which is the way of determining the schemes combination (S ) for each cell. For example, a phase combination in one cell may be S^,C₁={/₁ : +Ψ₁ , /₂ : +Ψ₁ ,/₃ : -Ψ₁ ,/₄ : -Ψ₁ }, wherein ft through f₄ are the sub-carriers of a specific sub-carrier signal, which are modulated by positive and negative symbols schemes as specified (ft and ft are modulated by positive symbol schemes +Ψι , while /₃ and ft are modulated by negative symbol schemes -Ψi).

For example, a minimal system comprising three cells, each cell having a unique | Ψ,- | and the following schemes combination (SC_j ):

: -Ψi , ft₂ ■ +Ψι , ft : -Ψj , ft : -Ψ_j }, for a first transmission point;

SC₂ -{ft ^'• +Ψ2 > fi ^'• ^~^2 > h '■ ^+ψ2 > Λ ^{: -ψ} ₂}_> f°^{r a} second transmission point; and

SC₃ ={ _j : +Ψ₃ , ft : -Ψ₃ , ft : -Ψ₃ , ft : +Ψ₃ }, for a third transmission point.

Each SC_t- (i=l, 2, 3) could be looked over as a serial (e.g. serial SC_l : + Ψj ,

+ Ψ_} , -Ψj , -Ψi) or a code. According to the present invention, the set/group of serials/codes SCj , SC₂ and SC₃ is chosen in a way that ensures that each serial code in this set is orthogonal to every other serial/code in the set.

These orthogonal serials/codes are selected from the known Walsh-Hadamard matrix, which is used to generate orthogonal codes. These Walsh-Hadamard orthogonal codes (i.e. serials) are generated recursively by:

Hn Tin

H2" where ι = [l] and H'i = -Ht Tin H' n

Thus,

H4 = etc., up to the desired order m„ which is an

integer power of 2. The number of disagreements and agreements is equal for each pair of rows in the matrix. Thus, each row (i.e. code/serial) is orthogonal to every other row (zero cross-correlation), and using this property enables to select each row as a Walsh-Hadamard code, which may be used for coding.

According to one embodiment of the present invention, a basic system comprises three cells (i.e. transmission points), each cell having the same four sub-carriers. As can be seen in matrix H₄ , it is comprised of four orthogonal serials/codes (i.e. SQ ,

1, 2, 3). However, since the first serial code SC₀ '=[1, 1, 1, 1] is impractical (i.e. there is no need to implement the inversion rule), only three orthogonal codes are available (i.e. SQ ', SC₂ ' and SC₃ '), which is adequate in this case, since there are only three cells in the above-mentioned example.

After selecting the orthogonal codes (SC_j ', S ' and SC₃ '), each symbol scheme | Ψ_; | is multiplied by SQ '. For example, cell 1 is assigned a basic symbol scheme | Ψ | and a serial/code SQ ' (see matrix H₄). Therefore, multiplying | Ψj | by S ' yields the following schemes combination (SQ):

SQ =1 Ψi ] *SQ'=| Ψj I *[+l ,-l,+l,-l] = [+Ψ_j -ψ_j.+ψ, -ψ,]

Cells 2 and 3 are assigned their schemes combinations (i.e. SC₂ and SC₃ ) in the same manner, after choosing a different SQ- ' code for each cell from the remaining orthogonal codes.

Utilizing the WALSH matrix by the present invention improves the selectivity of a receiver, thereby allowing said receiver to receive a signal from a selected transmission point without being interfered by other adjacent transmission points.

It should be noted that in cases wherein more than four sub -carriers are to be used in a cell and/or more than three cells utilize the same set of sub-carriers, a Walsh-Hadamard matrix of higher order should be considered, in order to generate more orthogonal codes to chose from.

Inversion of phases at a receiver

In order to allow a receiver to selectively receive a signal that is transmitted from a specific cell, the receiver is programmed to change the sign(s) of negative schemes of the sub-carriers in the selected composite carrier. For example, if a signal from cell 1 is to be received, of which composite signal comprises the schemes combination : +Ψj , ₂ : +Ψ_{l 5 3} : -Ψ_{1 ?} : -Ψ_j}, the receiver is programmed to change the sign of the phases of ft₃ and f₄. It should be noted that the sign of the phases of ft₃ and ft that are received from other adjacent cells, are also changed. However, by virtue of the orthogonal codes (i.e. SQ '), the effect of ft and ft of other cells are eliminated.

Summation of the phases

As a result of the above-mentioned inversion process (i.e. changing the sign of phases), the receiver has three sets of modified carriers, one of which is a set of 'all positive' symbol schemes, belonging to the selected transmission point, and the other two sets of symbol schemes, belonging to the adjacent/interfering cells, each one having at least one negative phase scheme. After the completion of the inversion process, the receiver sums-up the 'phase contribution' of every sub-carrier, from every adjacent transmission point. Assuming, for example, that there are three cells, in which each multi-carrier comprises m=4 sub-carriers, each one of the sub-carrier has, therefore, three phases contributions. For example, sub-carrier ft contributes (to the summation) +Ψ₁from cell A, + Ψ₂ from cell B and -Ψ₃ from cell C. Similarly, ft_x through ft contribute their respective phases.

At the end of the summation process, only the sub-carriers from the selected cell remain to be further processed (demodulated), while other signals cancel out each other. Since the selected composite carrier comprises sub-carriers that are modulated by the same data, the data can be extracted from either one of the sub-carriers. The data extraction is relatively easy, because, as is mentioned before, the sub-carriers in the multi-carrier signal are orthogonal, which provides for clear separation of the modulation signals of the sub-carriers in the receiver.

Compensating -for non-ideal conditions of the communication channel

The communication channel affects several factors, such as propagation delay, echoes etc. One additional major factor is that a mobile receiver may receive signals from three adjacent cells having essentially the same set of m sub-carriers. Since these signals are transmitted from different transmitters, the received signals may have different magnitudes, and as a consequence, calculating the summation of the phases will result in erroneous phase and data extraction. Therefore, the receiver has to periodically measure the transmission conditions in the relevant (i.e. the closest cells) environment. The measurement stage is carried out by the system in the following way: every predefined time interval each cell uses only one predefined carrier (from the set of m sub-carriers) to transmit a special coded signal, hereinafter called a 'training signal'. At the same time, or at other time(s), the other cells transmit their own training signals by using different sub-carriers. For example (assuming that m-4 and ft is used for other purpose), if cell A uses sub-carrier ft to transmit its training signal, then cell B can use, for instance, ft , in which case only ft is left to be used by cell C for sending its training signal. By receiving these .three training signals, the receiver measures their corresponding magnitudes, and makes calculations in order to compensate for the erroneous phase calculations. Continuing the above example, /₄ is used by each cell to transmit a training signal to the receiver, by which the receiver calculates the channel propagation delays for each cell (i.e. A, B and C), as well as the multi-path propagation. The above-mentioned measurements allow the receiver to apply various compensation factors in order to yield a signal that resembles the original data that is transmitted from a transmitter.

Guard interval

Guard interval is a technique of which purpose is to ehminate transient effects due to multi-path receptions, doppler effect and other factors. During the guard interval the receiver does not interpret the transmitted signal. This technique will not be further discussed, as it is well known in the art of wireless communication systems, such as Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB-T) terrestrial systems that use COFDM techniques. Whether guard intervals should be used in the invention depend on the signal's delay and the symbol(s) duration. If the signal's delay is Δt and the symbol duration is T_s , the duration of the guard interval At_g must comply with the rule Δt < At_g < (0.1 ÷ 02) * T_s .

According to the present invention, if the signal's delay is long in comparison to the symbol duration, the symbol transmission rate is reduced to allow a longer symbol duration, into which a guard interval of adequate duration can be affiliated. For example, the symbol rate can be doubled by doubling the number of the sub-carriers (e.g. from four to eight) while still maintaining the principle of having different scheme combinations in each group of sub-carriers in adjacent cells, which is the essence of this invention. Utihzing eight sub-carriers instead of four doubles the bandwidth of the channel. However, the channel's capacity is doubled as well, thereby maintaining the efficiency of the channel.

Signal power

Assuming a conventional system has a carrier with magnitude of V (volts) and a maximum transmission power R₀ , the new multi-carrier with m=4 sub -carriers has a maximum peak power transmission of

{(F72) *4}² =4*R₀. Therefore, the new multi-carrier requires as four times peak power as a conventional system does. Nevertheless, the new FBS system's power density is essentially one fourth comparing to the power density of signal that is transmitted by a conventional system, since utihzing m=4 (for example) sub-carriers allows reducing the magnitude of each individual sub-carrier. Signal to Noise ratio (S/N)

As is aforementioned, each of the sub-carriers transmitted power density is lower than the transmitted power density of a conventional system. However, since the same information/data is contained in each of the m=4 sub-carriers, the Signal-to Noise (S/N) ratio is essentially kept the same.

Signal to Interference ratio (S/I)

The same principle that holds in estimating the S/N ratio applies when estimating the S/I ratio, namely utihzing 'n' sub-carriers, each sub-carrier having lower transmission power density in comparison with conventional systems, results in improved S/I ratio. Consequently, the distance between two identical cells, i.e. having exactly the same set of sub-carriers and their corresponding combinations of schemes, can be reduced when compared with the distance between two identical cells in conventional systems, because the power density of the transmitted signals is significantly low. Therefore, according to one embodiment of the present invention, the basis of the (FBS) cellular system is formed from a cluster having three adjacent cells (each cell having a different composite carrier), of which pattern repeats itself as required without interference between two adjacent cells within the same cluster of cells, or between cells in adjacent clusters.

Svnphase Interference

Synphase interference is essentially ehminated in a FBS systems, since, according to the present invention, FBS system involves inverting half of the sub-carriers' phases, thereby causing to 'self-canceling' of such interference. Adjacent cells transmitting signals with high magnitudes

In some cases, adjacent cells may transmit signals with magnitudes that are essentially as high as the magnitude of the transmission from a cell, e.g. cell 1, of which data is to be received. For example, if the magnitude of the signal to be received is 1.0, a transmission signal received from adjacent cell 2 may have a magnitude of 0.9, and a transmission signal received from adjacent cell 3 may have a magnitude of 0.8. Consequently, a problem may arise, according to which several of the received sub-carriers may significantly fade, thereby increasing the effect of noise.

In order to solve this problem, every possible of modulated signal from cell 2, or cell 3, is added, at the receiver, to the signal received from cell 1. For example, in a system that utilizes e.g. the QPSK modulation technique (i.e. there are four symbols- [00], [01], [10] and [11]), the receiver simulates receiving simultaneously four additional (to the transmission received from cell 1) signals from cell 2, or from cell 3. The first signal relates to symbol [00], the second signal to symbol [01] etc. The receiver adds the signal received from cell 1 to the first, second, third and fourth simulation signals (representative of symbols [00], [01], [10] and [11]), thereby achieving a set of four new signals. The receiver then chooses from this set the best signal; i.e. the signal having sub-carriers with the largest magnitudes, after which it continues the same process of demodulation as is described before (i.e. applying the inversion rule, etc.). Modulation techniques

As is hereinbefore described, each sub-carrier is modulated by digital symbols. In the following detailed description, a digital modulation technique known in the art as Quadrature Phase Shift Keying (QPSK) is referred to. However, according to the present invention, the sub -carriers of the FBS system can be modulated by utihzing other currently known digital modulation techniques, such as Binary PSK (BPSK), Differential Binary PSK (DBQPSK), Differential QPSK (DQPSK), Minimum Shift Keying (MSK) , Minimum Phase Shift Keying (MPSK), Differential Minimum Phase Shift Keying (DMPSK) etc.

Efficient utilization of the communication band

As is mentioned before, conventional systems provide a communication band that allows only a relatively low number of mobile transceivers, located at the same sector within a cell, to utilize said band at the same time. This drawback is due to the maximum number of channels that might be used by a sector in a cell, which is rather low (e.g. 20 channels per sector in a GSM system). In several conventional systems communication channels are allocated to sectors in a cell according to the number of users in a sector; namely channels are allocated to a more 'busy' sector on the expanse of less busy sectors.

One cell in a cellular system currently occupies a communication band of 25 MHz, a principle that is coped by the present invention. Therefore, this kind of cell exploits a wideband RF channel. However, since, according to the present invention, the same set of m sub-carriers is reused in a multiple cells/base-stations, this wideband RF channel is practically utilized to transmit data with higher rate than in conventional cellular systems, wherein one cell comprises a channel of which band is narrower, but its data transmission rate is lower.

According to the present invention, the same set of m sub-carriers can be used by any number of adjacent transmission points without cross interference, due to the unique phases arrangements. In contrary to conventional systems, where the frequency is changed as one moves from one transmission point to another adjacent transmission point, in the new system the schemes combinations are changed from one point/area to another adjacent point/area. Implementing the newly disclosed method allows a better exploitation of the frequency spectrum/bands.

The essence of the present invention is that instead of reusing frequencies, as is the case in conventional systems, substantially only schemes combinations are reused, thus efficiently utilizing frequency bands.

It should be noted, that the method described in this disclosure can be advantageously utilized by systems other than cellular systems. For example, local radio and television stations can benefit from the- present invention as well.

The invention makes use of a communication technique known in the art as COFDM. The present invention makes use of some features of this technique; namely m orthogonal sub-carriers are also used, but unlike in the conventional COFDM systems, each of the m sub-carriers (simultaneously) transmits the same data. Another major difference is that according to the invention, every transmission point transmits the same m sub-carriers. However, each transmission point has a different schemes combination. Consequently, the same channel, comprising the same m sub-carriers, may be used by several adjacent transmission points without cross interference. A mobile receiver which is tuned to one transmission point is not affected, therefore, by other transmitting points.

Referring to Fig. 5A, it depicts a simple case of two cells (A and B), each transmission point comprises the same four sub-carriers (i.e. ft through ft) but different schemes combination (SQ and SC₂). As can be seen in the figure, ft has a negative symbol scheme (501a) in cell A (-Ψ_j) and a positive symbol scheme (502a) in cell B (+Ψ₂) . Similarly, f has a positive symbol scheme (503a) in cell A (+ Ψ_t) and negative symbol scheme (504a) in cell B (-Ψ₂) . The sign of the symbol scheme of f_λ and ft is the same in both cells; i.e. +Ψ,- (i=l, 2) for f₁ (505a) and -Ψ,. for ft₂ (506a).

A mobile phone might be located at a critical area between cells A and B (507a), and receive a signal b(t) described by Equation 1. This is the worst case since the magnitude of the interfering signal is as high as the selected signal. However, due to the phases arrangement (i.e. each transmission point having a unique schemes combination), the receiver can cancel out any interference of that kind.

Equation 1:

b(t) = A * Sin(wγ* + φ ) + A *Sin(w_lt + φ₂) + A * Sin(w₂t -φ_\) + A *Sin(w₂t - ₂) + A * Sin(w₃t -φ^ + A * Sin(w₃t + φ₂) + A * Sin(w₄t + φ ) + A * Sin(w₄t - φ₂)

wherein:

' w₍ '- denotes an angular speed of sub-carrier ft (i.e. w_{ = 2π * ft_t ) ' * ' - denotes a multiplication sign

'A' - denotes the sub-carriers magnitudes in both cells (i.e. A and B) in the worst case; that is when the interfering signal(s) is the strongest.

Usually the interfering signal has smaller magnitude (i.e. <'A').

Eq. 1 is rearranged to yield Eq. 2, in which the relation between the carriers (i.e. wl through w4) and their respective phases is more apparent.

Equation 2:

b(t) = 2A * Cos[(φ_x - φ₂) 12] * Siniwγt + (φ_l + φ₂)/2] + 2 A * CosK-φγ +φ₂)/2]* Sin[w₂t ~(φ_l + φ₂)l2] + 2A * Cos[-(φ_{ + φ₂) 12] * Sin[w₃t + (-φ_λ + φ₂) 12] + . 2A * CosKφ + φ₂)/2]* Sin[w₄t + (φχ -φ₂)l 2]

Activation of the inversion rule

According to the invention, the receiver is capable of changing its inversion rule according to a specific transmission point which is to be received.

Assuming that the mobile receiver receives a slightly stronger signal from cell A, it is set to mode 'receiver A', which means that the receiver is programmed to invert the symbol scheme of carriers ft₂ and ft₃ in order to get 'all positive' phases (508a). Additionally, the symbol schemes of ft and ft₃ , which are received from cell B, are also inverted according to the same inversion rule. The significance of this symbol scheme inversion process will be understood by further inspecting the various phases in Equation 2. The phases of the received four carriers, before applying the inversion rule, are:

f_\ "» ι+P₂)/2 ; ₂ - -(^ι+^₂)/2 ;/₃-» -(^-^₂)/2

and ft -» (fh.-<)ll

The phases of /₂ and /₃ after applying the inversion rule are:

Only the symbol schemes of ft and /₃ are inverted in this example. The symbol schemes of ft and ft remain unchanged, since they are already received from cell A as positive symbol schemes (505a and 503a, respectively). Therefore, no inversion step is required in their case. After implementing the inversion rule as described, the four phases are then summed-up to yield (see Eq.3):

Equation 3:

(φ_l+φ₂)/2+(φ_l+φ₂)/2+(φ_l-φ₂)/2-(φ_l-φ₂)/2= (4*ft)/2

Since the mobile receiver is programmed, according to this example, to apply the inversion rule that allows it to receive signals from cell A, the resultant signal phase depends only on the phase of the sub-carriers in cell A (i.e. ψι), while it is not affected by (any of the sub-carriers of) the signal transmitted from cell B. Similarly, if the receiver's mode is switched to receive signals from cell B, after inverting ft₂ and ft₄ the resultant phase is (4* φ₂)l2.

It should be noted, that the resultant divisor in Eq. 3 (i.e. '2') is a private case, where the sub-carriers have the same magnitude 'A' (see Eq. 1). In a general case, wherein each sub-carrier has a different magnitude, the result of Eq. 3 is (4 * φχ)l q ('q' : a real number). In order to calculate q, a training signal is transmitted from each cell's base station, of which magnitude is measured by/in a receiver.

Fig. 5B illustrates a more realistic example of three cells (i.e. A, B and C) in a cellular system, each cell uses the same four sub-carriers (i.e. ft through f_A) but with different symbol schemes, thus creating three distinguished sets of multi-carriers. In cell A sub-carriers ft_l and ft have a symbol scheme + Ψ^ , and sub -carriers ft and ft have a symbol scheme -Ψ_A . In cell B sub -carriers f_x and ft have a symbol scheme + Ψ_# , and sub -carriers f₂ and f₄ have a symbol scheme ~Ψ_# . Similarly, in cell C sub-carriers f[ and f₄ have a symbol scheme + Ψ , and sub-carriers ft and ft have a symbol scheme -Ψ .

According to the invention, receiver 501b may be switched over three modes of operations in order to allow it to receive signal coming from either cell (i.e. A, B or C). The decision to which cell it will be tuned to is taken according to the strongest signal, which is received in the receiver. Assuming that a mobile phone is closer to cell A, its receiver is forced by the transmitter in cell A to switch to 'receiver A' mode; namely receiver A is programmed to invert the phases of carriers ft and ft to yield 'all positive' phases (502b). Since transmission points B and C are allowed, according to the invention, to be rather close to transmission point A, receiver A detects their signal, which is almost as strong as the signal coming from point A. However, since receiver A is programmed to invert ft and ft (i.e. to yield 'all positive' symbol schemes), it also inverts ft and ft₄ in signals which are received from cells B and C. Inverting signal B by receiver A results in negative symbol schemes (504b), and the same applies to signal C whenever received by receiver A (503b).

Fig. 6 illustrates one exemplary method for measuring the magnitude of representative sub-carriers; i.e. one representative carrier from each transmission point (cell). According to this method, a cell (e.g. cell A) transmits only one (non- modulated) sub-carrier (e.g. f[) from the set of m available sub-carriers, which is received in a receiver. The magnitude of the received signal is measured and recorded in a memory in the receiver, while the phases, which are embedded in the transmitted signal(s), are 'known' to the receiver; i.e. the receiver 'knows' what phases it expects to receive. It should be noted that since the same transmitter is used in cell A to transmit sub-carriers ft through ft , all these sub-carriers are received, during normal operation, at the receiver at essentially the same magnitude. Therefore, transmitting one sub-carrier (e.g. ft) is adequate for measuring the magnitude of the multi-carrier signal transmitted from this cell. Similarly, another cell (e.g. cell B) transmits an other sub-carrier (e.g. ft), and the last cell C transmits an other sub-carrier ft . After having measured the relevant signals' magnitudes, the receiver has a means for using these measurements to calculate the required compensating factors in order to make the necessary adjustments in the demodulated si nal. Sub-carrier ft is used to measure the signal propagation delays, after which a further adjustment can be made, in the receiver, to the interpreted signal. Measuring the changes in the phases of the four sub-carriers, as compared to the 'known'/expected phases, allows the receiver to calculate multi-path propagation, as well as propagation delays.

Fig. 7A illustrates a block diagram of a transmitter, according to a preferred embodiment of the invention. The output of an analog source 601 is sampled and digitized by an Analog-to-Digital (A/D) converter 602. The stream of binary bits enters a Digital Signal Processor (DSP), which is comprised of two main elements. The first element (604) is a module which assembles a predefined constant portion of the digital data stream into symbols. The symbols are the data by which the 'n' RF sub-carriers 603 are modulated, for example, in a technique known as Quadrature Phase Shift Keying (QPSK). Obviously other modulation techniques may be utilized by the present invention, such as Binary PSK (BPSK), Differential Binary PSK (DBQPSK), Differential QPSK (DQPSK), Minimum Shift Keying (MSK) etc. A digital signal compression feature is also included in the transmitter, although not shown in Fig. 7 A.

The second element of the DSP is the Inverse Fast Fourier Transform (IFFT) 605. Implementing 604 results in having spectral components of the m sub-carriers with their respective phases. Therefore, the IFFT is required to transforms the digitized samplings X(k) back to samples in the time domain X(n), after which a Digital-to -Analog (D/A) module 606 transforms the sampled signal to an analog signal. The analog signal comprises a multi-carrier signal, which comprises m sub-carriers, each sub-carrier phase changes according to the changes in the stream of bits (i.e. changing symbols). The modulated multi-carrier (i.e. composite signal) signal is then 'up-shifted' in frequency (607); i.e. from Intermediate Frequency (IF) to the final/transmitted frequency, and transmitted by antenna 608. It should be noted that the result of the IFFT 605a could be achieved by applying any type of 'frequency-domain' to 'time-domain' transformation.

Fig. 7B illustrates a block diagram of a receiver, according to a preferred embodiment of the invention. The received analog signal (by antenna 609) is 'down-shifted' (610) in frequency to IF frequency and digitized by A/D converter 611. The inversion rule of the receiver is implemented, for example, by FFT (612) in the DSP module, after which the phases of the received sub -carriers (from the selected transmission point and from other adjacent transmission points) are summed-up to cancel out the interfering sub -carriers from adjacent cells. After eliminating the interfering sub -carriers, the symbols (i.e. data) are extracted from the remaining (i.e. selected) m sub-carriers, and an analog signal is created by the D/A converter. A digital signal decompression feature is also included in the receiver, although not shown in Fig. 7B. . It should be noted that the result of the FFT 612 could be achieved by applying any type of to 'time-domain' to 'frequency-domain' transformation.

Fig. 7C illustrates an example of two adjacent cells A and -B, each cell having a transmission point with a multi-carrier signal comprising four sub-carriers (m=4); i.e. ft_x through ft₄. This figure further illustrates an example of two schemes combinations. Additionally, symbols comprising two binary bits are predefined. Therefore, four distinguished symbols are possible in such a system (i.e. '00', '01', '10' and '11').

For the sake of simplifying the description, the following is assumed: 1) there is only one transmission point (i.e. cell A), and the basic symbols representation (i.e. scheme) is as described under cell A; namely: 2) symbol [00] is represented by a phase of + 10 , symbol [01] by a phase of +100 _} symbol [10] by a phase of ^~80 _an(j symbol [11] by a phase of -170° .

As can be seen in Fig. 7C, f[ (in cell A) has a positive symbol scheme + Ψ . The practical implication in this case is that whenever sub-carrier ft is to be modulated by e.g. symbol [00] it is phase-shifted by +10° . However, whenever sub-carrier ft_λ is to be modulated by symbol [11], it is phase-shifted by -170° etc.

Continuing with the above-mentioned example, in contrary to ft and ft , f₃ and ft (see Fig. 7C: Cell A) have negative symbols schemes (-Ψi). For example, whenever symbol [00] is the modulating data, a phase shift of. -10° is added to ft and ft . Similarly, whenever symbol [10] is the modulating data, a phase shift of + 80° is added to ft₃ and ft₄.

Returning back to the two-cell case depicted in. Fig. 7c, each transmission point has a different basic symbol scheme. Unlike in cell A, cell B has a different basic symbol scheme (i.e. | Ψ₂ | ), namely, the symbol [00] is basically represented by a phase of +70 degrees, symbol [01] by a phase of +160 degrees, symbol [10] by a phase of -20 degrees and symbol [11] by a phase of -110 degrees. The same negation principle is applied to ft through ft . For example, whenever the symbol [00] is the modulation data, a phase of (-70) degrees is added to ₂and ft₃.

It should be noted that, within each cell, each data/symbol (e.g. in cell A) simultaneously modulates ft_λ through ft . Fig. 8 illustrates a flow chart of the receiving steps. This figure references a typical case of three transmission cells, each cell uses a multi-carrier signal comprising four sub-carriers. Since the received multi-carrier signal is digitally modulated, a synchronization function (801) must be apphed on it. If a guard interval is added to the transmitted signal, this block (i.e. 802) is required in order to identify it and to allow the receiver to extract the actual/effective data. The guard interval is used as recovery time, and the receiver regards as effective data only the signal that is received between two consecutive guard intervals.

As is hereinbefore described (see Fig. 6), every predefined time interval each cell transmits a training signal. For example, cell A may use for this purpose sub-carrier /j , cell B sub-carrier ft etc. Additionally, each cell uses ft₄ to transmit a training signal for measuring the signal propagation delay (s). After receiving all these training signals and measuring the corresponding magnitudes and delays, the required compensation factors (i.e. At,Aφ,q ) are calculated (803). The usage of them is described hereinafter.

There are two types of phases' corrections that must be carried out in the receiver. The first one is due to signal propagation delay during normal operation (i.e. whenever an actual data is received)- Δt (804) is used to make the necessary phases adjustments (807) in that respect. The second type of phases corrections is due to the variance in the sub-carriers magnitudes- 'g' (806) is used to make these necessary corrections.

The FFT (808) is apphed on the phase-corrected signal (807), in order to calculate the multi-carriers magnitudes 810 and phases 809 in the received signal. After having calculated the magnitudes and phases, the phases are summed up (811) to cancel out any interfering sub-carriers in a way that is described before. However, as is mentioned above, the resulting sum of phases (811) is erroneous due to the differences in the various sub-carriers magnitudes. Therefore, after measurements of the magnitudes have been made (803), and the correction factor 'g' has been calculated (805), based on these measurements, 'g' is used (806) to make the necessary phases corrections (812). Phase φ (812) represents, therefore, the calculated phase(s) representing symbols which are received from a selected transmitter (i.e. a base station in a cell). Together with φ , the calculated magnitudes (810) are used to correct errors in the stream of incoming digital bits, by applying a 'Soft Decision' technique (813), which is a technique well known in the art of decoding of digital data, and therefore will not be further described.

When summing up the phases (811), the result may be negative, which poses a problem whenever trying to interpret the received signal by a Soft Decision Decoding (813). In order to solve this problem, prior to applying the Soft Decision Decoder (813), a sum of the absolute values of the phases is calculated, and its result examined. Whenever the resulting sum is greater than 360° , it signifies that the sign of the calculated phase should be changed to yield the true phase, thereby allowing the receiver to yield a symbol(s) that resemble the symbols that were originally transmitted. The problem and corresponding solution are demonstrated in Example- 1.

Example 1:

Assuming that there are two cells/base stations using the same one sub-carrier frequency, and each sub-carrier is modulated by shifting its phase by a different shift (e.g. φ in cell A and φ₂ in cell B), we get the following composite signal:

cos(wt + φ ) + cos(wt + φ₂) =

2 * cos{(wt + #>₁ -wt -φ₂)l2} * co${(wt + φ_l + wt + φ₂)/2} =

2* cos{(φι -φ₂)/2} * cos{wt + (φι + φ₂)/2}

The term cos{(<z)₁ -^₂)/2} affects both magnitude and sign (+/-) of the overall received signal. The signal magnitude, therefore, becomes negative

whenever the condition <Pl ^~ <P2 > 90° is met; i.e. for φ_x » φ₂ , or

2

Σ I φ |> 360° . A negative magnitude results in an erroneous interpretation, by the receiver, of the received phase. For example, an originally transmitted phase of +10° , which may represent e.g. a symbol '01', may cause the receiver to yield, after some calculations in the receiver, a phase like -15.4° . However, since in this example the condition ∑ | > |> 360° is met, the sign of the resultant phase is changed to yield + 15.4° . Moreover, since the phases, which represent symbols, are widely spaced from each other (i.e. used in QPSK modulation), the receiver interprets + 15.4° as

+ 10° (and not e.g. as +100° ), which is the desired result.

After the Soft Decision Decoder (813) tests each symbol, it disassembles the symbols, to provide the corresponding reconstructed binary digits (814), after which they are converted to analog signal by a Digital-to-Analog converter.

Fig. 9 Schematically illustrates two exemplary FBS systems, one system having four sub-carriers (n=2) and a second system having eight sub-carriers (?ι=3). In the first case, in which there are four sub-carriers, only three schemes combinations are utilized (i.e. numbers 1, 2 and 3). Combination number 0 is impractical since there is nothing advantageous about this combination; i.e. a receiver is not required to apply any Inversion Rule.

As is explained hereinabove, the number of sub-carriers may be increased

(i.e. according to the rule 2ⁿ , n=l,2,3,..,) in order to increase data transmission rate etc.. Fig. 9 illustrates a second FBS system wherein τι=3. Eight possible schemes combinations are depicted (i.e. numbers 0 to 7). Combination number 0 is impractical for the same reason(s) as described for combination number 0 in the former case (i.e. for n=2, Fig. 9). Three larger cells are depicted in the figure (cells A, B and C), and two smaller cells within the larger cells; cell D in cell A and cell E in cell B. In this example, schemes combinations 1 and 2 are assigned to cell A, combinations 4 and 5 to cell B and combinations 6 and 6 to cell C. Additionally, schemes combination 3 is assigned to cells D and E. As can be seen in Fig.9 (n=3), the frequency band used by each cell is essentially doubled comparing to the case of four sub-carriers. However, data capacity is also doubled, thus maintaining frequency efficiency. The same schemes combination (i.e. combination 3) can be utilized in cells D and E since they are not considered adjacent cells. These cells' and their corresponding schemes combinations arrangements are only an example, as the present invention offers flexibility in assigning frequencies and schemes combinations to cells. The cluster of a FBS system may comprise three cells, as depicted in Fig 9 (n=2), or three major cells plus two minor cells, as depicted in Fig 9 (n=3), or any other combination. The FBS system may be extended by repeating (i.e. reusing) this cluster. Extended communication bandwidth

In order to facilitate the understanding of the way the bandwidth is extended and utilized, a reference to the following example is made. According to this example, the method disclosed in the present invention is compared with a conventional Coded Division Multi-Access (CDMA) system.

CDMA systems are the more advanced systems that are widely used today. However, the present invention presents a significant advantage over CDMA systems, as is demonstrated in the following example. CDMA systems have minimal 15 users per MHz per cell (see J. Gardiner, B. West, Personal Communication Systems and Technologies, Artech House, Boston). Consequently, a 25 MHz frequency band can serve up to 375 Voice Connections. According to the present invention, and assuming a bit rate of 20 kbit/s (like in CDMA-900) and a frequency efficiency in QPSK- FBS signals of 0.5 bit/Hz, the total of Voice Connections that can utilize the same 25 MHz frequency band, is 600.

It should be noted that due to lacking data regarding actual number of Voice Connections (VC) in CDMA systems, any comparison between said CDMA system and the method disclosed in the present invention is not straightforward. The minimal number of 15 users per MHz per cell is only a theoretical number. However, in practice a larger number is often used on the expense of service quality, since increasing the number of VC in CDMA systems results in increasing cross interference and multi-path propagation interference.

Referring now to GSM systems, the Bit Rate (BR) for each Voice Connection (VC) is 22.8 kbits/s. Assuming there are a maximum of 20 channels in each sector of a cell, each channel having 8 Voice Connection (VC), a maximum of 160 VC can be utilized in this system. In the same manner, assuming that one VC is transmitted, according to the present invention, at a rate of 25 kbits/s (including guard interval), and each channel having the same number of VC (i.e. 8, like in GSM), the channel bit rate is 200 kbits/s. Since according to present invention the frequency efficiency in the QPSK-FBS signal is 0.5 bit/Hz, the actual frequency channel occupies 400 kHz. Consequently a 25 MHz frequency band can contain 62 such channels. Therefore, a total of 62*8=496 VC can be used. The latter result indicates that a fur larger number of data sources can (simultaneously) utilize a single communication band.

Simulation results

An example of the principles described in this disclosure is described in table 1. The basis for the simulation is the following:

1. Three adjacent transmission points have been selected; i.e. SI, S2 and S3, each point having four sub-carriers (i.e. ft through ft);

2. The corresponding symbols schemes are: for Si + Ψ, = {+10°,+100\-170° -80°} , for S2 +Ψ₂ = {+40° ,+130° -140° -50°} and for S3 +Ψ₃ = {+70° ,+160°, -110° -20°} ;

3. The corresponding schemes combinations are: for Si -> SQ = {+Ψ₁,-Ψ₁,-Ψ₁,+Ψ₁} , for S2 -» SC₂ = {+Ψ₂,-Ψ₂,+Ψ₂,-Ψ₂} and for S3 ^ SC₃ = {+Ψ₃,+Ψ₃,-Ψ₃,-Ψ₃} ;

4. The training signals from these three transmission points, as received at a receiver located at equal distance from each said transmission point, are: φ_x = +10° , φ₂ = +40° and φ₃ = -110° , and Al=A2=A3 (each point transmits a training signal with the same magnitude;

5. The training result: g=2.027. 'g' is the factor of which purpose is to allow correcting the received phases representing the corresponding digital symbols; and

6. Transmission point Si is assumed to be the selected point from which the transmission is to be received, while S2 and S3 are the interfering transmission points.

Table 1 shows part of the simulation results, in which point Si constantly transmits the same symbol (i.e. represented by a constant φ_λ = +\0° , column 1 in the table), while S2 and S3 transmit various signals (columns 2 and 3 respectively). In the first line, for example, S2 and S3 transmit the same symbol as Si, said symbol being represented by >₂ = +40° and φ₃ = +70° . After implementing the inversion rule and summing up the various phases as described hereinbefore, a phase representing the original symbol, as is transmitted by point Si, is calculated. However, as is mentioned before, this calculation is expected to yield erroneous phase. Therefore, the calculated 'g' (g=2.027,- in this example) is used to correct the calculation. The corrected result appears in column 4 in the table. As can be seen in this column, there is a large variance in the resulting calculations (e.g. from 7 to 15.9 degrees). Nevertheless, since only four symbols are expected, being represented by four phases spaced 90° from each other; i.e. + _x = {+10°, +100° -170°, -80°} , it is an easy task for a Soft Decision Decoder to decide that the actual received symbol is the one which is represented by +10° , as can be seen in the table (column 6, final result). As is described hereinbefore (see Example- 1), the resulting phase may be negative. Example- 1 specifies the causes for this problem and suggests the corresponding solution for it. For example (see column 4), in line 6 the resulting phase is -15.1° . The negative sign is due to Σ^ - 529.9° (see column 5). However, according to the invention (and Example- 1), since the latter phase is greater than 360° , the sign of this phase is changed to plus ('+') sign.

Table 1;

Advantages of the invention

The present invention improves performance over other wireless communication systems, due to its new type of composite carrier. The main advantage of the present invention is that the communication capacity of the existing frequency range is significantly increased. Other advantages, that result from the main advantage, are:

1) Possibility to utilize the same frequencies in different cells, regardless the distance between the cells;

2) Low multipath propagation influence;

3) Decreasing power spectral density of radiation/transmission;

4) With the new composite carrier system (i.e. FBS system), there are no critical areas between adjacent cells;

5) No synphase interference influence; and

6) Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB-T) systems can also benefit from the mentioned advantages.

The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention.

Claims

1. A method for increasing the number of communication channels that can be simultaneously established within a cell being part of a group of cells, comprising: a) obtaining a frequency band to be used by each cell within said group; b) for all cells within said group, determining the number of m=2ⁿ

(n=l, 2,..,) identical orthogonal sub-carriers ft, f%,.., fm., over which individual data is transmitted in each communication channel; c) for each sub-carrier ft (i=l, 2,..,m) within a cell, allocating a corresponding phase φ_x- , wherein the absolute value | φ_t \ is determined by the data that is transmitted and the digital modulation scheme, and is identical for all ft_t within said cell, and the sign of each within said cell is identical to the sign of a corresponding element taken from a row in a mxm Walsh matrix that corresponds to said cell, such that neighboring/overlapping cells use different rows; d) in each cell, transmitting its data over said sub-carriers and their corresponding phases; e) receiving all transmissions carried by all sub-carriers and obtaining the total phase Φi of all the received signals at frequency ft; f) for each cell, changing the sign of total phase Φi for all i values that correspond to negative elements in the corresponding row of said cell; and g) obtaining the phase φi, of any transmitted sub-carrier ft, by summing the values of the total phase Φi, after changing phase signs, and dividing the result of the summation by a number determined according to m and/or the ratio between the magnitudes of the sub-carriers within a cell, and the magnitudes of the sub-carriers within its neighboring/overlapping cells.

2. A method according to claim 1, wherein the transmitter and receiver are a base station and a mobile, or a stationary, receiver of a cellular system, respectively.

3. A method according to claim 1, wherein the transmitter is a local TV or Radio station and the receiver is a mobile, or a stationary, TV receiver or Radio receiver, respectively.

4. A method according to claim 1, wherein the digital modulation is carried out by phase-shifting the sub-carriers' RF frequencies, said phases being uniformly spaced on a O ÷ 360° circle and representing digital symbols.

5. A method according to claim 4, wherein the modulation method is selected from the group of: BPSK, DBPSK, QPSK, DQPSK, MSK, MPSK, DMPSK.

6. A method according to claim 1, wherein the data consists of digital symbols.

7. A method according to claims 1 and 6, further comprising adding guard interval to each symbol before transmission of said symbol, for reducing the effect of transient interference in the receiver.

8. A method according to claim 1, further comprising identifying and removing guard interval from each symbol.

9. A system for increasing the number of communication channels that can be simultaneously estabhshed within a cell being part of a group of cells, comprising: a) means for generating the number of m=2ⁿ (n=l, 2,..,) identical orthogonal sub-carriers ft, fe,.., f_m, over which individual data is transmitted in each communication channel, for all cells within said group; b) means for allocating a corresponding phase φi for each sub-carrier ft (i=l, 2,..,m) within a cell, wherein the absolute value [ φi I is determined by the data that is transmitted and the digital modulation scheme, and is identical for all φi within said cell, and the sign of each φi within said cell is identical to the sign of a corresponding element taken from a row in a mxm Walsh matrix that corresponds to said cell, such that neighboring/overlapping cells use different rows; c) means for transmitting its data over said sub-carriers and their corresponding phases in each cell; d) means for receiving all transmissions carried by all sub -carriers and obtaining the total phase Φi of all the received signals at frequency ft; e) , means for changing, for each cell, the sign of total phase Φi for all i values that correspond to negative elements in the corresponding row of said cell; and f) means for obtaining the phase φi, of any transmitted sub-carrier ft, by summing the values of the total phase Φi, after changing phase signs, and dividing the result of the summation by a number determined according to m and/or the ratio between the magnitudes of the sub-carriers within a cell, and the magnitudes of the sub -carriers within its neighboring/overlapping cells.

10. A system according to claim 9, wherein the transmitted individual data is digital symbols.

11. A system according to claims 9 and 10, wherein transmitting digital symbols over all sub-carriers further comprising: a) means for simultaneously modulating each of said m sub-carriers by the same digital symbols; b) means for generating a digital composite carrier by the summation of said modulated sub-carriers; and c) circuitry for converting said digital composite carrier to an analog composite carrier, for transmitting said analog composite carrier.

12. A system according to claim 9, wherein receiving transmissions carried by all sub-carriers further comprising: a) means for digitizing the received transmissions carried by all sub-carriers; b) means for calculating the magnitudes and phases of the spectral components of said received signal; c) means for calculating the phases representation of the symbols of the selected transmission point, by summing up said calculated phases; d) means for receiving training signal for measuring its magnitude, propagation delay and multi-path propagation interference; e) means for generating a phase correction signal being capable of compensating undesired phase shifts in the received signal; f) means for generating a correction signal being capable of compensating undesired errors in. the calculated phases representing the symbols of the selected transmission point; g) means for reconstructing said symbols according to said corrected phases and said measured magnitudes; and h) means for converting said reconstructed symbols into an analog signal.

13. A system according to claim 12, wherein the modulation used is selected from the group of: BPSK, DBPSK, QPSK, DQPSK, MSK, MPSK, DMPSK.

14. A system according to claim 12, wherein the data carrying signal(s) is an RF signal.

15. A system according to claim 12, wherein the transmission point(s) and receiver are base station and mobile, or stationary, receiver of a cellular system, respectively.

16. A system according to claim 12, wherein the transmission point(s) is a local TV/Radio station and the receiver is a mobile, or a stationary, TV/Radio set, respectively.

17. A system according to claim 11, wherein step a) further comprising means for adding guard interval to each symbol before transmission of said symbol, for reducing the effect of transient interference in the receiver.

18. A system accordin to claim 12, wherein step a) further comprising means for identifying and removing guard interval from each symbol, for reducing the effect of transient interference in the receiver.

19. A system according to claim 12, wherein the sub -carriers' phases are shifted in accordance with their corresponding symbol schemes, and the modulation at the transmitter is carried out by a frequency-domain to time-domain transformation.

20. A system according to claim 12, wherein calculating the magnitudes and the corresponding phases of the received signal is carried out by first applying time-domain to frequency-domain transformation on said received signal, and then applying on the transformed, signal, by the receiver, the inversion rule.

21. A system according to claim 12, wherein inverting the negative phases schemes of selected sub-carriers having negative schemes, is carried out by time-domain to frequency-domain transformation of said negative phases schemes.

22. A system according to claim 12, wherein reconstructing the selected symbols is carried out by a Soft Decision decoder.