WO1998007241A2

WO1998007241A2 - Method for adjusting carrier frequency, and a radio system

Info

Publication number: WO1998007241A2
Application number: PCT/FI1997/000476
Authority: WO
Inventors: Harri Holma; Timo Laakso
Original assignee: Nokia Mobile Phones Limited
Priority date: 1996-08-14
Filing date: 1997-08-13
Publication date: 1998-02-19
Also published as: FI963190A; AU3852097A; WO1998007241A3; FI963190A0

Abstract

The invention relates to a method for adjusting carrier frequency in a radio system, and a radio system, a base station (200) comprising measuring means (201) for measuring the received carrier frequency and for comparing the measured carrier frequency with the nominal carrier frequency, and transmission means (202) for sending a command to a subscriber terminal equipment (100) on the basis of the comparison of the frequency if the measured frequency differs essentially from the nominal frequency. The subscriber terminal equipment (100) comprises means (101) for adjusting the carrier frequency used in transmission and reception on the basis of said command, and means (102) for measuring the frequency of the received carrier, and for comparing the measured carrier frequency with the nominal carrier frequency and for adjusting the carrier frequency used in reception on the basis of the comparison.

Description

METHOD FOR ADJUSTING CARRIER FREQUENCY, AND A RADIO SYSTEM

FIELD OF THE INVENTION

The invention relates to a method for adjusting carrier frequency in a radio system comprising at least one base station and subscriber terminal equipment operating as transceivers, and which when transmitting and receiving, multiply a signal by a carrier.

The invention also relates to a method for adjusting carrier frequency in a radio system comprising at least one base station and subscriber terminal equipment operating as transceivers, and which when transmitting and receiving, multiply a signal by a carrier.

The invention further relates to a radio system comprising at least one base station and subscriber terminal equipment operating as transceivers, and which when transmitting and receiving, multiply a signal by a carrier.

BACKGROUND OF THE INVENTION In the Code Division Multiple Access method, that is, in the CDMA method, a user's narrowband signal is modulated by a spreading code having a broader band than a data signal to a relatively broad band. In known systems, bandwidths of 1.25 MHz, 10 MHz and 50 MHz, for example, have been used. A spreading code is generally formed of a long pseudo-random bit sequence. Each user has a specific spreading code. Several users send simultaneously on the same frequency band and data signals are separated from one another in receivers on the basis of a pseudo-random spreading code.

It is typical for a cellular network environment that a signal conveyed between the user and the base station does not travel directly but, depending on the characteristics of the environment, it propagates along paths of different lengths from a transmitter to a receiver. This kind of multipath propagation takes place even if there was a direct visual contact between the base station and the mobile station. This multipath propagation is mainly caused by signal reflections from surrounding surfaces. Signals propagating via different paths have a propagation delay of a different length, and they arrive at the receiver with different phases. The subscriber terminal equipment may be a mobile phone, for example, in which case the movement of the subscriber terminal equipment causes varying reflections. The relative movement of the subscriber terminal equipment also causes a frequency change of the size of a Doppler shift with respect to the nominal frequency of the signal.

In a cellular network environment users are located at random with respect to the base station and one another. The attenuation of a signal modulated to the carrier taking place between the base station and the subscriber terminal equipment is described by path loss which will increase at least like square when the distance grows. The movement of a subscriber terminal equipment with respect to a base station causes a Doppler shift. A rotating phase is produced when the frequency of the received signal has a different phase than the carrier frequency of the receiver. Reflections bring about multipath propagations. Attenuation is caused if multipath components propagating via different paths to the receiver are summed in the receiver. The carrier sent by subscriber terminal equipments close to the base station propagates fairly directly to the base station. In the transmission of a carrier far from the base station the carrier can, however, be reflected, whereby carriers propagate to the base station via various different paths. Carrier reflections cause various problems, such as rotating phase errors to the signal received by the CDMA receiver. There are particular problems with signal reception when the subscriber terminal equipments are moving, in which case the frequencies of the carriers change because of the Doppler shift.

In the cellular radio system, a base station receives a signal sent by a subscriber terminal equipment, the signal being modulated to the carrier. The base station demodulates the signal received by it and modulated to the carrier. In the CDMA radio system the base station demodulates all the signals it receives on the same carrier. There are, however, problems with signal reception as because of multipath propagation, for example, the carriers received at the base station do not have the same frequency but they differ from the nominal carrier frequency of the base station. The frequency changes of carriers make signal reception and channel estimation more difficult especially when interference cancellation is used in signal reception. If the frequencies of the received carriers differ from one another, it will bring about a phase component changing as constant at some of the subscriber terminal equipments, the component having an effect on the complex attenuation factor of the signal. CHARACTERISTICS OF THE INVENTION

The object of the present invention is to provide a method with which frequency change that disturbs signal estimation and reception will be diminished. This will be attained with a method as described in the preamble that is characterized in that the base station measures the carrier frequency it has received and compares the carrier frequency it has measured with the nominal carrier frequency, and if the measured frequency differs essentially from the nominal frequency, the base station will send a command to the subscriber terminal equipment to adjust the carrier frequency, and that on the basis of the command sent by the base station, the subscriber terminal equipment adjusts the carrier frequency used by it both in transmission and in reception.

This will also be attained with a method as described in the preamble that is characterized in that the subscriber terminal equipment measures the carrier frequency it has received and compares the carrier frequency it has measured with the nominal carrier frequency, and if the measured frequency differs essentially from the nominal frequency, the subscriber terminal equipment adjusts the carrier frequency used by it in reception.

The radio system of the invention is characterized in that the base station comprises measuring means for measuring the received carrier frequency and for comparing the measured carrier frequency with the nominal carrier frequency, and transmission means for sending a command to the subscriber terminal equipment on the basis of the comparison of the frequency if the measured frequency differs essentially from the nominal frequency, and that the subscriber terminal equipment comprises means for adjusting on the basis of the command the carrier frequency used in transmission and in reception, and means for measuring the received carrier frequency and for comparing the measured carrier frequency to the nominal carrier frequency and for adjusting the carrier frequency used in reception on the basis of the comparison.

Considerable advantages will be attained with the method of the invention. In the invention, the carrier frequency used in transmission and reception by the carrier of the subscriber terminal equipment is adjusted. By frequency adjustment, the carrier frequency received by the base station remains close to the nominal carrier frequency of the base station. In this way, the effect of the Doppler shift will be diminished, for example. Furthermore, phase errors in CDMA receivers are removed with the method of the invention. Removal of phase errors will improve channel estimation, and thus the quality of the connection between the subscriber terminal equipment and the base station will be amended.

The preferred embodiments of the method of the invention will also appear from the appended dependent claims, and the preferred embodiments of the receiver of the invention will appear from the appended dependent claims.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be explained in more detail with reference to the examples of the accompanying drawings, where

Figure 1 illustrates the essential parts of a block diagram of a subscriber terminal equipment operating as a transceiver used in the solution of the invention, and

Figure 2 illustrates the radio system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows the essential parts of a transceiver 100 used in the solution of the invention. The transceiver 100 is the exemplary case a subscriber terminal equipment 100, such as a mobile telephone. A base station 200 can also operate as a similar transceiver 100. The subscriber terminal equipment 100 includes a reception side A and a transmission side B. The reception side A comprises an antenna 111 , radio frequency parts 112, a demodulator 113, a decoder 114 and a handset 115. The transmission part B comprises a microphone 121 , a coder 122, a modulator 123, radio frequency parts 124 and an antenna 125. The subscriber terminal equipment 100 further comprises control means 131 which are common to the reception side A and transmission side B. The subscriber terminal equipment 100 also comprises means 101 for adjusting the carrier frequency and means 102 for measuring the carrier frequency and for comparing the carrier frequency. The means 102 measure signal strength.

Figure 2 shows a radio system comprising a subscriber terminal equipment 100 and a base station 200 operating as a transceiver. The base station 200 comprises an antenna 211 operating as a transceiver antenna. The subscriber terminal equipment 100 of the figure comprises means 101 and means 102. In principle, the base station 200 comprises almost the same parts as the subscriber terminal equipment 100. The base station 200 does not, however, include a microphone 121 or a handset 115. The base station 200 also comprises measuring means 201 for measuring the carrier frequency and transmission means 202 for sending commands to the subscriber terminal equipment 100. The means 101 of the subscriber terminal equipment 100 receive commands sent by the base station 200.

The reception side A of the subscriber terminal equipment 100 in Figure 1 operates in the following way. The radio frequency parts 112 of the subscriber terminal equipment 100 transfer a radio frequency analog signal from the antenna 111 to an intermediate frequency and filter the signal. The radio frequency parts 112 also detect the intermediate frequency signal. The demodulator 113 restores a broadband signal into a narrowband data signal. The data signal is decoded in a suitable manner with the decoder 114. The decoder 114 typically decodes a convolution-encoded signal, and the operation of the decoder 114 is based on the Viterbi algorithm, for example. The decoder 114 generally also de-encrypts and de-interleaves a pre- processed signal. From the decoder 114 the signal is further carried to the handset 115. The signal received at the antenna 111 is also conveyed to the means 101 which receive commands sent by the transmission means 202 of the base station 200 and adjust the carrier frequency. The means 102 measure the carrier frequency and measure the carrier frequency they use in reception. The transmission side B of the subscriber terminal equipment 100 operates in the following way. The microphone 121 receives an audio signal and the sends an electronic response to the coder 122. The coder 122 convolution-encodes and typically encrypts the signal. The coder 122 also interleaves the bits of the signal or bit groups. The convolution-encoded narrowband signal is pseudonoise-encoded into a broadband spread spectrum signal in the modulator 123. After this, the spread spectrum signal is transformed into a radio frequency signal in accordance with prior art in the radio frequency parts 124 and is sent via the antenna 125 to the radio path.

The control means 31 control the operation of both the reception side A and the transmission side B of the subscriber terminal equipment 100. The antennas 111 and 125 are transmission and reception antennas of the prior art subscriber terminal equipment 100. The operations of the transmission and reception antennae 111 and 125 are in practice combined to the same antenna. The microphone 121 , the handset 115, the radio frequency parts 112 and 124 are prior art parts, also used in other radio systems. In the radio system of Figure 2, the subscriber terminal equipment

100 sends a signal to the base station 200. The subscriber terminal equipment 100 sets up a connection to the base station 200 by means of the signal modulated to a carrier. The carrier frequency may change slightly in carriers that have propagated via different routes, in which case the reception of the carrier will become more difficult in the base station 200. The base station 200 shown also in the figure, operating as a transceiver, sends a signal to the subscriber terminal equipment 100. The base station 200 sets up a connection to the subscriber terminal equipment 100 by means of the signal modulated to a carrier. The carrier formed in the base station 200 can also propagate via several different routes to the subscriber terminal equipment 100.

The solution shown in Figure 2 is in the following explained in connection with uplink transmission direction. Uplink transmission refers to a transmission direction where the subscriber terminal equipment 100, such as a mobile phone in the cellular radio system, sends a signal modulated to a carrier to the base station 200. The signals modulated to carriers received at the base station 200 are demodulated by using one carrier. Because the frequencies of some carriers received by the base station 200 differ from the nominal carrier frequency, the carrier frequency is adjusted. The frequency is adjusted in such a manner that the carrier frequencies received by the base station 200 are as close as possible to the nominal frequency. It is assumed that the subscriber terminal equipment 100 is moving and it first comes closer to the base station 200 and after that it moves away from the base station 200.

When the subscriber terminal equipment 100 comes closer to the base station 200, the carrier frequency received by the base station 200 will grow because of the effect of the Doppler shift. However, when the subscriber terminal equipment 100 moves away from the base station 200, the carrier frequency received by the base station 200 will diminish. To eliminate the effect of the Doppler shift on frequency, frequency change is taken into account in the transmission of the carrier. In the following, formulae (1) and (2) are given for calculating the transmission frequency of the subscriber terminal equipment 100 in uplink transmission. The effect of the Doppler shift on the transmission frequency of the subscriber terminal equipment 100 has been taken into account in the formulae. Formula (1) is used when the subscriber terminal equipment 100 comes closer to the base station 200. Formula (2) is used when the subscriber terminal equipment 100 moves away from the base station 200. In the formula, f_u1 and f_u2 refer to the transmission frequency of the subscriber terminal equipment 100. The nominal frequency in the uplink direction is indicated in the formulae by reference f_unom and the Doppler shift by reference Δ_u1 and Δ_u2.

(1 ) f_u1 = f_unom - Δ_u1

The measuring means 201 of the base station 200 measure the carrier frequency sent by the subscriber terminal equipment 100 and compare the measured frequency to the nominal carrier frequency. The measuring means 201 also measure signal strength. If the frequency measured by the measuring means 201 differs from the nominal frequency, the transmission means 202 of the base station 200 send a command to the subscriber terminal equipment 100 on the basis of the comparison of frequencies by the measuring means 201 to adjust the transmission frequency of the carrier. The transmission means 202 will send a command to raise the frequency if the frequency measured by the measuring means 201 is lower than the nominal carrier frequency. The transmission means 202 will, however, send a command to lower the frequency if the frequency measured by the measuring means 201 is higher than the nominal carrier frequency.

If the base station 200 receives carriers that arrive at the base station 200 via different routes, the transmission means 202 of the base station send a command which is based on the measurement of the carrier which arrive first at the base station 200. The frequency adjustment command sent by the transmission means 202 can also be based on the carrier measured by the measuring means 201 with the greatest signal strength. The measuring means 201 also estimate the carrier frequency and send frequency adjustment commands on the basis of frequency estimation. Frequency estimation can in practice be realized in connection with channel estimation, for example. The adjustment command changes the transmission frequency of the subscriber terminal equipment 100 by a predetermined frequency step. Adjustment commands are in practice sent a few times in a second, for example.

The means 102 change the carrier frequency the subscriber terminal equipment 100 uses in reception and transmission on the basis of the commands sent by the transmission means 202 of the base station 200 The signals received by the subscriber terminal equipment 100 from various different base stations 200 cause problems especially when the signals have almost the same frequency This problem is lessened by keeping the transmission frequencies of the base stations 200 nominally the same The problem is further diminished in such a manner that the algorithm used in channel estimation has been planned to endure rotating phase errors caused by a frequency error

In the following, the solution of the invention shown in Figure 2 is explained in downlink transmission direction Downlink transmission refers to a transmission direction where the base station 200, for example, sends to the subscriber terminal equipment 100 a signal modulated to a carrier. The signals received by the subscriber terminal equipment 100, modulated to carriers are demodulated by using one carrier If the frequencies of the received carriers differ essentially from the nominal carrier frequency, carrier frequency will be adjusted. The object of the adjustment is that the carrier frequency received by the subscriber terminal equipments 100 of the base station 200 would be as close as possible to the nominal frequency It is assumed that the subscriber terminal equipment 100 in the figure is moving, comes closer to the base station 200 and after that moves away from the base station 200.

In the following, formulae (3) and (4) are given for calculating the carrier frequency used by the subscriber terminal equipment 100 in reception in downlink transmission. The effect of the Doppler shift on the transmission frequency of the subscriber terminal equipment 100 has been taken into account in the formulae. Formula (3) is used when the subscriber terminal equipment 100 comes closer to the base station 200. Formula (4) is used when the subscriber terminal equipment 100 moves away from the base station 200. In the formula, f_d1 and f_d2 refer to the reception frequency of the subscriber terminal equipment 100 The nominal frequency in the downlink direction is indicated in the formulae by reference f_dnom and the Doppler shift by reference Δ_d1 and Δ_d2. ) 'd1 ^— 'dnom ^" ^ 1

vv *d2 ^_ 'dnom ⁺ Δ_d2

The means 101 of the subscriber terminal equipment 100 measure the carrier frequency sent by the base station 200. The measuring means 101 also measure signal strength. The subscriber terminal equipment 100 adjusts the carrier frequency used especially in the radio frequency parts 112 and the modulator 113 on the reception side on the basis of the carrier received by it. This adjustment is used even if interference cancellation was not used on the reception side. The means 101 compare the carrier frequency received by it and on the basis of the comparison, adjust the carrier frequency they use in the receiver. In case the means 102 measure carriers received via different routes, the means 102 adjust the carrier frequency they use in the receiver on the basis of the carrier which arrives first at the subscriber terminal equipment 100. The carrier frequency in the receiver may also be based on the carrier with the greatest signal strength. The means 102 will diminish the carrier frequency used by them in the reception if the carrier frequency measured by the means 102 is lower than the nominal carrier frequency. The means 102 will, however, raise the carrier frequency they use in reception if the carrier frequency is higher than the nominal carrier frequency.

Although the invention is above explained with reference to the example of the accompanying drawings, it is evident that the invention is not restricted thereto, but it can be modified in various ways in the scope of the inventive idea disclosed in the appended claims.

Claims

1. A method for adjusting carrier frequency in a radio system comprising at least one base station (200) and subscriber terminal equipment (100) operating as transceivers, and which when transmitting and receiving, multiply a signal by a carrier, characterized in that the base station (200) measures the carrier frequency it has received and compares the carrier frequency it has measured with the nominal carrier frequency, and if the measured frequency differs essentially from the nominal frequency, the base station (200) will send a command to the subscriber terminal equipment (100) to adjust the carrier frequency, and that on the basis of the command sent by the base station (200), the subscriber terminal equipment (100) adjusts the carrier frequency used by it both in transmission and in reception.

2. A method for adjusting carrier frequency in a radio system comprising at least one base station (200) and subscriber terminal equipment (100) operating as transceivers, and which when transmitting and receiving, multiply a signal by a carrier, characterized in that the subscriber terminal equipment (100) measures the carrier frequency it has received and compares the carrier frequency it has measured with the nominal carrier frequency, and if the measured frequency differs essentially from the nominal frequency, the subscriber terminal equipment (100) adjusts the carrier frequency used by it in reception.

3. A method according to claim 1, characterized in that the base station (200) sends a command to the subscriber terminal equipment (100) to raise the frequency if the carrier frequency measured by the base station (200) is essentially lower than the nominal carrier frequency.

4. A method according to claim 1, characterized in that the base station (200) sends a command to lower the frequency if the carrier frequency measured by the base station (200) is essentially higher than the nominal carrier frequency.

5. A method according to claim 1, characterized in that the base station (200) sends a command to adjust the frequency on the basis of the measurement to the carrier which arrives first at the base station (200).

6. A method according to claim 1, characterized in that the base station (200) measures the strength of the signal received by it and sends a command to adjust the frequency on the basis of the measurement to the carrier which has the greatest signal strength when received at the base station (200).

7. A method according to claim 1 , characterized in that the base station (200) estimates the carrier frequency received by it and sends a command to adjust the frequency on the basis of the estimation by a predetermined frequency step.

8 A method according to claim 2, characterized in that the subscriber terminal equipment (100) adjusts the carrier frequency used by it in reception on the basis of the carrier that arrives first at the subscriber terminal equipment (100)

9 A method according to claim 2, characterized in that the subscriber terminal equipment (100) measures the strength of the signal received by it and adjusts the carrier frequency used by it in reception on the basis of the carrier which has the greatest signal strength when received at the base station (200)

10. A method according to claim 2, characterized in that the subscriber terminal equipment (100) raises the carrier frequency used by it in reception if the measured carrier frequency is greater than the nominal carrier frequency.

11. A method according to claim 2, characterized in that the subscriber terminal equipment (100) lowers the carrier frequency used by it in reception if the measured carrier frequency is lower than the nominal carrier frequency

12. A method according to claim 1 or 2, characterized in that the method is used in connection with the CDMA multiple access method.

13. A radio system comprising at least one base station (200) and subscriber terminal equipment (100) operating as transceivers, and which when transmitting and receiving, multiply a signal by a carrier, characterized in that the base station (200) comprises measuring means (201) for measuring the received carrier frequency and for comparing the measured carrier frequency with the nominal carrier frequency, and transmission means (202) for sending a command to the subscriber terminal equipment (100) on the basis of the comparison of the frequency if the measured frequency differs essentially from the nominal frequency, and that the subscriber terminal equipment (101) comprises means (101) for adjusting on the basis of the command the carrier frequency used in transmission and in reception, and means (102) for measuring the received carrier frequency and for comparing the measured carrier frequency to the nominal carrier frequency and for adjusting the carrier frequency used in reception on the basis of the comparison.

14. A radio system according to claim 13, characterized in that the transmission means (202) send a command to raise the frequency if the carrier frequency measured by the measuring means (201) is essentially lower than the nominal carrier frequency.

15. A radio system according to claim 13, characterized in that the transmission means (202) send a command to lower the frequency if the carrier frequency measured by the measuring means (201) is essentially higher than the nominal carrier frequency.

16. A radio system according to claim 13, c h a ra cte rized in that the means (102) raise the carrier frequency used in reception if the carrier frequency measured by the means (102) is higher than the nominal carrier frequency.

17. A radio system according to claim 13, characterized in that the means (102) lower the carrier frequency used in reception if the carrier frequency measured by the means (102) is lower than the nominal carrier frequency.

18. A radio system according to claim 13, characterized in that the measuring means (201) are arranged to measure the signal strength of the received carrier.

19. A radio system according to claim 18, characterized in that the transmission means (202) send a command to adjust the frequency on the basis of the carrier which has the greatest signal strength measured by the measuring means (201).

20. A radio system according to claim 13, characterized in that the transmission means (202) send a command to adjust the frequency on the basis of the carrier which arrives first at the base station (200).

21. A radio system according to claim 13, characterized in that the means (102) adjust the carrier frequency used by them in reception on the basis of the carrier which arrives first at the subscriber terminal equipment (100).

22. A radio system according to claim 13, characterized in that the means (102) adjust the carrier frequency used by them in reception on the basis of the carrier which has the greatest signal strength measured by the means (102).