WO2004017539A1

WO2004017539A1 - Wireless communication system and a method of operating a wireless communication system

Info

Publication number: WO2004017539A1
Application number: PCT/GB2003/003208
Authority: WO
Inventors: Simon Reza Saunders; Vasileios Nikolopoulos
Original assignee: University Of Surrey
Priority date: 2002-08-16
Filing date: 2003-07-29
Publication date: 2004-02-26
Also published as: AU2003285690A1; GB0219170D0

Abstract

A wireless communication system has a distributed antenna arrangement and has the capability to deselect an antenna element thereby excluding its antenna signal from processing if the antenna signal level exceeds a predetermined threshold.

Description

WIRELESS COMMUNICATION SYSTEM AND A METHOD OF

OPERATING A WIRELESS COMMUNICATION SYSTEM

This invention relates to a wireless communication system and a method of operating

a wireless communication system.

The invention concerns wireless communication systems having distributed antenna

arrangements, as may be deployed, for example, to provide coverage in an indoor

environment, such as an office.

In a WCDMA system, the power levels transmitted by mobile users are subjected to

constant power control (PC) by their closest base station. This creates a problem,

because the PC dynamic range limits the minimum power that a mobile user can

transmit. Thus, if the mobile user is operating at this minimum power level and is

close to the base station, the signal strength received at the base station may well be

too high in relation to receiver sensitivity. If another mobile user is located much

further away from the base station, at the boundary of the cell, for example, the signal

levels received from both mobile users may differ by an amount exceeding the linear

dynamic range of the base station even if the more distant mobile user is operating at

the maximum available power level. This results in either distortion of the stronger

signal, which may degrade the performance of all users in the cell, or adjustment of

the dynamic range of the base station to accommodate linear operation in response to the higher strength signals. However, an adjustment of dynamic range is generally

undesirable because the noise floor of the receiver will then increase and so mobile

users producing weak signals (e.g. users at the cell edge) will be lost from coverage.

This leads to cell shrinkage and potential system degradation. Similar considerations

also apply in the case of mobile users operating on adjacent channels. Although such

users derive some protection from operation of the receiver's adjacent channel

rejection ratio, the aforementioned problems may be even greater because the power

control imposed on one channel is independent of the power control imposed on an

adjacent channel.

In general, the described problems are expected to be greatest, for both co-channel and

adjacent-channel mobile users, for in-building scenarios where there are likely to be

users located very close to an indoor base station antenna whose signals will mask

those of more distant users.

When a distributed antenna arrangement is deployed to provide coverage in an indoor

environment, the signal produced by each mobile user is received by a plurality of

discrete antenna elements distributed around a building.

The inventors have discovered that characteristics of a distributed antenna

arrangement can be exploited to eliminate or reduce the aforementioned signal

blocking. In effect, improvement may be accomplished by deselecting any antenna element

encountering a signal that is too strong for the required dynamic range of the base

station.

According to one aspect of the invention there is provided a wireless communication

system including a base station and an antenna arrangement having a plurality of

distributed antenna elements each capable of supplying an antenna signal to the base

station for processing, wherein at least one said antenna element can be deselected by

selectively excluding the respective antenna signal or antenna signals from processing

by the base station.

According to another aspect of the invention there is provided a method of operating

a wireless communication system including a base station and an antenna arrangement

having a plurality of distributed antenna elements each capable of supplying an

antenna signal to the base station for processing, the method including deselecting at

least one said antenna element by selectively excluding the respective antenna signal

or antenna signals from processing by the base station.

Although deselection of one or more antenna element reduces the effectiveness of the

overall distributed antenna arrangement when operating in normal conditions, it serves

to avoid the catastrophic impact of receiver signal blocking and will only compromise

coverage in a localised region around the or each deselected antenna element. An embodiment of a wireless communication system according to the invention is

described, by way of example only, with reference to the accompanying drawings of

which:

Figure 1 illustrates a distributed antenna arrangement of which the individual antenna

elements (DAE) are connected to a base station in the form of a CDMA receiver and

receive signals from a single mobile user (MS),

Figure 2 illustrates deployment of two distributed antenna arrangements in a building,

Figure 3 shows test results obtained using one of the distributed antenna arrangements

having eight antenna elements, and

Figure 4 shows test results obtained using another of the distributed antenna

arrangements having six antenna elements.

Referring to Figure 1, the mobile user (MS) is very close to one of the antenna

elements DAE¹. Thus, the corresponding antenna signal received at the base station

will tend to mask antenna signals produced by mobile users that are more distant from

the antenna elements. This is illustrated by the following example.

The minimum output power P^ that can be transmitted by a mobile user (MS) is typically -44dBm. At close range e.g. 5m from a nearby antenna element (DAE¹) the

free space path loss at 2 GHz is approximately 52.4 dB. Assuming an indoor DAE

effective gain (Gain^) of 2.5dBi and an effective MS gain (Gain - f OdBd

(=2.15dBi) the received signal strength (Rx) at the antenna element and consequently

at the central base station location (assuming that additional cable losses are overcome

by a low noise amplifier) is:

R_χ =EIRP_MS-PathLoss +Gain ^ = -91 5dBm

where EIRP_MS is the effective isotropic radiated power of the MS given by:

EIRP_MS=P_τχ+Gain^MS

Hitherto, it has been customary to reduce the gain of the base station amplifier so as

to operate in the linear region in response to higher signal strength, and so in this

example the maximum level would need to be adjusted to - 91.75dBm. Since the

required signal-to-interference-plus-noise ratio requirement (SINR) for e.g. VOICE

SERVICE is approximately -18dB, the required receiver sensitivity at the base station,

RxSens would become -109.75dBm (i.e. -91.75dBm-18dB) which, of course, is much

higher than the original receiver sensitivity of approximately -121.1 dBm for which

only thermal noise levels are considered and no blocking effects are present.

Therefore, mobile users producing weaker signals would be lost from coverage. By contrast, in accordance with this embodiment of the invention, the nearby antenna

element DAE¹ is deselected by excluding its antenna signal from processing by the

base station, thereby enabling the receiver sensitivity to be maintained at a relatively

low level.

Wireless communication systems having active and passive distributed antenna

arrangements may differ somewhat in their modes of operation.

In the case of an active distributed antenna arrangement, each antenna element has a

receiver (or at least a low noise amplifier) which can detect the received signal level.

The base station monitors the detected signal levels and deselects any antenna element

at which the detected signal level exceeds a preset threshold value commensurate with

a required dynamic range.

In the case of a passive distributed antenna arrangement, the base station monitors the

level of a resultant signal formed by combining the individual antenna signals. If the

resultant signal exceeds a preset threshold level, the base station would deselect

antenna elements sequentially, one-by-one, until the resultant signal is sufficiently

small.

This can be accomplished faster by subdividing the elements into two groups,

examining the combined signal in each group, then subdividing the group with the higher combined signal and repeating the process until a single element is obtained.

This reduces the number of steps from N, where N is the number of elements, to less

then or equal to:

1 +ce//[log₂(N)]

where ceil[.] represents the next highest integer.

Quantification of performance of a system according to the invention in which one or

more antenna element can be deselected to avoid the blocking effect in the uplink due

to a nearby user producing a very strong signal which can potentially mask the weaker

signals of other users near the edge of the cell is now illustrated with reference to a

particular implementation shown in Figures 2 to 4.

Referring to Figure 2, two distributed antenna arrangements having eight and six

antenna elements respectively were deployed to provide coverage in a 13-storey

building. The building includes a number of offices with different dimensions and is

considered to be typical of an office environment. The average floor distances were

assumed to be 4 metres, whereas attenuations of 4 dB/wall and 15dB/floor were

assumed, as discussed, for example, in "Antennas and Propagation for Wireless

Communication Systems", Simon R. Saunders, Wiley & Sons, 1999 and "Radio

Coverage in buildings" J.M. Keenan et al, Br Telecom Journal, Vol 8, No 1, pp 19 -

24, January 1990. The propagation model proposed in the latter reference was used for the analysis which follows. The antennas were located on the ceilings of the

different stories at either point A or point B shown in Figure 2, chosen to give best

coverage for a given number of antennas. With the arrangement including six antenna

elements only point A was used to locate the antennas. However, when eight antenna

elements were deployed, it was possible to locate the additional elements (7^th and 8^th)

at point B, on different floors of the building to obtain better coverage. The radiation

pattern of the antennas was a typical indoor omni-directional one, providing coverage

mainly downwards but with significant upward pointing lobes as well. Antenna gains

of the antenna elements were 2 dBi, whereas the antenna gain of the mobile station

was 0 dBd with maximum transmitted power of 21 dBm. Power control dynamic

range of 65 dB has been assumed providing a minimum transmitting power of -44

dBm (i.e. 21dBm-65dB). The analysis presented here is based on active distributed

antenna systems, for which an increase of the noise levels by an amount equal to

101og(N)_> where N is the number of antenna elements occurs in the uplink.

As described in "CDMA Systems Engineering Handbook", J. S. Lee et al, 1998 the

maximum number of video (R = 144 Kbps) users that can be accommodated by a W-

CDMA system is approximately 13, as given by the following capacity equation:

Λ „. = +1

*" v.( ME_b/N_o]_Mm

where M^x is the maximum number of users of a W-CDMA system, W is the W- CDMA chip rate, R is the bit rate of the service, v is the voice activity factor (v = 1

for video users), f is the inter-cell interference factor (other cell-to-home-cell

interference), and [Eb/No]_MIN is the minimum signal energy per bit over the noise

spectral density that is required to meet a predefined quality of service (QoS).

Inter-cell interference factor values, f, between the interval 0.4-0.6 have been

proposed. However, due to improved isolation of buildings to the outdoor

environment the inter-cell interference factor was assumed to be 0.2, as proposed in

"Estimation of Capacity and required transmission power of WCDMA downlink

based on a downlink pole equation", K. Sipila et al, IEEE Vehicular Technology

Conference, Spring 2000, Tokyo, Vol 2, pp 1002-1005. The outage probabilities of

the deployed distributed antenna arrangement having eight antenna elements are

represented in Figure 3 by the dashed starred line 1. It is seen that for a coverage

requirement of 95% (or 5% outage) the maximum number of users has been reached.

The effect of a nearby user is shown by lines 2, 3, 4 and 5 for different distances 1.5,

2, 3 and 4 metres respectively from the closest antenna element. This shows that

without deselection, capacity reductions may be significant for distances up to 4

metres away from the antenna element. With a user standing at 1.5 metres away from

the antenna element, the capacity has more than halved (6 users instead of 13)

compared to when no blocking effects are present, whereas the capacity increases as

the user moves away from the affected antenna element. The effect of deselecting the antenna element receiving the strongest signal, can be

seen with the solid starred lines 6,7. Each line corresponds to a different scenario

where one antenna element is deselected at a time. Based on a uniform user

distribution, it is seen that the differences regarding which antenna element is

deselected are not significant, provided that the deselected antenna element is the one

that receives the strongest signal. Degradation in terms of coverage and capacity is

small, since it is still possible to accommodate 12 users instead of 13, when no

blocking effects are present.

Figure 4 illustrates the effect of deselection using a distributed antenna arrangement

having six antenna elements. In this case, without deselection, the effect of a user

being very close to one of the antenna elements can be detrimental, since at distances

less than 2 metres it is not possible to provide coverage to any user at 95% of the

locations. By de-selecting the antenna element which receives the strongest signal

these detrimental effects are alleviated, as shown by the solid starred lines 6,7 of

Figure 4.

Claims

1. A wireless communication system including a base station and an antenna

arrangement having a plurality of distributed antenna elements each capable of

supplying an antenna signal to the base station for processing, wherein at least one

said antenna element can be deselected by selectively excluding the respective antenna

signal or antenna signals from processing by the base station.

2. A system according to claim 1 wherein said base station is arranged to deselect

a said antenna element according to a signal level received at the antenna element.

3. A system according to claim 2 wherein each said antenna element includes

detection means for detecting the received signal level.

4. A system according to claim 3 wherein said detection means comprises a low

noise amplifier.

5. A system according to any one of claims 2 to 4 wherein said base station is

arranged to deselect a said antenna element if a signal level received at the antenna

element exceeds a threshold level.

6. A system according to claim 1 wherein said base station is arranged to combine said antenna signals to generate a resultant signal, monitor a level of said resultant

signal and deselect one or more of said antenna elements according to the level so

monitored.

7. A system according to claim 6 wherein said antenna elements are deselected

sequentially until said level is less than a predetermined threshold level.

8. A system according to claim 6 wherein said base station is arranged to divide

the antenna elements into two groups and to combine the antenna signals received at

the antenna elements of each group to generate respective resultant signals, and to

identify the group giving the strongest resultant signal and repeat the process with the

antenna elements of the group so identified until a single antenna element receiving

the strongest antenna signal has been identified.

9. A method of operating a wireless communication system including a base

station and an antenna arrangement having a plurality of distributed antenna elements

each capable of supplying an antenna signal to the base station for processing, the

method including deselecting at least one said antenna element by selectively

excluding the respective antenna signal or antenna signals from processing by the base

station.

10. A method according to claim 9 including monitoring a signal level received at each antenna element and deselecting an antenna element according to the monitored

signal level.

11. A method according to claim 9 including combining the antenna signals to

generate a resultant signal, monitoring a level of the resultant signal and deselecting

an antenna element according to the level so monitored.

12. A method according to claim 11 including deselecting the antenna elements

sequentially until the monitored level is less than a predetermined threshold.

13. A wireless communication system substantially as herein described with

reference to the accompanying drawings.

14. A method of operating a wireless communication system according to claim

9 and substantially as herein described.