WO2001018993A1

WO2001018993A1 - Transmission using an antenna array in a cdma communication system

Info

Publication number: WO2001018993A1
Application number: PCT/US2000/022559
Authority: WO
Inventors: David Mesecher
Original assignee: Interdigital Technology Corporation
Priority date: 1999-09-10
Filing date: 2000-08-17
Publication date: 2001-03-15
Also published as: EP1475900A2; BR0013889A; EP1475900B1; US6115406A; US7684469B2; CN1897482A; EP2230772A2; US6373877B1; ATE465561T1; IL181375A; ES2223579T3; NO20021177L; ATE279818T1; JP2005260977A; ES2225197T3; NO326753B1; DE60012944D1; JP5175324B2; DK1210777T3; US6983008B2

Abstract

The invention provides for transmission and reception of a data signal using a plurality of transmitting antennas. Each antenna transmits a different pilot signal having a pseudo random chip code sequence. A receiver filters each transmitted pilot using that pilot's chip code. The filtered pilots are weighted and combined. Each pilot signal's weight is adaptively adjusted in part on a signal quality of the combined signal. A data signal is transmitted such that different spread spectrum versions of the data signal are transmitted from each transmitting antenna. Each version having a different chip code identifier. Upon reception, each version is filtered with its associated chip code. The filtered versions are weighted in accordance with the adjusted weights associated with the pilot signal of the respective antenna.

Description

TRANSMISSION USING AN ANTENNA ARRAY IN A CDMA COMMUNICATION SYSTEM

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to signal transmission and reception

in a wireless code division multiple access (CDMA) communication system. More

specifically, the invention relates to a system and method of transmission using an

antenna array to improve signal reception in a wireless CDMA communication

system.

Description of the Prior Art

A prior art CDMA communication system is shown in Figure 1. The

communication system has a plurality of base stations 20-32. Each base station 20

communicates using spread spectrum CDMA with user equipment (UEs) 34-38

within its operating area. Communications from the base station 20 to each UE 34-

38 are referred to as downlink communications and communications from each UE

34-38 to the base station 20 are referred to as uplink communications.

Shown in Figure 2 is a simplified CDMA transmitter and receiver. A data

signal having a given bandwidth is mixed by a mixer 40 with a pseudo random chip

code sequence producing a digital spread spectrum signal for transmission by an

antenna 42. Upon reception at an antenna 44, the data is reproduced after correlation

at a mixer 46 with the same pseudo random chip code sequence used to transmit the

data. By using different pseudo random chip code sequences, many data signals use the same channel bandwidth. In particular, a base station 20 will communicate

signals to multiple UEs 34-38 over the same bandwidth.

For timing synchronization with a receiver, an unmodulated pilot signal is

used. The pilot signal allows respective receivers to synchronize with a given

transmitter allowing despreading of a data signal at the receiver. In a typical CDMA

system, each base station 20 sends a unique pilot signal received by all UEs 34-38

within communicating range to synchronize forward link transmissions. Conversely,

in some CDMA systems, for example in the B-CDMA™air interface, each UE 34-38

transmits a unique assigned pilot signal to synchronize reverse link transmissions.

When a UE 34-36 or a base station 20-32 is receiving a specific signal, all the

other signals within the same bandwidth are noise-like in relation to the specific

signal. Increasing the power level of one signal degrades all other signals within the

same bandwidth. However, reducing the power level too far results in an undesirable

received signal quality. One indicator used to measure the received signal quality is

the signal to noise ratio (SNR). At the receiver, the magnitude of the desired

received signal is compared to the magnitude of the received noise. The data within

a transmitted signal received with a high SNR is readily recovered at the receiver.

A low SNR leads to loss of data.

To maintain a desired signal to noise ratio at the minimum transmission power

level, most CDMA systems utilize some form of adaptive power control. By

minimizing the transmission power, the noise between signals within the same bandwidth is reduced. Accordingly, the maximum number of signals received at the

desired signal to noise ratio within the same bandwidth is increased.

Although adaptive power control reduces interference between signals in the

same bandwidth, interference still exists limiting the capacity of the system. One

technique for increasing the number of signals using the same radio frequency (RF)

spectrum is to use sectorization. In sectorization, a base station uses directional

antennas to divide the base station's operating area into a number of sectors. As a

result, interference between signals in differing sectors is reduced. However, signals

within the same bandwidth within the same sector interfere with one another.

Additionally, sectorized base stations commonly assign different frequencies to

adjoining sectors decreasing the spectral efficiency for a given frequency bandwidth.

Accordingly, there exists a need for a system which further improves the signal

quality of received signals without increasing transmitter power levels.

SUMMARY OF THE INVENTION

The invention provides for transmission and reception of a data signal using

a plurality of transmitting antennas. Each antenna transmits a different pilot signal

having a pseudo random chip code sequence. A receiver filters each transmitted

pilot using that pilot's chip code. The filtered pilots are weighted and combined.

Each pilot signal's weight is adaptively adjusted in part on a signal quality of the

combined signal. A data signal is transmitted such that different spread spectrum

versions of the data signal are transmitted from each transmitting antenna. Each version having a different chip code identifier. Upon reception, each version is

filtered with its associated chip code. The filtered versions are weighted in

accordance with the adjusted weights associated with the pilot signal of the

respective antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a prior art wireless spread spectrum CDMA communication

system.

Figure 2 is a prior art spread spectrum CDMA transmitter and receiver.

Figure 3 is the transmitter of the invention.

Figure 4 is the transmitter of the invention transmitting multiple data signals.

Figure 5 is the pilot signal receiving circuit of the invention.

Figure 6 is the data signal receiving circuit of the invention.

Figure 7 is an embodiment of the pilot signal receiving circuit.

Figure 8 is a least mean squarred weighting circuit.

Figure 9 is the data signal receiving circuit used with the pilot signal

receiving circuit of Figure 7.

Figure 10 is an embodiment of the pilot signal receiving circuit where the

output of each RAKE is weighted.

Figure 11 is the data signal receiving circuit used with the pilot signal

receiving circuit of Figure 10. Figure 12 is an embodiment of the pilot signal receiving circuit where the

antennas of the transmitting array are closely spaced.

Figure 13 is the data signal receiving circuit used with the pilot signal

receiving circuit of Figure 12.

Figure 14 is an illustration of beam steering in a CDMA communication

system.

Figure 15 is a beam steering transmitter.

Figure 16 is a beam steering transmitter transmitting multiple data signals.

Figure 17 is the data receiving circuit used with the transmitter of Figure 14.

Figure 18 is a pilot signal receiving circuit used when uplink and downlink

signals use the same frequency.

Figure 19 is a transmitting circuit used with the pilot signal receiving circuit

of Figure 18.

Figure 20 is a data signal receiving circuit used with the pilot signal receiving

circuit of Figure 18.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with reference to the drawing

figures where like numerals represent like elements throughout. Figure 3 is a

transmitter of the invention. The transmitter has an array of antennas 48-52,

preferably 3 or 4 antennas. For use in distinguishing each antenna 48-52, a different

signal is associated with each antenna 56-60. The preferred signal to associate with each antenna is a pilot signal as shown in Figure 3. Each spread pilot signal is

generated by a pilot signal generator 56-60 using a different pseudo random chip

code sequence and is combined by combiners 62-66 with the respective spread data

signal. Each spread data signal is generated using data signal generator 54 by mixing

at mixers 378-382 the generated data signal with a different pseudo random chip

code sequence per antenna 48-52, D,-D_N. The combined signals are modulated to

a desired carrier frequency and radiated through the antennas 48-52 of the array.

By using an antenna array, the transmitter utilizes spacial diversity. If spaced

far enough apart, the signals radiated by each antenna 48-52 will experience different

multipath distortion while traveling to a given receiver. Since each signal sent by an

antenna 48-52 will follow multiple paths to a given receiver, each received signal

will have many multipath components. These components create a virtual

communication channel between each antenna 48-52 of the transmitter and the

receiver. Effectively, when signals transmitted by one antenna 48-52 over a virtual

channel to a given receiver are fading, signals from the other antennas 48-52 are used

to maintain a high received SNR. This effect is achieved by the adaptive combining

of the transmitted signals at the receiver.

Figure 4 shows the transmitter as used in a base station 20 to send multiple

data signals. Each spread data signal is generated by mixing at mixers 360-376 a

corresponding data signal from generators 74-78 with differing pseudo random chip

code sequences D_π-D_NM. Accordingly, each data signal is spread using a different

pseudo random chip code sequence per antenna 48-52, totaling N x M code sequences. N is the number of antennas and M is the number of data signals.

Subsequently, each spread data signal is combined with the spread pilot signal

associated with the antenna 48-52. The combined signals are modulated and radiated

by the antennas 48-52 of the array.

The pilot signal receiving circuit is shown in Figure 5. Each of the

transmitted pilot signals is received by the antenna 80. For each pilot signal, a

despreading device, such as a RAKE 82-86 as shown in the Figure 5 or a vector

correlator, is used to despread each pilot signal using a replica of the corresponding

pilot signal's pseudo random chip code sequence. The despreading device also

compensates for multipath in the communication channel. Each of the recovered

pilot signals is weighted by a weighting device 88-92. Weight refers to both

magnitude and phase of the signal. Although the weighting is shown as being

coupled to a RAKE, the weighting device preferably also weights each finger of the

RAKE. After weighting, all of the weighted recovered pilot signals are combined

in a combiner 94. Using an error signal generator 98, an estimate of the pilot signal

provided by the weighted combination is used to create an error signal. Based on the

error signal, the weights of each weighting device 88-92 are adjusted to minimize the

error signal using an adaptive algorithm, such as least mean squared (LMS) or

recursive least squares (RLS). As a result, the signal quality of the combined signal

is maximized.

Figure 6 depicts a data signal receiving circuit using the weights determined

by the pilot signal recovery circuit. The transmitted data signal is recovered by the antenna 80. For each antenna 48-52 of the transmitting array, the weights from a

corresponding despreading device, shown as a RAKE 82-86, are used to filter the

data signal using a replica of the data signal's spreading code used for the

corresponding transmitting antenna. Using the determined weights for each

antenna's pilot signal, each weighting device 106-110 weights the RAKE's despread

signal with the weight associated with the corresponding pilot. For instance, the

weighting device 88 corresponds to the transmitting antenna 48 for pilot signal 1.

The weight determined by the pilot RAKE 82 for pilot signal 1 is also applied at the

weighting device 106 of Figure 6. Additionally, if the weights of the RAKE's

fingers were adjusted for the corresponding pilots signal's RAKE 82-86, the same

weights will be applied to the fingers of the data signal's RAKE 100-104. After

weighting, the weighted signals are combined by the combiner 112 to recover the

original data signal.

By using the same weights for the data signal as used with each antenna's pilot

signal, each RAKE 82-86 compensates for the channel distortion experienced by

each antenna's signals. As a result, the data signal receiving circuit optimizes the

data signals reception over each virtual channel. By optimally combining each

virtual channel's optimized signal, the received data signal's signal quality is

increased.

Figure 7 shows an embodiment of the pilot signal recovery circuit. Each of

the transmitted pilots are recovered by the receiver's antenna 80. To despread each

of the pilots, each RAKE 82-86 utilizes a replica of the corresponding pilot's pseudo random chip code sequence, P,-P_N. Delayed versions of each pilot signal are

produced by delay devices 114-124. Each delayed version is mixed by a mixer 126-

142 with the received signal. The mixed signals pass through sum and dump circuits

424-440 and are weighted using mixers 144-160 by an amount determined by the

weight adjustment device 170. The weighted multipath components for each pilot

are combined by a combiner 162-164. Each pilot's combined output is combined by

a combiner 94. Since a pilot signal has no data, the combined pilot signal should

have a value of 1+jO. The combined pilot signal is compared to the ideal value,

1+jO, at a subtractor 168. Based on the deviation of the combined pilot signal from

the ideal, the weight of the weighting devices 144-160 are adjusted using an adaptive

algorithm by the weight adjustment device 170.

A LMS algorithm used for generating a weight is shown in Figure 8. The

output of the subtractor 168 is multiplied using a mixer 172 with the corresponding

despread delayed version of the pilot. The multiplied result is amplified by an

amplifier 174 and integrated by an integrator 176. The integrated result is used to

weight, W_1M, the RAKE finger.

The data receiving circuit used with the embodiment of Figure 7 is show for

a base station receiver in Figure 9. The received signal is sent to a set of RAKEs

100-104 respectively associated with each antenna 48-52 of the array. Each RAKE

100-104, produces delayed versions of the received signal using delay devices 178-

188. The delayed versions are weighted using mixers 190-206 based on the weights

determined for the corresponding antenna's pilot signal. The weighted data signals for a given RAKE 100-104 are combined by a combiner 208-212. One combiner

208-212 is associated with each of the N transmitting antennas 48-52. Each

combined signal is despread M times by mixing at a mixer 214-230 the combined

signal with a replica of the spreading codes used for producing the M spread data

signals at the transmitter, D , , -D_NM. Each despread data signal passes through a sum

and dump circuit 232-248. For each data signal, the results of the corresponding sum

and dump circuits are combined by a combiner 250-254 to recover each data signal.

Another pilot signal receiving circuit is shown in Figure 10. The despreading

circuits 82-86 of this receiving circuit are the same as Figure 7. The output of each

RAKE 82-86 is weighted using a mixer 256-260 prior to combining the despread

pilot signals. After combining, the combined pilot signal is compared to the ideal

value and the result of the comparison is used to adjust the weight of each RAKE's

output using an adaptive algorithm. To adjust the weights within each RAKE 82-86,

the output of each RAKE 82-86 is compared to the ideal value using a subtractor

262-266. Based on the result of the comparison, the weight of each weighting device

144-160 is determined by the weight adjustment devices 268-272.

The data signal receiving circuit used with the embodiment of Figure 10 is

shown in Figure 11. This circuit is similar to the data signal receiving circuit of

Figure 9 with the addition of mixers 274-290 for weighting the output of each sum

and dump circuit 232-248. The output of each sum and dump circuit 232-248 is

weighted by the same amount as the corresponding pilot's RAKE 82-86 was

weighted. Alternatively, the output of each RAKE's combiner 208-212 may be weighted prior to mixing by the mixers 214-230 by the amount of the corresponding

pilot's RAKE 82-86 in lieu of weighting after mixing.

If the spacing of the antennas 48-52 in the transmitting array is small, each

antenna's signals will experience a similar multipath environment. In such cases, the

pilot receiving circuit of Figure 12 may be utilized. The weights for a selected one

of the pilot signals are determined in the same manner as in Figure 10. However,

since each pilot travels through the same virtual channel, to simplify the circuit, the

same weights are used for despreading the other pilot signals. Delay devices 292-

294 produce delayed versions of the received signal. Each delayed version is

weighted by a mixer 296-300 by the same weight as the corresponding delayed

version of the selected pilot signal was weighted. The outputs of the weighting

devices are combined by a combiner 302. The combined signal is despread using

replicas of the pilot signals' pseudo random chip code sequences, P₂-P_n> by the mixers

304-306. The output of each pilot's mixer 304-306 is passed through a sum and

dump circuit 308-310. In the same manner as Figure 10, each despread pilot is

weighted and combined.

The data signal recovery circuit used with the embodiment of Figure 12 is

shown in Figure 13. Delay devices 178-180 produce delayed versions of the

received signal. Each delayed version is weighted using a mixer 190-194 by the

same weight as used by the pilot signals in Figure 12. The outputs of the mixers are

combined by a combiner 208. The output of the combiner 208 is inputted to each

data signal despreader of Figure 13. The invention also provides a technique for adaptive beam steering as

illustrated in Figure 14. Each signal sent by the antenna array will constructively

and destructively interfere in a pattern based on the weights provided each antenna

48-52 of the array. As a result, by selecting the appropriate weights, the beam 312-

316 of the antenna array is directed in a desired direction.

Figure 15 shows the beam steering transmitting circuit. The circuit is similar

to the circuit of Figure 3 with the addition of weighting devices 318-322. A target

receiver will receive the pilot signals transmitted by the array. Using the pilot signal

receiving circuit of Figure 5, the target receiver determines the weights for adjusting

the output of each pilot's RAKE. These weights are also sent to the transmitter, such

as by using a signaling channel. These weights are applied to the spread data signal

as shown in Figure 15. For each antenna, the spread data signal is given a weight

by the weighting devices 318-322 corresponding to the weight used for adjusting the

antenna's pilot signal at the target receiver providing spatial gain. As a result, the

radiated data signal will be focused towards the target receiver. Figure 16 shows the

beam steering transmitter as used in a base station sending multiple data signals to

differing target receivers. The weights received by the target receiver are applied to

the corresponding data signals by weighting devices 324-340.

Figure 17 depicts the data signal receiving circuit for the beam steering

transmitter of Figures 15 and 16. Since the transmitted signal has already been

weighted, the data signal receiving circuit does not require the weighting devices

106-110 of Figure 6. The advantage of the invention's beam steering are two-fold. The transmitted

data signal is focused toward the target receiver improving the signal quality of the

received signal. Conversely, the signal is focused away from other receivers

reducing interference to their signals. Due to both of these factors, the capacity of

a system using the invention's beam steering is increased. Additionally, due to the

adaptive algorithm used by the pilot signal receiving circuitry, the weights are

dynamically adjusted. By adjusting the weights, a data signal's beam will

dynamically respond to a moving receiver or transmitter as well as to changes in the

multipath environment.

In a system using the same frequency for downlink and uplink signals, such

as time division duplex (TDD), an alternate embodiment is used. Due to reciprocity,

downlink signals experience the same multipath environment as uplink signals send

over the same frequency. To take advantage of reciprocity, the weights determined

by the base station's receiver are applied to the base station's transmitter. In such a

system, the base station's receiving circuit of Figure 18 is co-located, such as within

a base station, with the transmitting circuit of Figure 19.

In the receiving circuit of Figure 18, each antenna 48-52 receives a respective

pilot signal sent by the UE. Each pilot is filtered by a RAKE 406-410 and weighted

by a weighting device 412-416. The weighted and filtered pilot signals are

combined by a combiner 418. Using the error signal generator 420 and the weight

adjustment device 422, the weights associated with the weighting devices 412-416

are adjusted using an adaptive algorithm.

Claims

The transmitting circuit of Figure 19 has a data signal generator 342 togenerate a data signal. The data signal is spread using mixer 384. The spread datasignal is weighted by weighting devices 344-348 as were determined by thereceiving circuit of Figure 19 for each virtual channel.The circuit of Figure 20 is used as a data signal receiving circuit at the basestation. The transmitted data signal is received by the multiple antennas 48-52. Adata RAKE 392-396 is coupled to each antenna 48-52 to filter the data signal. Thefiltered data signals are weighted by weighting devices 398-402 by the weightsdetermined for the corresponding antenna's received pilot and are combined atcombiner 404 to recover the data signal. Since the transmitter circuit of Figure 19transmits the data signal with the optimum weights, the recovered data signal at theUE will have a higher signal quality than provided by the prior art. CLAIMSWhat is claimed is:

1. A method for use in a spread spectrum communication system having

a plurality of transmitting antennas, the method comprising:

transmitting from each transmitting antenna, a pilot signal having a

pseudo random chip code sequence uniquely associated with that antenna;

receiving at the receiver all of said transmitted pilot signals;

filtering each said transmitted pilot signal using that pilot signal's

pseudo random chip code sequence;

weighting each said filtered pilot signal by a particular weight;

combining said weighted pilot signals as a combined signal;

adaptively adjusting each said pilot signal's particular weight based in

part on a signal quality of the combined signal;

transmitting a data signal such that different spread spectrum versions

of the data signal are transmitted from each antenna, each version having a different

chip code identifier for the respective transmitting antenna; and

receiving the data signal via filtering each version with its associated

chip code and combining the filtered versions, wherein the different data signal

versions are weighted in accordance with the adjusted weights associated with the

pilot signal of the respective antenna.

2. The method of claim 1 wherein the different data signal versions are

weighted in accordance with the adjusted weights prior to transmission.

3. The method of claim 1 wherein the different data signal versions are

weighted in accordance with the adjusted weights after reception.

4. The method of claim 1 wherein adaptively adjusting is by using a least

mean squared algorithm.

5. The method of claim 1 wherein adaptively adjusting is by using a

recursive least squares algorithm.

6. The method of claim 1 wherein adaptively adjusting is by comparing

the combined signal with an ideal value to produce an error signal and adjusting each

said pilot signal's particular weight based in part on the error signal.

7. The method of claim 6 wherein the ideal value is 1 + jO.

8. The method of claim 1 wherein the steps of filtering and weighting

occur concurrently.

9. The method of claim 1 wherein the filtering of each said transmitted

pilot signal and of each said version of the data signal is performed by a RAKE.

10. The method of claim 1 wherein the filtering of each said transmitted

pilot signal and of each said version of the data signal is performed by a vector

correlator.

11. The method of claim 9 wherein the weighting of each said pilot signal

is by weighting each finger of that pilot's RAKE by a particular amount and the

weighting of each finger of each said version of the data signal's RAKE is in

accordance with the adjusted weights associated with the respective finger of the

pilot signal's RAKE for the pilot signal of the respective antenna.

12. The method of claim 9 wherein the weighting of each said pilot signal

is by weighting each finger of that pilot's RAKE and weighting an output of that

pilot's RAKE and the weighting of each said version of the data signal is by

weighting each finger and an output of each said version of the data signal's RAKE

in accordance with the adjusted weights associated with the respective finger and the

output of the pilot signal's RAKE for the pilot signal of the respective antenna.

13. A spread spectrum communication system comprising:

a transmitter comprising:

a plurality of transmitting antennas;

means for transmitting from each transmitting antenna, a pilot

signal having a pseudo random chip code sequence uniquely associated with that

antenna; and

means for transmitting a data signal such that different spread

spectrum versions of the data signal are transmitted from each transmitting antenna,

each version having a different chip code identifier for the respective transmitting

antenna; and

a receiver comprising:

a receiving antenna;

means coupled to said receiving antenna for filtering each said

transmitted pilot signal using that pilot signal's pseudo random chip code sequence

and for weighting each said filtered pilot signal by a particular weight;

means for combining said weighted pilot signals as a combined

signal;

means for adaptively adjusting each said pilot signal's particular

weight based in part on a signal quality of the combined signal; and

means for receiving the data signal via filtering each version

with its associated chip code and combining the filtered versions, wherein the different data signal versions are weighted in accordance with the adjusted weights

associated with the pilot signal of the respective antenna.

14. The system of claim 13 wherein the transmitter further comprises

means for weighting the different data signal versions in accordance with the

adjusted weights.

15. The system of claim 13 wherein the receiver further comprises means

for weighting the different data signal versions in accordance with the adjusted

weights.

16. The system of claim 13 wherein the adaptively adjusting means uses

a least mean squared algorithm to adaptively adjust each said pilot signal's particular

weight.

17. The system of claim 13 wherein the adaptively adjusting means uses

a recursive least squares algorithm to adaptively adjust said pilot signal's particular

weight.

18. The system of claim 13 wherein said adaptively adjusting means

comprises means for comparing the combined signal with an ideal value to produce

an error signal and adaptively adjusting each said pilot signal's particular weight is

based in part on the error signal.

19. The system of claim 18 wherein the ideal value is 1+jO.

20. The system of claim 13 wherein the pilot signal filtering and weighting

means comprises a plurality of RAKEs for respectively filtering each said transmitted

pilot signal and the data signal receiving means comprises a plurality of RAKEs for

respectively filtering each version of the data signal.

21. The system of claim 13 wherein the pilot signal filtering and weighting

means comprises a plurality of vector correlators for respectively filtering each said

transmitted pilot signal and the data signal receiving means comprises a plurality of

vector correlators for respectively filtering each version of the data signal.

22. The system of claim 20 wherein the pilot signal filtering and weighting

means weights each said pilot signal by weighting each finger of that pilot's RAKE

by a particular amount and the weighing of each finger of each said version of the

data signal's RAKE is in accordance with the adjusted weights associated with the

respective finger of the pilot signal's for the pilot signal of the respective antenna.

23. The system of claim 20 wherein the pilot signal filtering and weighting

means weights each said pilot signal by weighing each finger of that pilot's RAKE

and weighting an output of the pilot's RAKE and the weighting of each said version

of the data signal is by weighing each finger and an output of each said version of

the data signal's RAKE in accordance with the adjusted weights associated with the

respective finger and the output of the pilot signal's RAKE for the pilot signal of the

respective antenna.