AU2002240010A1 - Method and system for forward link beam forming in wireless communications - Google Patents

Description

METHOD AND SYSTEM FOR FORWARD LINK BEAM FORMING IN WIRELESS COMMUNICATIONS

BACKGROUND

1. FIELD

The present invention generally relates to the field of wireless

communication systems. More specifically, the invention relates to adaptive

antenna array techniques for code division multiple access communication

systems.

2. RELATED ART

In wireless communication systems several users share a common

communication channel. To avoid conflicts arising from several users

transmitting information over the communication channel at the same time

requires the use of some form of multiple access protocol, such as code division

multiple access (CDMA). In addition to providing multiple access allocation to a

channel of limited capacity, a protocol can serve other functions, for example,

providing isolation of users from each other, i.e. limiting interference between

users, and providing security by making interception and decoding difficult for a

non-intended receiver, also referred to as low probability of intercept.

In CDMA systems each signal is separated from those of other users by

coding the signal. Each user uniquely encodes its information signal into a

transmission signal. The intended receiver, knowing the code sequences of the

user, can decode the transmission signal to receive the information. The encoding of the information signal spreads its spectrum so that the bandwidth of

the encoded transmission signal is much greater than the original bandwidth of

the information signal. For this reason CDMA is also referred to as "spread

spectrum" modulation or coding.

The energy of each user's signal is spread across the channel bandwidth so

that each user's signal appears as noise to the other users. So long as the

decoding process can achieve an adequate signal to noise ratio, i.e. separation of

the desired user's signal from the "noise" interference of the other users' signals,

the information in the signal can be recovered. Other factors which affect

information recovery of the user's signal are different conditions in the

environment for each subscriber, such as fading due to shadowing and multipath.

Briefly, shadowing is interference caused by a physical object interrupting the

signal transmission path between the transmitter and receiver, for example, a

large building. Multipath is a signal distortion which occurs as a result of the

signal traversing multiple paths of different lengths and arriving at the receiver at

different times. Multipath is also referred to as "time dispersion" of the

communication channel. Multipath fading may also vary with time. For

example, the amount of multipath fading can vary rapidly as a result of the signal

reflecting off of moving objects such as cars or trees blowing in the wind.

In wireless communications, especially in voice communications, it is

desirable to provide communication between two users in both directions

simultaneously, referred to as duplexing or full-duplexing. One method used to

provide duplexing in CDMA systems is frequency division duplexing. In

frequency division duplexing, one frequency band is used for communication from a base station to a user or "subscriber unit", called the "forward" channel,

and another frequency band is used for communication from the user or

subscriber unit to the base station, called the "reverse" channel. A forward

channel may also be referred to as a "downlink" channel, and a reverse channel

may also be referred to as an "uplink" channel or a "return link". Specific

implementation of coding and modulation may differ between forward and

reverse channels.

One collection of techniques which can be used to increase signal

reliability is referred to as "power control". Simply stated, power control adjusts

the power of the signal at the transmitter while the signal is being transmitted in

order to compensate for varying conditions in the communication channel, such

as relative movement of different users and multipath fading. One object of

power control is to keep the power transmitted to each user at a minimum level

needed to maintain a reliable communication link, thereby minimizing the

interference between different users' signals discussed above. For example,

power control can be implemented on a forward link by having each user

receiving unit periodically send power control information to the base station

transmitter. In one typical CDMA system, the power control information is in

the form of a bit which is sent every 1.25 milliseconds. For example, if the bit

has values of +1 and -1, the +1 can be interpreted as information that the receiver

has a weak signal, i.e. the receiver needs more power transmitted to it, and the -1

can be interpreted as information that the receiver has a strong signal, i.e. the

receiver can get by on less power transmitted to it. Thus, the base station can

adjust the signal power transmitted to each user separately in order to minimize the interference between users by optimizing the signal power for each user.

Adaptive antenna array technology can also be used to minimize the

interference between users. Adaptive antenna array technology can be used to

focus and direct the energy in a transmitted signal into a "beam" which can be

used to dramatically increase the efficiency of signal transmission. The shape of

the beam can be altered. For example, the beam can be made narrow or wide.

The beam can also be "pointed" or "steered" in different directions. The

particular shape and direction of a beam transmitted from an antenna array is

referred to as the "antenna beam pattern". By narrowing the antenna beam

pattern directed to a specific user, interference is reduced or eliminated for other

users which are outside the specific user's antenna beam pattern, thereby helping

to minimize the interference between users. As interference between users is

reduced, system capacity is increased, that is, a greater number of users are able

to simultaneously use the system.

There are a number of approaches for using adaptive antenna array

technology in wireless communication systems. Some of the approaches are

described in U.S. patent 6,006,110 entitled "Wireless Communication Network

Using Time-Varying Vector Channel Equalization For Adaptive Spatial

Equalization" and assigned to Cisco Technology, Inc., San Jose, California.

In one approach for using adaptive antenna array technology at the base

station, the base station estimates the angle of arrival of the signal received from

the user. The base station then uses the angle of arrival estimate to form an

estimate for an angle of transmission. With frequency duplexing, as discussed

above, the received signal and the transmitted signal are on different frequencies, so that the radio frequency channel differs for the two signals. Depending on the

degree of correlation in direction between the two channels at different

frequencies, the estimate for the angle of transmission is a better or worse

approximation. To the extent that the approximation is inaccurate, a wider

antenna beam pattern must be used, limiting the reduction in interference

between users with this approach.

In a second approach, the base station transmits an orthogonal pilot signal

on each antenna element of the adaptive antenna array. The user's subscriber

unit determines the amplitude and phase of each pilot signal, and the subscriber

unit reports that information back to the base station on the return link. The base

station uses the information to form a narrow beam directed toward the user.

This approach requires extra bandwidth on the return link to transmit the pilot

signal amplitude and phase information, i.e. non-user information, on the return

link, i.e. the reverse channel. To the extent that channel capacity is used to

transmit non-user information, less channel capacity is available for transmitting

user information. In other words, an "overhead" is incurred for signal

transmission on the return link, and the effective bandwidth efficiency of

transmission on the return link is reduced. Moreover, this approach entails added

complexity at the subscriber unit, which can involve added cost for the subscriber

unit. Thus, this second approach is not "backward compatible" in the sense that

introducing it into an existing CDMA communication system would require

modification or replacement of a large number of subscriber units for the

improvement to be effective.

Thus, there is a need in the art for reducing interference between users and for increasing system capacity by providing signal transmission using accurate

antenna beam patterns. There is also a need in the art to form accurate antenna

beam patterns without incurring signal transmission overhead on the return link.

Further, there is a need in the art to provide signal transmission systems using

accurate antenna beam patterns which are backward compatible with existing

systems and which do not increase the complexity and cost of subscriber units.

SUMMARY

The present invention is directed to method and system for forward link

beam forming in wireless communications. In one aspect of the invention

overcomes the need in the art for reducing interference between users and for

increasing system capacity by providing signal transmission using accurate

antenna beam patterns. According to one embodiment of the invention, accurate

antenna beam patterns are formed on the forward link without incurring signal

transmission overhead on the return link. In one aspect of the invention provides

accurate antenna beam patterns for signal transmission systems in a way which

does not increase the complexity and cost of subscriber units and which is

backward compatible with existing systems.

According to an aspect of the invention an antenna beam pattern is used to

send a communication signal to a user. For example, the antenna beam pattern

can be formed using adaptive antenna array technology on the forward link of a

wireless communication system. A control signal from the user is used to

determine a statistic. For example, the control signal can be a power control bit

sent by the user on the return link. The statistic can be an average of the power

control bit values over a specified time interval. The statistic is used to narrow

the antenna beam pattern and direct it toward the user. For example, a dithering

algorithm can be used to optimize the antenna beam pattern. After a call has

been completed, an optimized antenna beam pattern specific to each user can be

stored for the user's next call in order to make the antenna beam pattern

optimizing process faster and more efficient. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an illustrative representation of a sectored cell, with an

example of an antenna beam pattern directed toward a user in an exemplary

wireless communication system.

Figure 2 is a block diagram showing the flow of information in an

example beam optimizing module in accordance with one embodiment of the

present invention in an exemplary wireless communication system.

Figure 3 is a flowchart describing one example of a process of providing

accurate beam forming in accordance with one embodiment of the present

invention in an exemplary wireless communication system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The presently disclosed embodiments are directed to method and system

for forward link beam forming in wireless communications. The following

description contains specific information pertaining to the implementation of the

present invention. One skilled in the art will recognize that the present invention

may be implemented in a manner different from that specifically discussed in the

present application. Moreover, some of the specific details of the invention are

not discussed in order not to obscure the invention. The specific details not

described in the present application are within the knowledge of a person of

ordinary skill in the art.

The drawings in the present application and their accompanying detailed

description are directed to merely example embodiments of the invention. To

maintain brevity, other embodiments of the invention which use the principles of

the present invention are not specifically described in the present application and

are not specifically illustrated by the present drawings.

Figure 1 illustrates an example of wireless communication in a sectored

cell using adaptive antenna array beam forming in accordance with one

embodiment. Exemplary system 100 shown in Figure 1 constitutes part of an

exemplary cellular communication system, which can be, for example, a

wideband code division multiple access ("WCDMA") communication system. A

WCDMA communication system is also referred to as a "spread spectrum

communication system".

The general principles of CDMA communication systems, and in

particular the general principles for generation of spread spectrum signals for transmission over a communication channel are described in U.S. patent

4,901,307 entitled "Spread Spectrum Multiple Access Communication System

Using Satellite or Terrestrial Repeaters" and assigned to the assignee of the

present invention. The disclosure in that patent, i.e. U.S. patent 4,901,307, is

hereby fully incorporated by reference into the present application. Moreover,

U.S. patent 5,103,459 entitled "System and Method for Generating Signal

Waveforms in a CDMA Cellular Telephone System" and assigned to the

assignee of the present invention, discloses principles related to PN spreading,

Walsh covering, and techniques to generate CDMA spread spectrum

communication signals. The disclosure in that patent, i.e. U.S. patent 5,103,459,

is also hereby fully incorporated by reference into the present application.

Further, the present invention may utilize time multiplexing of data and various

principles related to "high data rate" communication systems, and the present

invention can be used in "high data rate" communication systems, such as that

disclosed in U.S. patent application entitled "Method and Apparatus for High

Rate Packet Data Transmission" Serial No. 08/963,386 filed on November 3,

1997, and assigned to the assignee of the present invention. The disclosure in

that patent application is also hereby fully incorporated by reference into the

present application.

Continuing with Figure 1, exemplary system 100 comprises cell 102. Cell

102 can be, for example, one of a number of cells in a WCDMA system. Cells in

a WCDMA system are typically divided into 3 sectors. Sectoring of the cells

provides various benefits such as more efficient allocation of orthogonal

spreading codes, for example. In the example used to illustrate one embodiment in the present application, cell 102 is a sectored cell and includes sector 104.

Base station 106 provides communication for users in all sectors of cell 102, and,

in particular in the present example, provides communication to user 108 in

sector 104. The present example illustrates communication taking place in a

forward channel, i.e. from base station 106 to the subscriber unit of user 108.

Adaptive antenna array technology is used at base station 106 to form antenna

beam pattern 110 which is directed toward user 108.

Antenna beam pattern 110 is used to transmit a communication signal

which includes user information that is to be sent across the communication

channel to user 108. The communication channel can be, for example, radio

frequency transmission between transmit and receive antennas in a wireless

communication system. Antenna beam pattern 110 has been narrowed to focus

the signal transmission energy in the direction of user 108, rather than spreading

the signal transmission energy across the entire width of sector 104. The

interference for users other than user 108 caused by the communication signal

transmitted in antenna beam pattern 110, which is intended for user 108, is

thereby reduced. Thus, Figure 1 illustrates one example of a system in which

adaptive antenna array technology is used to minimize interference between

multiple users in a WCDMA wireless communication system.

Figure 2 illustrates an example antenna beam optimizing module in

accordance with one embodiment. Beam optimizing module 200 shown in

Figure 2 constitutes part of a transmitter which may generally reside in a base

station, gateway, or satellite repeater when communication is taking place in a

forward channel. Beam optimizing module 200 can be part of a base station transmitter, for example, in a WCDMA communication system or spread

spectrum communication system. Beam optimizing module 200, shown in

Figure 2, comprises control signal monitoring module 202, signal statistic

computation module 204, antenna beam pattern storing module 206, antenna

beam pattern optimizing module 208, and adaptive antenna array module 210.

The flow of information between modules is indicated in the block diagram of

Figure 2 by the arrows between modules, which also indicate the direction of

information flow.

Continuing with Figure 2, control signal monitoring module 202

comprises hardware for reading a control signal returned by the user on the return

link and making the information in the signal available at the output of control

signal monitoring module 202. For example, the control signal can be a power

control bit which appears on the return link approximately every 1.25

milliseconds, as discussed above. The power control bits can be stored in a

buffer or register, for example, or can be passed directly to other modules in the

system. Although the present example uses power control signals to illustrate

one embodiment, it is manifest that other existing user feedback control signals

can be used without sacrificing the invention's advantages of not increasing

complexity at the subscriber unit and backward compatibility. For example, the

data rate control ("DRC") signal can be used in a wireless data communication

system such as a high data rate ("HDR") communication system. The details of

implementing control signal monitoring module 202 for a particular type of

control signal are apparent to a person of ordinary skill in the art.

Continuing with the present example, power control signal bits are output by control signal monitoring module 202 to signal statistic computation module

204. The output of control signal monitoring module 202 is also referred to as a

"sequence of monitored signals". As described above, the power control bit can

be configured to alternate between +1 and -1 depending on whether the

subscriber unit requires more or less signal power. Thus, the power control

signal bits are responsive to noise in the channel such as multipath fading. The

power control signal is, therefore, a noisy signal which requires filtering in order

to be useful. In the present example, the power control signal is filtered by

averaging the power control bits over time. An average value of the power

control bits can be output by signal statistic computation module 204

periodically, for example, every 100 milliseconds. As another example, signal

statistic computation module 204 can keep a running average which is output and

then reset each time the value is read by another module. In the present example,

signal statistic computation module 204 outputs the average value of the power

control bits for a time interval which begins after each time antenna beam pattern

optimizing module 208 reads an average value from the output of signal statistic

computation module 204.

Continuing with Figure 2, antenna beam pattern storing module 206 stores

an optimized antenna beam pattern specific to each user. The optimized antenna

beam pattern resulting from each user's previous call is saved for each specific

user. Then, at the beginning of the user's next call, the optimized antenna beam

pattern specific to that user, from the user's previous call, can immediately be

used to transmit communication signals from the base station to that specific

user. The antenna beam pattern is re-optimized during the call, and at the end of the call the latest optimized antenna beam pattern is stored for the user. If there

was no previous call for a user, or the user's optimized antenna beam pattern was

not previously stored, a standard antenna beam pattern can be used. For

example, a standard antenna beam pattern can be a 120 degrees wide pattern

which transmits to the entire sector. The standard antenna beam pattern is then

optimized during the call, and at the end of the call the optimized antenna beam

pattern is stored for the user. Thus, storing an optimized antenna beam pattern

specific to each user increases the efficiency of the system, and shortens the time

required to achieve an optimized antenna beam pattern for users that have a

previously stored optimized antenna beam pattern.

Continuing with Figure 2, antenna beam pattern optimizing module 208

begins with either a standard antenna beam pattern or the optimized antenna

beam pattern specific to the user from the user's previous call, as described

above, and optimizes the antenna beam pattern. In the present example, the

algorithm used by antenna beam pattern optimizing module 208 to optimize the

antenna beam pattern is a "dithering" algorithm. Operation of the dithering

algorithm can be illustrated by the following example. A known antenna beam

pattern is used to transmit a communication signal to the user. The known

antenna beam pattern can be the standard antenna beam pattern, as described

above, or the user's previously stored pattern, or the antenna beam pattern used at

any intermediate stage of the optimization. The known antenna beam pattern is

then narrowed by removing a specific portion or "slice" of the beam from one

side of the pattern.

Antenna beam pattern optimizing module 208 then reads the average value from the output of signal statistic computation module 204. If the value

indicates that no change in power is required by the user (the user is still "inside"

the pattern), then another slice of the beam is removed from one side of the

pattern. If the value indicates that an increase in power is required by the user

(the user is suddenly "outside" the pattern), then the previously removed slice of

the beam is restored to the pattern, and a slice is removed from the other side of

the pattern. Antenna beam pattern optimizing module 208 reads the average

value from the output of signal statistic computation module 204 after each

change to the antenna beam pattern. In this manner, the antenna beam pattern is

simultaneously narrowed and directed toward the user. The process of narrowing

and directing the antenna beam pattern, i.e. optimizing the antenna beam pattern,

continues throughout the duration of the call. At the termination of the call, the

optimized antenna beam pattern is stored by antenna beam pattern storing

module 206, as described above.

Continuing with Figure 2, adaptive antenna array module 210 translates

the antenna beam pattern information in the form of complex number valued

weights into amplitude and phase information provided to each antenna element

of the antenna array in order to form the antenna beam pattern of the signal

transmitted by the antenna array. For example, the antenna beam pattern can be

used to transmit a signal conforming to the antenna beam pattern from the

antenna array as known in the art.

Thus, Figure 2 shows an example of a system that can be used to provide

optimized formation of antenna beam patterns for communication signal

transmission in a wireless communication system in order to minimize interference between multiple users and thereby increase system capacity.

Figure 3 shows flowchart 300 describing one example of a process of

providing accurate beam forming in accordance with one embodiment.

Flowchart 300 shown in Figure 3 describes a process which can be performed at

a transmitter which may generally reside in a base station, gateway, or satellite

repeater when communication is taking place in a forward channel. The process

shown in flowchart 300 can be performed by a beam optimizing module, for

example, in a base station transmitter in a WCDMA communication system or

spread spectrum communication system.

Referring to Figure 3, at step 302 the process for optimizing an antenna

beam pattern begins with a user acquiring the system. For example, acquiring

the system may comprise several steps including the transmission of probe

signals, completing various "handshake" protocols and "training periods", and

allocation of various system resources such as orthogonal spreading codes.

When a user acquires the system, the system has information that can be used to

identify the specific user, for example, a user specific mask or electronic serial

number ("ESN"). The identifying information can be used at step 304 to

determine whether an antenna beam pattern has been stored for the specific user

that has acquired the system.

When a pattern has not been previously stored, then the process proceeds

at step 306, and a standard antenna beam pattern is used to transmit a

communication signal back to the user prior to beginning optimization of the

antenna beam pattern. When a pattern has been previously stored, then the

process proceeds at step 308, and a previously stored antenna beam pattern specific to the user is used to transmit a communication signal back to the user

prior to beginning optimization of the antenna beam pattern. As described above,

the use of a previously stored pattern can decrease the time required to optimize

the antenna beam pattern.

At step 310 the process starts power control bit monitoring. Power control

bit monitoring comprises accessing information from the power control

subsystem of the wireless communication system as described above. Although

power control is used to illustrate the present example, other types of feedback

information can be used. For example, data rate control or DRC information can

be used in a data communication system, as apparent to a person of ordinary skill

in the art. The process of power control bit monitoring continues throughout the

duration of the call.

At step 312 the process starts power control bit statistics computation. In

the present example, successive values of the power control bit are averaged. As

described above, a number of different methods for averaging exist. For

example, the average can be computed over a specific interval of time, 100

milliseconds for example, and the average value can be output every 100

milliseconds. As another example, the averaging computation can be restarted

each time an average value is read and continued until the next time an average

value is read, restarted again and so forth. Although a simple average is used to

illustrate the present example, other types of averaging can be used, for example,

weighted averages. The process of power control bit statistics computation

continues throughout the duration of the call.

At step 314 the process starts antenna beam pattern optimization using a dithering algorithm as described above. The process of antenna beam pattern

optimization continues throughout the duration of the call. At step 316 the user

terminates the call, relinquishing the system resources which have been allocated

to the user. In a communication system where user locations are stationary, the

final optimized beam pattern of the call correlates to the antenna beam pattern

needed for the next call. Therefore, the efficiency and initial response time of the

system is improved by storing the final optimized antenna beam pattern from the

call at step 318. After the optimized antenna beam pattern from the call is stored

at step 318, the process for optimizing an antenna beam pattern ends.

Thus, Figure 3 shows an example of a process that can be used to provide

optimized formation of antenna beam patterns for communication signal

transmission in a wireless communication system in order to minimize

interference between multiple users and thereby increase system capacity.

It is appreciated by the above description that the invention provides

method and system for forward link beam forming in wireless communications.

According to an embodiment of the embodiments are described above, user

information is transmitted in a communication signal from a base station to a user

using adaptive antenna array technology to form a narrow antenna beam pattern

directed toward a user. The antenna beam pattern is optimized for each particular

user, thus minimizing signal interference between multiple users in a wireless

communication system and resulting in increased system capacity. Moreover,

according to an embodiment of the invention described above, the method of

narrowing and directing the antenna beam pattern toward a user provides more

accurate beam forming and better optimization than other methods. Although the invention is described as applied to communications in a CDMA system, it will

be readily apparent to a person of ordinary skill in the art how to apply the

invention in similar situations where accurate antenna beam pattern forming is

needed to increase system capacity and reduce interference between users in a

wireless communication system.

From the above description, it is manifest that various techniques can be

used for implementing the concepts of the present invention without departing

from its scope. Moreover, while the embodiments are has been described with

specific reference to certain embodiments, a person of ordinary skill in the art

would recognize that changes can be made in form and detail without departing

from the spirit and the scope of the invention. For example, different existing

feedback control signals, such as data rate control (DRC) signals, can be used as

well as the power control signal used in the present example. Also, for example,

the dithering algorithm presented in one embodiment described here, can be

replaced by other algorithms for optimizing the beam pattern. The described

embodiments are to be considered in all respects as illustrative and not

restrictive. It should also be understood that the invention is not limited to the

particular embodiments described herein, but is capable of many rearrangements,

modifications, and substitutions without departing from the scope of the

invention.

Thus, method and system for forward link beam forming in wireless

communications have been described.

WHAT IS CLAIMED IS:

Claims (23)

1. A method comprising the steps of:

using an antenna beam pattern to send a communication signal to a user;

determining a statistic using a control signal from said user;

utilizing said statistic to narrow said antenna beam pattern and to direct

said antenna beam pattern to said user.

2. The method of claim 1 further comprising storing said antenna

beam pattern after said utilizing step.

3. The method of claim 1 wherein said utilizing step comprises using

a dithering algorithm to optimize said antenna beam pattern.

4. The method of claim 1 wherein said control signal is a power

control signal.

5. The method of claim 1 wherein said control signal is a data rate

control signal.

6. The method of claim 1 wherein said statistic is an average of said

control signal over a specified interval of time.

7. The method of claim 1 wherein said statistic is a running average of

said control signal.

8. The method of claim 1 wherein said statistic is a weighted average

of said control signal.

9. The method of claim 1 wherein said antenna beam pattern is

formed using an adaptive antenna array.

10. The method of claim 1 wherein said communication signal is sent

over a forward link of a wireless communication system.

11. The method of claim 10 wherein said wireless communication

system is a wideband code division multiple access communication system.

12. A system comprising:

a control signal monitoring module configured to access a control signal

from a user;

a signal statistic computation module configured to determine a statistic

from a sequence of monitored signals output by said signal monitoring module;

an antenna beam pattern optimizing module configured to utilize said

statistic to narrow an antenna beam pattern to be directed to said user.

13. The system of claim 12 further comprising an adaptive antenna array module configured to output and direct said antenna beam pattern to said

user.

14. The system of claim 12 further comprising an antenna beam pattern

storing module configured to store said antenna beam pattern.

15. The system of claim 12 wherein said antenna beam pattern

optimizing module uses a dithering algorithm to optimize said antenna beam

pattern.

16. The system of claim 12 wherein said control signal is a power

control signal.

17. The system of claim 12 wherein said control signal is a data rate

control signal.

18. The system of claim 12 wherein said statistic is an average of said

sequence of monitored signals over a specified interval of time.

19. The system of claim 12 wherein said statistic is a running average

of said sequence of monitored signals.

20. The system of claim 12 wherein said statistic is a weighted average

of said sequence of monitored signals.

21. The system of claim 12 wherein said antenna beam pattern is used

to send a communication signal to said user.

22. The system of claim 21 wherein said communication signal is sent

over a forward link of a wireless communication system.

23. The system of claim 22 wherein said wireless communication

system is a wideband code division multiple access communication system.