WO2000036760A1

WO2000036760A1 - Channel estimation for a cdma system using pre-defined symbols in addition to pilot symbols

Info

Publication number: WO2000036760A1
Application number: PCT/SE1999/002398
Authority: WO
Inventors: Hans Cavander
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1998-12-16
Filing date: 1999-12-16
Publication date: 2000-06-22
Also published as: DE19983814T1; MY130820A; GB2363553A; GB2363553B; GB0116741D0; AU3092600A; CN1334997A; JP2002533007A; CN1192506C

Abstract

Systems and methods for improved channel estimation for a WCDMA system. In a RAKE receiver, predefined pilot symbols and a Transmitter Power Control (TPC) symbol are received in a time slot and correlated to determine the transmitted predefined pilot symbols and the transmitted TPC symbol. The correlated predefined pilot symbols and the correlated TPC symbol are compared to the transmitted predefined pilot symbols and TPC symbol. Based upon the difference between the received version and correlated version of the symbols an estimation of channel impairments can be calculated. Based upon the estimation the received data symbols may be more accurately correlated to their correct values.

Description

CHANNEL ESTIMATION FOR A CDMA SYSTEM USING PRE-DEFINED

SYMBOLS IN ADDITION TO PDLOT SYMBOLS

BACKGROUND

This invention relates to cellular telephony, and in particular to cellular

code division multiple access (CDMA) systems.

In a typical CDMA system, an information data stream to be transmitted is impressed upon a much higher bit rate data stream generated by a pseudo-random

code generator. The information data stream and the higher bit rate data stream

are typically multiplied together, and such combination of the higher bit rate signal

with the low bit rate signal is called direct-sequence spreading the information

signal. Each information data stream or channel is allocated a unique spreading

code. A plurality of spread spectrum signals are transmitted on radio frequency

carrier waves and jointly received as a composite signal at a receiver. Each of the

spread spectrum signals overlaps all of the other spread spectrum signals, as well

as noise-related signals, in both frequency and time. By correlating the composite

signal with one of the unique spreading codes, the corresponding information

signal is isolated and despread. A particular problem in cellular communications is multipath propagation.

Multipath propagation, in which a radio signal takes many paths from a transmitter

to a receiver, can be addressed in spread spectrum and other digital communication

systems by using a RAKE receiver. RAKE receivers are described in U.S. Patent

No. 5,305,349 to Dent for "Quantized Coherent Rake Receiver" and U.S. Patent

No. 5,237,586 to Bottomley for "Rake Receiver with Selective Ray Combining", both of which are expressly incorporated by reference. The signal energies from

the several propagation paths, as received from several fingers, are combined, or

"raked together", by the RAKE receiver before decoding. The spreading codes

are used in order to avoid correlation between successive chips. Thus, if the

multiple paths are delayed more than one chip apart, they appear as uncorrelated

noise when they are demodulated by the RAKE fingers. To optimally decode the

original transmitted symbols (bits), the received signal energies must be combined

in an appropriate way, which involves scaling and aligning the phases of the

received signals before they are combined. The combining technique typically

used for the received signals in RAKE receivers is known as maximum ratio

combining (MRC).

One problem with scaling and aligning the phases of received signals is

caused by the channel's influence on the frequency, the amplitude and the phase of

the transmitted data. Therefore, in order to properly scale and align the received signal, it is necessary to calculate a channel estimate and then compensate for the

influence of the channel on the signal by using the conjugate of the channel

estimate. Since channel estimates are used in order to weight the different

multipaths in the MRC step, it is important that the channel estimates accurately reflect how much the channel is actually affecting the transmitted data.

One method of deterrriining how a particular transmission channel is

affecting the transmitted data is to transmit a predefined symbol from the

transmitter to the receiver. A correlation is performed at the receiver using what was received (i.e., the received predefined symbol) and what was known to have

been transmitted (i.e., the transmitted predefined symbol). In an ideal

transmission channel the received predefined symbol would be an exact replica of

the transmitted predefined symbol. However, due to transmission channel

impairments, such as those mentioned above, there will typically be a difference

between the two symbols. This difference illustrates how the transmission channel

is affecting the transmitted data and can be quantified by correlation.

In wireless communications radio channel conditions do not remain

constant. Accordingly, the channel estimates need to be continually updated. One

method of continually updating the estimate is to transmit the predefined symbol in each time slot, calculate the channel estimate for each time slot and keep the

channel estimate constant over the time slot. The more predefined symbols in each time slot the more accurate the channel estimate becomes. However, each

additional predefined symbol in each time slot displaces a data symbol in the time

slot. Therefore, the additional predefined symbols restricts the effective

bandwidth for transmitting data symbols. Since bandwidth is a precious resource

in a radiotelephone environment, it is desirable, therefore, to use a minimum

number of predefined symbols for channel estimation.

One method of using a known symbol sequence in a radiotelephone system

is described in U.S. Patent No. 5,297,169 to Backstrόm et al. for "Equalizer

Training in a Radiotelephone System" , which is herein incorporated by reference.

Backstrom et al. describes a method for improving the reception of a transmission

in a radiotelephone system by fraining an equalizer using a synchronization portion

of the transmission having a data pattern chosen for its correlation properties and

retraining the equalizer using a portion of the transmission transmitted for a

different purpose, i.e., the digital verification color code.

In CDMA systems predefined symbols termed "pilot symbols", may be

sent on a separate pilot signal (i.e., using it's own spreading code) or may be

embedded along with user-data symbols. The use of four pilot symbols as

predefined symbols for channel estimation is discussed in ARIB specification IMT-

2000 Study Committee Air-Interface Document Number: AIF/SWG2-121-2(S),

which is herein incorporated by reference. Applicant has found that although four pilot symbols are adequate for channel estimation under normal carrier power to

interference power (C/I) conditions, for low C/I conditions four pilot symbols do

not provide an acceptable channel estimate. In order to improve the channel

estimate, the number of pilot symbols could be increased. However, this would decrease the number of data symbols and thus, reduce the transmission bit rate for

user data.

Accordingly, what is needed is a way to increase the accuracy of channel

estimation in a RAKE receiver while minimizing the additional overhead used for

the channel estimation.

SUMMARY

Systems and methods for deterπuning channel estimates in a spread

spectrum radio receiver are described. According to an exemplary embodiment of

the present invention, channel estimation, in a RAKE receiver, is improved

without adding additional overhead by using a symbol, already present in each time slot, which is transmitted for a purpose other than channel estimation, in the

channel estimation. In accordance with the present invention an improved method

and apparatus for deterrmning channel estimates in a RAKE receiver is disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and features of the present invention will be more

apparent from the following description of the preferred embodiments with

reference to the accompanying drawings, wherein:

Figure 1 illustrates a simplified diagram of a channel estimator in a RAKE

receiver according to an exemplary embodiment of the present invention;

Figure 2 illustrates a simplified diagram of a maximum ratio combiner in a

RAKE receiver according to an exemplary embodiment of the present invention;

Figure 3 illustrates the composition of a single time slot;

Figure 4 illustrates one method of correlating and compensating for channel

impairments;

Figure 5 illustrates an alternative method of correlating and compensating

for channel impairments.

DETAΠ ED DESCRIPTION

In exemplary CDMA systems pre-defined pilot symbols are transmitted in

each time slot to a receiving station. Although, there are a number of ways of

performing channel estimation, a purely exemplary system for calculating the

channel estimation is illustrated in FIG. 1. The channel estimation of figure 1 is

individually performed for each finger of a RAKE receiver. Multipliers X, , ... , X_M calculate the product of the transmitted pilot symbols PS_T1, ..., PS™ and the

complex conjugate of their respective received pilot symbol PS_R1\ ... , PS_RM ^*. The

output of multipliers X, , ... , X_M are summed by summing junction 200. Multiplier

202 takes the product of the summation from summing junction 200 and the inverse value of the number of pilot symbols, i.e. , 1/M. The output of multiplier

202 is the channel estimate. Alternatively, the purely exemplary system for

calculating channel estimation can be described by the equation below:

where M is the number of pilot symbols, P(i) is the known pilot symbol,

i.e., a complex value, (P(i))* is the complex conjugate of the received version of

P(i), and C(k) is the channel estimate. According to this exemplary embodiment, C(k) is kept constant over one time slot.

An exemplary apparatus for performing the MRC is illustrated in FIG. 2.

Multipath signals S_ls ..., S_N are received by the RAKE receiver by fingers F^...,

F_N respectively. Accordingly, the RAKE receiver uses one finger to demodulate

each multipath signal. Further, since the RAKE receiver can dynamically allocate

the number of fingers according to the power profile of the multipath channel, the

RAKE receiver can handle delay spreads of various lengths, i.e., from long delay spreads to short delay spreads, without inducing additional interference and without adding to the receivers complexity.

The received signals are passed from fingers F_t, ..., F_N to respective

multipliers m,, ..., m_N where the received multipath signals are multiplied by the

conjugate of the corresponding channel estimate C,^*, ..., C_N ^*. The conjugate of

the channel estimate is determined based upon the channel estimation illustrated in

figure 1. The product of the respective multipliers are input to summing junction

1 which outputs a signal representing the result of the MRC. Alternatively, the

apparatus of FIG. 2 which performs the MRC can be described by the following

formula:

where N is the number of RAKE fingers, C(k)^* is the complex conjugate of the

channel estimate, x(k) is the symbol received by the RAKE finger and y is the output from the MRC. Since the symbols from the RAKE fingers are added

together, the output signal-to-noise ratio (SNR) from the MRC is the sum of the

individual SNRs of the fingers.

A purely exemplary time slot is illustrated in FIG. 3. As can be seen there

are four pilot symbols in the time slot. Following the pilot symbols, is the transmission power control (TPC) symbol. The TPC symbol is used by a

transmitter, i.e., a base station, to control the transmitting power of a receiver,

i.e., a mobile station. The TPC symbol can, for example, be a binary phase shift

key (BPSK) modulated signal and has either a (+ 1, + 1) or (-1, -1) value in an I/Q diagram. The base station, through the use of the TPC symbol, either commands

the mobile station to increase the transmission power by ldB (+ 1, + 1) or it

commands the mobile station to decrease the transmission power by 1 dB (-1, -1).

In each time slot the data symbols follow the TPC and pilot symbols.

A first embodiment of the present invention is illustrated in FIG. 4. In step 405, the pilot symbols and the TPC symbol are identified, e.g. , as a result of

synchronizing to the time slot boundaries of the received signal. Next, in step 410, the pilot symbols and the TPC symbol are used to calculate a channel

estimate, e.g., using the calculation of equation (1) wherein P(l)-P(4) are pilot

symbols and P(5) is the TPC symbol. Finally, in step 415, the complex conjugate

of the channel estimate is used to compensate for the channel impairments, for

example through the use of the MRC in a RAKE receiver.

A second exemplary embodiment of the present invention is illustrated in

FIG. 5. In step 505 the pilot symbols are identified. Next, in step 510, the pilot

symbols are used to calculate a first channel estimate, e.g., using equation (1). In

step 515, the complex conjugate of the first channel estimate is used to compensate the received time slot for the channel impairments. In step 520 the TPC symbol is demodulated/decoded. Using the pilot symbols and the decoded TPC symbol, an

improved channel estimate is calculated in step 525. Finally, in step 530, the

complex conjugate of the improved channel estimate is used to compensate for the

channel impairments and the data symbols can then be demodulated. According to an exemplary embodiment, the method of FIG. 5 is performed in a RAKE

receiver.

Applicants have found that by using the TPC symbol in addition to the pilot

symbols in the calculation of the channel estimate, there is an increase of 10*log(5/4) or approximately 1 dB, although at the expense of an extra correlator,

i.e., an extra finger in the RAKE receiver. This performance improvement

provides two options, (1) it is possible to lower the transmitted power during the

pilot symbols and the TPC symbols by ldB and still obtain the same channel

estimates as with four pilot symbols, or (2) the transmitted power can remain the

same and the channel estimate can be improved using the extra pre-defined

symbol.

While the present invention has been described with respect to the

aforedescribed exemplary embodiments, one skilled in the art will appreciate that

the invention can be embodied in other ways. Thus, many variants and

combinations of the techniques taught above may be devised by a person skilled in -l i ¬

the art without departing from the spirit or scope of the invention as described by

the following claims.

Claims

WHAT IS CLAIMED IS:

1. A RAKE receiver comprising:

a plurality of fingers for receiving a corresponding plurality of signals;

a channel estimator for calculating a channel estimation for each of said

plurality of signals; and

wherein said channel estimation is based upon a plurality of pilot symbols

and at least one other predefined symbol, in a time slot associated with one of said

plurality of signals.

2. The RAKE receiver of claim 1, wherein said at least one other

predefined symbol is also used for a purpose other than channel estimation.

3. The RAKE receiver of claim 2, wherein said at least one other

predefined symbol is a Transmitter Power Control symbol.

4. The RAKE receiver of claim 1, further comprising:

a multiplier for applying a complex conjugate of said channel estimation to

a respective output of said plurality of fingers.

5. In a RAKE receiver, a method for channel estimation comprising the steps of:

detecting a plurality of predefined received symbols; and

calculating a channel estimate based upon said predefined received symbols

and predefined transmitted symbols;

wherein both said predefined received symbols and said predefined transmitted symbols include a plurality of pilot symbols and at least one other

predefined symbol.

6. A method in accordance with claim 5, wherein said at least one

other predefined symbol is also used for a purpose other than channel estimation.

7. A method in accordance with claim 6, wherein said at least one

other predefined symbol is a Transmitter Power Control symbol.

8. A method in accordance with claim 5, further comprising the step

of:

multiplying a complex conjugate of said channel estimate by a respective

output of a plurality of RAKE fingers.

9. In a RAKE receiver, a method for channel estimation comprising

the steps of:

detecting a plurality of predefined received symbols including a plurality of

pilot symbols and at least one other known symbol in a received time slot; calculating a first channel estimate using said plurality of pilot symbols;

demodulating said at least one other known symbol based upon a complex

conjugate of said first channel estimate; and

calculating a second channel estimate using said pilot symbols and said at

least one other known symbol.

10. A method in accordance with claim 9, wherein said at least one

other known symbol is also used for a purpose other than channel estimation.

11. A method in accordance with claim 10, wherein said at least one

other predefined symbol is a Transmitter Power Control symbol.

12. A method in accordance with claim 9, further comprising the step

of:

compensating a received time slot using the complex conjugate of said

channel estimate.

13. A RAKE receiver for channel estimation comprising:

means for detecting a plurality of predefined received symbols; and

means for calculating a channel estimate based upon said predefined

received symbols and said predefined transmitted symbols;

predefined symbol.

14. The RAKE receiver of claim 13, wherein said at least one other

predefined symbol is also used for a purpose other than channel estimation.

15. The RAKE receiver of claim 14, wherein said at least one other

predefined symbol is a Transmitter Power Control symbol.

16. The RAKE receiver of claim 13, further comprising:

means for compensating a received time slot using the complex conjugate

of said channel estimate.

17. A RAKE receiver for channel estimation comprising: 0/36760

-16-

means for detecting a plurality of predefined received symbols including a

plurality of pilot symbols and at least one other known symbol in a received time slot;

means for calculating a first channel estimate using said plurality of pilot symbols;

means for demodulating said at least one other known symbol based upon a

complex conjugate of said first channel estimate; and

means for calculating a second channel estimate using said pilot symbols

and said at least one other known symbol.

18. The RAKE receiver of claim 17, wherein said at least one other

known symbol is also used for a purpose other than channel estimation.

19. The RAKE receiver of claim 18, wherein said at least one other

predefined symbol is a Transmitter Power Control symbol.

20. The RAKE receiver of claim 17, further comprising:

means for compensating a received time slot using the complex conjugate

of said channel estimate.