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

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

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Publication number
WO2000036760A1
WO2000036760A1 PCT/SE1999/002398 SE9902398W WO0036760A1 WO 2000036760 A1 WO2000036760 A1 WO 2000036760A1 SE 9902398 W SE9902398 W SE 9902398W WO 0036760 A1 WO0036760 A1 WO 0036760A1
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Prior art keywords
symbol
predefined
symbols
rake receiver
pilot symbols
Prior art date
Application number
PCT/SE1999/002398
Other languages
French (fr)
Inventor
Hans Cavander
Original Assignee
Telefonaktiebolaget Lm Ericsson (Publ)
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Priority to US21225898A priority Critical
Priority to US09/212,258 priority
Application filed by Telefonaktiebolaget Lm Ericsson (Publ) filed Critical Telefonaktiebolaget Lm Ericsson (Publ)
Publication of WO2000036760A1 publication Critical patent/WO2000036760A1/en

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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W52/00Power management, e.g. TPC [Transmission Power Control], power saving or power classes
    • H04W52/04TPC
    • H04W52/38TPC being performed in particular situations
    • H04W52/42TPC being performed in particular situations in systems with time, space, frequency or polarisation diversity
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B1/00Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69Spread spectrum techniques
    • H04B1/707Spread spectrum techniques using direct sequence modulation
    • H04B1/7097Interference-related aspects
    • H04B1/711Interference-related aspects the interference being multi-path interference
    • H04B1/7115Constructive combining of multi-path signals, i.e. RAKE receivers
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; Arrangements for supplying electrical power along data transmission lines
    • H04L25/0202Channel estimation
    • H04L25/0204Channel estimation of multiple channels
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; Arrangements for supplying electrical power along data transmission lines
    • H04L25/0202Channel estimation
    • H04L25/0212Channel estimation of impulse response
    • H04L25/0214Channel estimation of impulse response of a single coefficient
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00Baseband systems
    • H04L25/02Details ; Arrangements for supplying electrical power along data transmission lines
    • H04L25/0202Channel estimation
    • H04L25/0224Channel estimation using sounding signals
    • H04L25/0228Channel estimation using sounding signals with direct estimation from sounding signals
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B2201/00Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
    • H04B2201/69Orthogonal indexing scheme relating to spread spectrum techniques in general
    • H04B2201/707Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
    • H04B2201/70701Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation featuring pilot assisted reception

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, , ... , XM calculate the product of the transmitted pilot symbols PST1, ..., PS™ and the

complex conjugate of their respective received pilot symbol PSR1\ ... , PSRM *. The

output of multipliers X, , ... , XM 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:

Figure imgf000009_0001

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 Sls ..., SN are received by the RAKE receiver by fingers F^...,

FN 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 Ft, ..., FN to respective

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

conjugate of the corresponding channel estimate C,*, ..., CN *. 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:

Figure imgf000010_0001

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;
wherein both said predefined received symbols and said predefined transmitted symbols include a plurality of pilot symbols and at least one other
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.
PCT/SE1999/002398 1998-12-16 1999-12-16 Channel estimation for a cdma system using pre-defined symbols in addition to pilot symbols WO2000036760A1 (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
US21225898A true 1998-12-16 1998-12-16
US09/212,258 1998-12-16

Applications Claiming Priority (4)

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AU30926/00A AU3092600A (en) 1998-12-16 1999-12-16 Channel estimation for a cdma system using pre-defined symbols in addition to pilot symbols
DE1999183814 DE19983814T1 (en) 1998-12-16 1999-12-16 Channel estimation for a CDMA system and use of predefined symbols in addition to pilot symbols
GB0116741A GB2363553B (en) 1998-12-16 1999-12-16 Channel estimation for a CDMA system using pre-defined symbols in addition to pilot symbols
JP2000588906A JP2002533007A (en) 1998-12-16 1999-12-16 Channel estimation for CDMA systems using predefined symbols in addition to pilot symbols

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CN (1) CN1192506C (en)
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WO2002025848A1 (en) * 2000-09-25 2002-03-28 Huawei Technologies Co., Ltd. Method for multiple time slot power control
WO2002039686A1 (en) * 2000-11-09 2002-05-16 Linkair Communications, Inc. A channel estimating method and apparatus thereof
EP1263179A1 (en) * 2001-05-29 2002-12-04 Lucent Technologies Inc. Channel estimation for a CDMA system using coded control symbols as additional pilot symbols
EP1298814A2 (en) * 2001-09-27 2003-04-02 Nec Corporation CDMA receiver and channel estimation method
WO2004014029A1 (en) * 2002-08-06 2004-02-12 Zte Corporation A method of channel estimation in wideband cdma communication
FR2851383A1 (en) * 2003-02-17 2004-08-20 Wavecom Wireless data transmitting process for universal mobile telecommunication system, involves estimating response of transmission channel of data transmission signal by taking into account simple carrier pilot signal
WO2004112337A1 (en) * 2003-05-31 2004-12-23 Qualcomm Incorporated Signal-to-noise estimation in wireless communication devices with receive diversity
CN100431317C (en) * 2000-09-14 2008-11-05 法国电讯公司 Method for optimal estimation of propagation channel relying solely on pilot symbols and corresponding estimator
EP2753036A1 (en) 2013-01-08 2014-07-09 Freescale Semiconductor, Inc. Channel estimation in wireless communication
US9118515B2 (en) 2014-01-08 2015-08-25 Freescale Semiconductor, Inc. Channel estimation in wireless communication

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CN100362762C (en) * 2003-12-29 2008-01-16 华为技术有限公司 An apparatus and method for implementing data signal mode decision
CN105282059A (en) 2014-06-30 2016-01-27 深圳市中兴微电子技术有限公司 Multi-path selection method and equipment

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EP0715440A1 (en) * 1994-06-22 1996-06-05 Ntt Mobile Communications Network Inc. Synchronous detector and synchronizing method for digital communication receiver
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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100431317C (en) * 2000-09-14 2008-11-05 法国电讯公司 Method for optimal estimation of propagation channel relying solely on pilot symbols and corresponding estimator
WO2002025848A1 (en) * 2000-09-25 2002-03-28 Huawei Technologies Co., Ltd. Method for multiple time slot power control
WO2002039686A1 (en) * 2000-11-09 2002-05-16 Linkair Communications, Inc. A channel estimating method and apparatus thereof
EP1263179A1 (en) * 2001-05-29 2002-12-04 Lucent Technologies Inc. Channel estimation for a CDMA system using coded control symbols as additional pilot symbols
EP1298814A3 (en) * 2001-09-27 2004-03-31 Nec Corporation CDMA receiver and channel estimation method
EP1298814A2 (en) * 2001-09-27 2003-04-02 Nec Corporation CDMA receiver and channel estimation method
WO2004014029A1 (en) * 2002-08-06 2004-02-12 Zte Corporation A method of channel estimation in wideband cdma communication
FR2851383A1 (en) * 2003-02-17 2004-08-20 Wavecom Wireless data transmitting process for universal mobile telecommunication system, involves estimating response of transmission channel of data transmission signal by taking into account simple carrier pilot signal
WO2004112337A1 (en) * 2003-05-31 2004-12-23 Qualcomm Incorporated Signal-to-noise estimation in wireless communication devices with receive diversity
US7366137B2 (en) 2003-05-31 2008-04-29 Qualcomm Incorporated Signal-to-noise estimation in wireless communication devices with receive diversity
EP2753036A1 (en) 2013-01-08 2014-07-09 Freescale Semiconductor, Inc. Channel estimation in wireless communication
US9118515B2 (en) 2014-01-08 2015-08-25 Freescale Semiconductor, Inc. Channel estimation in wireless communication

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AU3092600A (en) 2000-07-03
GB0116741D0 (en) 2001-08-29
DE19983814T1 (en) 2001-12-13
GB2363553B (en) 2003-12-03
JP2002533007A (en) 2002-10-02
GB2363553A (en) 2001-12-19
CN1192506C (en) 2005-03-09
MY130820A (en) 2007-07-31
CN1334997A (en) 2002-02-06
DE19983814T0 (en)

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