U.S. patent application number 13/967228 was filed with the patent office on 2013-12-12 for channel quality estimation from raw bit error rate.
This patent application is currently assigned to TELEFONAKTIEBOLAGET L M ERICSSON (PUBL). The applicant listed for this patent is TELEFONAKTIEBOLAGET L M ERICSSON (PUBL). Invention is credited to George Jongren, Yi-Pin Eric Wang.
Application Number | 20130329570 13/967228 |
Document ID | / |
Family ID | 44243026 |
Filed Date | 2013-12-12 |
United States Patent
Application |
20130329570 |
Kind Code |
A1 |
Wang; Yi-Pin Eric ; et
al. |
December 12, 2013 |
CHANNEL QUALITY ESTIMATION FROM RAW BIT ERROR RATE
Abstract
Channel quality metrics (such as SINR, BLER, and the like) are
derived from a raw bit error rate (RBER), defined as the error rate
of raw bits output by a demodulator. These initial raw bits are
decoded and error-checked (or error-corrected). The error-free
decoded bits are re-encoded, and the regenerated raw bits are
compared to the initial raw bits to determine the RBER. The RBER is
then converted to SINR, BLER, or other channel quality metric. The
RBER-based metrics are derived from a data channel rather than
reference signals, and hence more accurately reflect deviations
from nominal transmission power level, and include receiver
demodulator impairments.
Inventors: |
Wang; Yi-Pin Eric; (Fremont,
CA) ; Jongren; George; (SUNDBYBERG, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TELEFONAKTIEBOLAGET L M ERICSSON (PUBL) |
Stockholm |
|
SE |
|
|
Assignee: |
TELEFONAKTIEBOLAGET L M ERICSSON
(PUBL)
Stockholm
SE
|
Family ID: |
44243026 |
Appl. No.: |
13/967228 |
Filed: |
August 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12779172 |
May 13, 2010 |
8537936 |
|
|
13967228 |
|
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Current U.S.
Class: |
370/242 |
Current CPC
Class: |
H04W 24/08 20130101;
H04L 1/0036 20130101; H04L 1/20 20130101; H04L 1/0032 20130101 |
Class at
Publication: |
370/242 |
International
Class: |
H04W 24/08 20060101
H04W024/08 |
Claims
1. A method of estimating channel quality in receiver operative in
a wireless communication network, comprising: receiving a wireless
signal from a transmitter; demodulating the received signal to
generate initial raw bits; decoding the initial raw bits to
generate decoded bits; performing an error check on the decoded
bits; generating regenerated raw bits by encoding the decoded bits
that pass the error check; comparing the initial raw bits to the
regenerated raw bits to determine a raw bit error rate (RBER); and
using the RBER to generate a channel quality metric.
2. The method of claim 1 further comprising: storing the initial
raw bits; transmitting a retransmission request; receiving a
retransmitted signal; demodulating the retransmitted signal to
generate retransmitted raw bits; decoding the retransmitted raw
bits to generate retransmitted decoded bits; performing an error
check on the retransmitted decoded bits; generating regenerated
retransmitted raw bits by encoding the retransmitted decoded bits
that pass the error check; comparing the initial raw bits and the
retransmitted raw bits to the regenerated retransmitted raw bits to
determine a raw bit error rate (RBER) over both the received signal
and retransmitted signal transmission durations.
3. The method of claim 1 further comprising using the channel
quality metric from a current transmission duration and at least
one prior transmission duration to jointly predict the channel
quality metric for a future transmission duration having the same
frequency allocation.
4. The method of claim 1 wherein the received wireless signal was
generated using a high order modulation and heavy encoding to
increase the number of raw bit errors.
5. The method of claim 4 wherein high order modulation comprises
16-QAM or 64-QAM.
6. The method of claim 4 wherein heavy encoding comprises the use
of repetition code to extend the coding rate to below rate 1/3.
7. The method of claim 1 further comprising terminating the method
when the relative speed between the transmitter and receiver
exceeds a predetermined threshold.
8. The method of claim 1 further comprising terminating the method
if the number of initial raw bits in error over a predetermined
duration is below a predetermined threshold.
9. The method of claim 1 wherein using the RBER to generate a
channel quality metric comprises converting the RBER to a
RBER-based signal to interference and noise ratio (SINR).
10. The method of claim 9 wherein converting the RBER to a
RBER-based SINR comprises indexing a mapping table using the RBER
and obtaining a corresponding RBER-based SINR.
11. The method of claim 10 wherein indexing a mapping table
comprising indexing a mapping table corresponding to the modulation
format under which the received signal was transmitted.
12. The method of claim 1 wherein using the RBER to generate a
channel quality metric comprises converting the RBER to a
RBER-based block error rate (BLER).
13. A method of estimating channel quality in receiver operative in
a wireless communication network, comprising: receiving a wireless
information-bearing signal from a transmitter, the received signal
modulated using one of 16-QAM or 64-QAM modulation and encoded
using a repetition code to extend the coding rate to below rate
1/3; demodulating the received signal to generate initial raw bits;
decoding the initial raw bits to generate decoded bits; performing
an error check on the decoded bits; generating regenerated raw bits
by encoding the decoded bits that pass the error check; comparing
the initial raw bits to the regenerated raw bits to determine a raw
bit error rate (RBER); and using the RBER to generate a channel
quality metric.
14. A receiver operative in a wireless communication network,
comprising: a demodulator operative to demodulate a received
wireless communication signal to generate initial raw bits; a
decoder operative to decode the initial raw bits to generate
decoded bits; an error checker operative to detect or correct
errors in the decoded bits; an encoder operative to encode
error-free decoded bits to generate regenerated raw bits; a
comparator operative to compare the initial raw bits to the
regenerated raw bits and determine a raw bit error rate (RBER); and
a converter operative to convert the RBER to a channel quality
metric.
15. The receiver of claim 14 further comprising memory operative to
store an RBER value calculated over a first transmission
duration.
16. The receiver of claim 14 further comprising memory operative to
store initial raw bits from a signal for which the receiver
requests retransmission.
17. The receiver of claim 14 wherein the comparator includes a bit
selector operative to select only a high error probability subset
of initial and regenerated raw bits per symbol for comparison.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuation of U.S. patent application Ser. No.
12/779,172, filed May 13, 2010, which is incorporated by reference
herein in its entirety.
TECHNICAL FIELD
[0002] The present invention relates generally to wireless
communication network receivers, and in particular to a method and
apparatus of estimating channel quality by calculating a raw bit
error rate.
BACKGROUND
[0003] Wireless communication systems provide voice and data
communications between one or more fixed transceivers, known as
base stations or Node Bs, and a plurality of mobile transceivers,
or User Equipment (UE), across an air interface. The voice or data
information (herein collectively referred to as data) are encoded,
modulated onto a radio frequency (RF) carrier. The modulated RF
signal is amplified, transmitted across the air interface,
received, demodulated, and the demodulated bits are decoded and
further processed to recover the data. The physical channel, which
includes circuits in the transmitter, the air interface, and
circuits in the receiver, introduces impairments (interference and
noise), making the received signal differ, often significantly,
from the transmitted signal. The inverse of the extent to which the
channel impairs the transmitted signal is referred to as channel
quality. A common metric of channel quality is the Signal to
Interference and Noise Ratio (SINR); another is the block error
rate (BLER).
[0004] One method of improving spectral efficiency for a given
channel quality is link adaptation, also known as adaptive coding
and modulation (ACM). In one form of link adaptation, a
transmission format (TF), which specifies the modulation type,
forward error correction (FEC) coding rate, number of transmit
antennas to employ (i.e., space-time code), number of spatial
multiplexing streams, and other parameters, is adaptively selected
from a fixed number of possibilities, in response to dynamic
measurements of the channel quality. To enable assessment of the
channel quality, known data patterns, referred to as reference
signals or pilots, are transmitted across the air interface. By
comparing received reference signals to their known value, a
receiver can assess and quantify the impairment characteristics,
and report the channel quality.
[0005] Several types of reference signals are used for channel
characterization. For example, in LTE uplink, two types of
reference signals are employed. A sounding reference signal (SRS)
is used to facilitate frequency dependent scheduling. A
demodulation reference signal (DMRS) facilitates coherent
demodulation. Particularly in a rapidly changing channel, sending
more reference signals increases channel characterization accuracy
and hence improves link adaptation. However, reference signals
decrease spectral efficiency, as they consume air interface
bandwidth yet do not transmit user data. Additionally, in practice,
impairments arising from imperfections in the transmitter and
receiver circuits are difficult to accurately estimate using
reference signals.
SUMMARY
[0006] According to one or more embodiments described and claimed
herein, channel quality metrics (such as SINR, BLER, and the like)
are derived from a raw bit error rate (RBER), defined as the error
rate of raw bits output by a demodulator. These initial raw bits
are decoded and error-checked (or error-corrected). The error-free
decoded bits are re-encoded, and the regenerated raw bits are
compared to the initial raw bits to determine the RBER. The RBER is
then converted to SINR, BLER, or other channel quality metric.
[0007] One embodiment relates to a method of estimating channel
quality in receiver operative in a wireless communication network.
A wireless signal is received from a transmitter. The received
signal is demodulated to generate initial raw bits. The initial raw
bits are decoded to generate decoded bits. An error check is
performed on the decoded bits. Regenerated raw bits are generated
by encoding the decoded bits that pass the error check. The initial
raw bits are compared to the regenerated raw bits to determine a
RBER. The RBER is used to generate a channel quality metric.
[0008] Another embodiment relates to a receiver operative in a
wireless communication network. The receiver includes a demodulator
operative to demodulate a received wireless communication signal to
generate initial raw bits; a decoder operative to decode the
initial raw bits to generate decoded bits; an error checker
operative to detect or correct errors in the decoded bits; an
encoder operative to encode error-free decoded bits to generate
regenerated raw bits; a comparator operative to compare the initial
raw bits to the regenerated raw bits and determine a RBER; and a
converter operative to convert the RBER to a channel quality
metric.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a flow diagram of a method of estimating channel
quality in wireless communication network receiver.
[0010] FIG. 2 is a functional block diagram of a wireless
communication network receiver implementing the method of FIG.
1.
[0011] FIG. 3 is a graph depicting the relationship between raw bit
error rate and SINR for three different modulation techniques.
[0012] FIG. 4 is a graph depicting the relationship between raw bit
error rate and SINR for high and low probability bit subsets for
16-QAM modulation.
[0013] FIG. 5 is a graph depicting the relationship between raw bit
error rate and SINR for high, medium, and low probability bit
subsets for 64-QAM modulation.
[0014] FIG. 6 is a functional block diagram of a prior art wireless
communication network.
DETAILED DESCRIPTION
[0015] The present invention relates to channel estimation, using
the Raw Bit Error Rate (RBER), in a wireless communication network.
To provide a complete and enabling disclosure, embodiments of the
present invention are described herein in the context of the Long
Term Evolution (LTE, or LTE Advanced) of the Universal Mobile
Telecommunications System (UMTS). However, it should be noted that
the present invention is not limited to LTE, or any other
particular wireless communication protocol.
[0016] FIG. 6 depicts a representative LTE network 10. The network
10 comprises a core network 12 interconnecting and controlling a
plurality of Node Bs, or base stations, 16. Each Node B 16 includes
radio transceivers to effect wireless communications with a
plurality of User Equipment (UE), or mobile terminals, 18 within a
geographic region, or cell, 20. The network 10 provides voice and
data communications between UEs 18 in the network 10, and also
between UEs 18 and one or more external networks 22, such as the
Public Switched Telephone Network (PSTN), the Internet, another
wireless network, or the like.
[0017] As discussed above, characterization of the physical channel
between the Node B 16 is traditionally performed using known
reference signals, which consume valuable air interface resources
(transmit power, bandwidth, and the like), and themselves generate
interference in communication signals to other UEs 18. According to
embodiments of the present invention, channel quality (e.g., SINR)
is estimated from the Raw Bit Error Rate (RBER) of a received
communication signal, such as the physical uplink shared channel
(PUSCH). Not only does this approach reduce the need for reference
signal overhead, it captures all impairments in the channel,
including loss incurred during the modulation. The RBER may be
mapped to a SINR, block error rate (BLER), or other channel quality
metric.
[0018] FIG. 1 depicts a method 100 of estimating channel quality in
a wireless communication network receiver 200, depicted in FIG. 2.
The receiver 200, which may comprise the receiver side of a
transceiver, and which may reside in a Node B 16 or UE 18, receives
a wireless signal (block 102) at one or more antennas 202. The
received signal is processed by a front end receiver circuit 204,
including low-noise amplification, filtering, frequency
downconversion to baseband, analog to digital conversion, in-phase
(I) and quadrature-phase (Q) separation, and the like. The front
end processed signal is demodulated by a demodulator 206,
generating initial raw bits (block 104). The initial raw bits are
decoded by a decoder 208, generating decoded bits (block 106). The
decoded bits are error-checked (block 108), and if possible
error-corrected, such as by operation of the cyclic redundancy
check (CRC) 210. If the decoded bits pass the error check (block
110) (either by being error-free or being corrected), they are
passed on for further processing, as known in the art.
[0019] To determine the RBER, the initial raw bits generated by the
demodulator 206 are compared to their transmitted values. These are
obtained by encoding the decoded bits that pass the error check in
the encoder 212, generating regenerated raw bits (block 112). The
initial raw bits and the regenerated raw bits are compared in the
comparator 214, which determines the RBER (block 114). In some
embodiments, a bit selector 216 selects only a subset of the raw
bits (both initial and regenerated) per symbol for comparison, as
discussed in greater detail herein. In one embodiment, the
comparator 214 ensures that the RBER is based on a statistically
significant sample by only generating a RBER if the number of
initial raw bits in error exceeds a predetermined threshold.
[0020] A converter 218 converts the RBER to a useful metric of
channel quality, such as SINR, BLER, or the like. The converter 218
preferably compiles (or is provisioned with) and maintains a
separate mapping table between the RBER and the desired channel
quality metric for each modulation format. For example, FIG. 3
depicts the relationship between RBER and SINR for quadrature phase
shift keying (QPSK), 16-element quadrature amplitude modulation
(16-QAM), and 64-QAM. In operation, interpolation may be used for
RBER values between table entries. The converter 218 is operatively
connected to memory 220 to store the channel quality metric
conversion look-up tables.
[0021] The memory 220 may also store initial raw bits. If the
decoded bits fail a CRC check, the receiver may request a
retransmission (e.g., according to a HARQ protocol). In this case,
the initial raw bits may be stored to memory 220. When the receiver
208 later receives the retransmitted signal, and it passes the CRC
check, the comparator 214 compares the regenerated raw bits to the
initial raw bits for the retransmitted signal. In one embodiment,
the comparator 214 may additionally retrieve the stored raw bits
from the first received signal, and compare them to the regenerated
raw bits from the retransmitted signal. In this manner, the channel
quality for both subframes (that of the original signal and that of
the retransmitted signal) may be estimated.
[0022] The memory 220 may also store prior RBER values. In one
embodiment, the converter 218 may average the RBER over two or more
subframes (or other predefined transmission duration, depending on
the wireless communication protocol). Of course, the converter 218
may also store, and average, other metrics derived from the RBER,
such as SINR, BLER, or the like.
[0023] In one embodiment, the communication signal received at the
antenna 202 is a conventional data signal received over a data
channel, e.g., PUSCH. In one embodiment, to improve the accuracy of
channel estimation using the RBER, the transmitter sends a
high-order modulated and heavily encoded signal. Such a signal is
conceptually an information-bearing sounding reference signal, as
it facilitates channel characterization while also carrying useful
data. The signal may be modulated using 16-QAM or 64-QAM
modulation, which will generate a large number of erroneous initial
raw bits, particularly in poor channel conditions. The signal is
heavily encoded--for example, by using repetition codes to reduce
the coding rate to below rate 1/3--so that the data may be
recovered reliably in the face of the high RBER.
[0024] In one embodiment, the comparator 214 includes a bit
selector 216. The bit selector 216 increases the reliability of
RBER estimation by selecting, from the initial raw bits and
regenerated raw bits, a subset of bits per symbol having a higher
probability of error, prior to comparison of the subsets by the
comparator 214. Consider rectangular 16-QAM modulation. In this
modulation scheme, four bits are mapped to each symbol. A first
subset of the four bits identifies the sign of a symbol (+ or - for
each of I and Q axes), and a second subset of the four bits
identifies the magnitude of the symbol (1 or 3). Statistically, the
sign bits are demodulated with greater accuracy (as they identify
the quadrant of the IQ constellation diagram) than the magnitude
bits (which identify the position of the symbol within a quadrant).
This is depicted in FIG. 4. Similarly, as depicted in FIG. 5, in
rectangular 64-QAM modulation, two of the six bits in each symbol
have a low error probability, two have a medium error probability,
and two have a high error probability. By considering only the
subset of bits per symbol for each type of modulation that have the
highest error probability, the comparator 214 generates a more
reliable estimate of RBER, as it is derived from a more
statistically robust sample.
[0025] The estimation of RBER, and hence RBER-derived SINR or BLER,
is most accurate at low to intermediate relative speed between the
transmitter and receiver (i.e., UE 18 speed). Accordingly, in one
embodiment, the RBER method of channel quality estimation may be
selectively engaged, and in particular may be terminated at high UE
18 speed (that is, UE 18 speed, relative to the Node B 16, above a
predetermined threshold). UE 18 speed may be determined in the UE
18 by a positioning system, such as GPS, and reported to the Node B
16. Alternatively, UE 18 speed may be determined by the UE 18 or
the Node B 16 my measuring Doppler frequency shift, or by other
means, as known in the art.
[0026] In one embodiment, the RBER corrects a bias or offset in
conventional SINR estimation obtained from reference signals. For
example, a bias may occur due to circuit impairments in the
transmitter and/or receiver. In this embodiment, a receiver
estimates a conventional SINR from reference signals, as known in
the art. The receiver also generates an RBER-based SINR estimate,
as described herein. An SINR bias term is the difference between
the conventional SINR and the RBER-based SINR estimates. The SINR
bias term may be computed for each of a plurality of predetermined
transmission durations, such as frames or subframes, and averaged
over several such durations to improve estimation accuracy.
[0027] Due to non-linearity in the transmitter power amplifier, the
actual transmit power of a wireless communication network
transmitter often deviates from the nominal transmitter power. In
one embodiment, a RBER-based SINR estimate is used to generate and
maintain a correction term for each nominal transmit power
level.
[0028] LTE uplink imposes loose restrictions on the tolerance
between the transmission towers of different transmission.
Accordingly, an SRS transmission by a UE 18, used to predict the
SINR in a subsequent subframe, might use a transmission power
(nominal transmit power) widely different from the transmission
power actually used to transmit data in the subsequent subframe.
Since the Node B 16 does not have accurate information about the
transmission power of the UE 18, it is difficult to compensate the
conventional SINR to reflect the difference in transmission power.
In this case, an RBER-based SINR estimate (or BLER estimate),
obtained from the data channel (PUSCH) is more accurate than a
conventional SINR estimate obtained from reference signals. For
example, the SINR estimated from SRS transmitted at power level
p.sub.1 is .gamma..sub.1. The UE 18 is allowed to transmit PUSCH at
power level p.sub.2. Nominally,
[0029] one would expect the SINR for PUSCH to be
.gamma. 1 p 2 p 1 . ##EQU00001##
However, due to PA nonlinearity, the actual PUSCH transmit power
from UE 18 is
p 2 .DELTA. . ##EQU00002##
According to embodiments of the present invention, the power
deviation .DELTA. can be estimated from RBER-based SINR estimation.
Thus, the Node B 16 can already adjust the SINR estimation for
PUSCH as
.gamma. 1 p 2 p 1 .DELTA. . ##EQU00003##
[0030] Embodiments of the present invention provide more accurate
channel quality metrics than conventional reference signal-based
approaches, as the RBER estimate includes receiver circuit
impairments, including the demodulator 206. Furthermore,
embodiments of the present invention improve spectral efficiency,
as the channel quality is estimated from a data channel, reducing
the required number of reference signal transmissions. The accurate
channel quality estimation includes the accuracy of link
adaptation, which in turn maximizes spectral efficiency for a given
channel quality.
[0031] The present invention may, of course, be carried out in
other ways than those specifically set forth herein without
departing from essential characteristics of the invention. The
present embodiments are to be considered in all respects as
illustrative and not restrictive, and all changes coming within the
meaning and equivalency range of the appended claims are intended
to be embraced therein.
* * * * *