U.S. patent application number 13/597207 was filed with the patent office on 2013-10-17 for low-complexity channel noise reduction method and apparatus for multi-carrier mode in wireless lans.
The applicant listed for this patent is Yun Zhang. Invention is credited to Yun Zhang.
Application Number | 20130272364 13/597207 |
Document ID | / |
Family ID | 46774266 |
Filed Date | 2013-10-17 |
United States Patent
Application |
20130272364 |
Kind Code |
A1 |
Zhang; Yun |
October 17, 2013 |
LOW-COMPLEXITY CHANNEL NOISE REDUCTION METHOD AND APPARATUS FOR
MULTI-CARRIER MODE IN WIRELESS LANS
Abstract
Low-complexity channel noise reduction method and apparatus for
multi-carrier mode in wireless LANs are disclosed. The method
selects an optimal frequency domain channel impulse response by
using a known long training sequence and a highly protected
signaling sequence of the multi-carrier mode frame structure to
ensure the receiver to have a good operation threshold in different
time-delay spread environments at the cost of a low complexity.
Instead of detecting time domain channel responses, the method
directly performs noise reduction to a noise-containing frequency
domain channel by using preset Wiener filtering coefficients to
obtain multiple frequency domain channel responses, among which
there must be a relatively optimal frequency domain channel
response. The relatively optimal frequency domain channel response
can be selected by using the highly protected signaling sequence to
calculate the signaling frequency domain channel.
Inventors: |
Zhang; Yun; (Shanghai,
CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zhang; Yun |
Shanghai |
|
CN |
|
|
Family ID: |
46774266 |
Appl. No.: |
13/597207 |
Filed: |
August 28, 2012 |
Current U.S.
Class: |
375/231 |
Current CPC
Class: |
H04L 25/022 20130101;
H04L 25/0232 20130101; H04L 25/03159 20130101 |
Class at
Publication: |
375/231 |
International
Class: |
H04L 27/01 20060101
H04L027/01 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2012 |
CN |
201210109447.3 |
Claims
1. A low-complexity channel noise reduction method for a
multi-carrier mode in a wireless local area network, comprising the
following steps: dividing a received frequency domain long training
sequence by a locally stored long training sequence to obtain an
original noise-containing frequency domain channel; performing
Wiener filtering to the original noise-containing frequency domain
channel by using a plurality of groups of prestored Wiener
filtering coefficients according to their corresponding filtering
methods to obtain a plurality of groups of noise-reduced frequency
domain channels; performing equalization to received frequency
domain signaling symbols by using a frequency domain channel
response provided by the original noise-containing frequency domain
channel, and demodulating, deinterleaving and convolutionally
decoding the equalized signaling symbols to obtain effective
signaling bits; performing encoding, interleaving, modulation and
OFDM framing to the obtained effective signaling bits according to
the order at a transmitting end to obtain OFDM signaling symbols,
and dividing the received frequency domain signaling symbols by the
OFDM signaling symbols to obtain a signaling frequency domain
channel; and selecting a group of noise-reduced frequency domain
channel that is closest to a response of the signaling frequency
domain channel from the plurality of groups to be an optimal
frequency domain channel.
2. A low-complexity channel noise reduction apparatus for a
multi-carrier mode in a wireless local area network, comprising a
first sequence divider, a channel equalizer, a
demodulator/deinterleaver/convolutional-decoder, a
convolutional-encoder/interleaver/modulator, an OFDM framer, a
second sequence divider, a Wiener filtering processer and a channel
analyzer/selector, wherein the first sequence divider is configured
to receive a frequency domain long training sequence and divide it
by a locally stored long training sequence to obtain an original
noise-containing frequency domain channel; the channel equalizer is
configured to receive frequency domain signaling symbols and
perform channel equalization to the frequency domain signaling
symbols with the original noise-containing frequency domain channel
to obtain equalized signaling symbols; the
demodulator/deinterleaver/convolutional-decoder is configured to
demodulate, deinterleave and convolutionally decode the equalized
signaling symbols to obtain effective signaling bits; the
convolutional-encoder/interleaver/modulator and the OFDM framer are
configured to perform convolutional encoding, interleaving,
modulation and OFDM framing to the effective signaling bits to
obtain OFDM signaling symbols; the second sequence divider is
configured to receive the frequency domain signaling symbols and
dividing the frequency domain signaling symbols by the OFDM
signaling symbols to obtain a signaling frequency domain channel;
the Wiener filtering processer is prestored with a plurality of
groups of Wiener filtering coefficients and is configured to
perform Wiener filtering to the obtained original noise-containing
frequency domain channel by using filtering methods corresponding
to the plurality of groups of Wiener filtering coefficients to
obtain a plurality of groups of noise-reduced frequency domain
channels; and the channel analyzer/selector is configured to select
a group of noise-reduced frequency domain channel that is closest
to a response of the signaling frequency domain channel from the
plurality of groups to be an optimal frequency domain channel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority of Chinese patent
application number 201210109447.3, filed on Apr. 13, 2012, the
entire contents of which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to the field of wireless
communications, and more particularly, to a low-complexity channel
noise reduction method and apparatus used in multi-carrier mode in
wireless local area networks (LANs).
BACKGROUND
[0003] In wireless communication systems, inevitable influence of
terrains or obstacles on signals causes the occurrence of multipath
distortion. As a time-varying channel impulse response is generally
modeled as a time-domain discrete finite impulse response (FIR)
filter denoted by
h ( .tau. ; t ) = n .alpha. n ( t ) - j 2 .pi. f c .tau. n ( t )
.delta. ( .tau. - .tau. n ( t ) ) , ##EQU00001##
complex multipath interference always exists in received wideband
signals, which appears as frequency selective fading in the
frequency domain. For this reason, single-carrier systems usually
employ time domain equalization to eliminate the multipath
influence, which results in a very high complexity of
receivers.
[0004] The orthogonal frequency division multiplexing (OFDM)
technology has been widely used in wireless wideband communication
systems. One significant advantage of OFDM technology is its
capability of dividing a carrier with a relatively wide bandwidth
into multiple parallel subcarriers, each subcarrier having a
bandwidth far less than the coherence bandwidth of a channel.
Therefore, the channel frequency fading that each subcarrier signal
undergoes is flat, which overcomes the adverse effect of channel
frequency selective fading. If channel frequency response
characteristics at different subcarriers can be obtained by channel
estimation technology, a receiver will be capable of realizing
coherent demodulation to correctly recover transmitted signals. In
order to improve demodulation threshold of the receiver and the
quality of received signals, noise reduction is generally performed
to the estimated channel response.
[0005] Wiener filtering is a commonly used channel noise reduction
technology in multi-carrier OFDM systems. In consideration of the
implementation complexity, Wiener filtering methods are usually
designed according to several groups of preset channel power delay
characteristics, and the corresponding Wiener filtering
coefficients are stored. The receiver selects appropriate Wiener
filtering methods and coefficients according to the preset channel
power delay characteristic in the practical transmission
environment. The above concept is applied in the China mobile
multimedia broadcasting (CMMB) system to perform noise reduction
and interpolation by selecting the most appropriate group of Wiener
filtering coefficients through analyzing the multipath delay spread
in a transmission channel. However, this method is only suitable
for CMMB systems or the like, which adopt a low quadrature
amplitude modulated (QAM) transmission mode, as the low QAM
transmission mode is not sensitive to weak paths in reception
environment, and ignorance or loss of several weak paths will not
effect the determination of constellation points. Moreover, as the
energy of noise in the environment where a low QAM reception is
located is relatively high, it is advantageous for the reception
threshold if some weak paths together with the noise are inhibited.
With the increase of transmission rate, great QAM constellation
points are more frequently used in practical systems, for example,
the 64-QAM constellation points used in wireless LANs and the
256-QAM constellation points used in the European DVB-T2 system.
These constellation points all have high demodulation thresholds,
so that weak multipath in the transmission environment will have
significant effect on their determination. Nevertheless, virtual
carriers employed in OFDM systems that include cyclic prefix make
weak paths of low energies sometimes be drowned in energy leakage
of strong paths and cannot be easily detected, making the channel
noise reduction of high QAM reception systems more difficult. In
fact, the channel noise reduction process will lead to channel
distortion while reducing the noise. As noise reduction is good for
demodulation and channel distortion is bad for demodulation, the
result of the channel noise reduction process is depended on the
combined effect of the above two opposing effects.
[0006] FIG. 1 illustrates the frame structure of a multi-carrier
transmission mode in 802.11a/g systems. The frame consists of four
parts wherein a first part is a short training sequence; a second
part is a long training sequence; a third part is a signaling
sequence; and a fourth part is a data sequence. The short training
sequence is composed of 10 duplicate short training symbols and is
mainly used for signal gain adjustment, signal capture and coarse
estimation of carrier frequency offset. The following long training
sequence is composed of 2.5 duplicate long training symbols and is
mainly used for accurate estimation of carrier frequency offset,
accurate timing synchronization of OFDM symbols and multipath
estimation of transmission channel. The following signaling
sequence is used for transmitting indication information which is
necessary for the demodulation of the data sequence. Such
indication information may include the length of the data sequence
as well as the modulation mode used in transmission and the coding
efficiency. Once the receiver obtains the above mentioned necessary
information, it will be able to correctly demodulate and detect the
data sequence.
[0007] In the 802.11a/g multi-carrier mode, channel estimation is
accomplished during the period of receiving the long training
sequence. The frequency domain channel response is obtained by
dividing the received long training sequence by the locally stored
frequency domain long training sequence. Since the frequency domain
channel contains noise in most cases, if the channel is directly
used for subsequent demodulation operation, it will result in a
poor demodulation performance, therefore a noise reduction process
is usually performed to the channel to improve operation threshold
of system.
SUMMARY OF THE INVENTION
[0008] An objective of the present invention is to provide a
low-complexity channel noise reduction method applicable for the
802.11a/g/n multi-carrier mode in wireless local area networks
(LANs). The present invention selects an optimal frequency domain
channel impulse response by using a known long training sequence
portion and a highly protected signaling sequence portion of the
multi-carrier mode frame structure to ensure the receiver to have a
good operation threshold in environments with different time-delay
spreads at the cost of a low complexity.
[0009] To achieve the above objective, the present invention
provides a low-complexity channel noise reduction method for a
multi-carrier transmission mode in a wireless local area network,
the method includes:
[0010] divide a received frequency domain long training sequence by
a locally stored long training sequence to obtain an original
noise-containing frequency domain channel;
[0011] perform Wiener filtering to the original noise-containing
frequency domain channel by using a plurality of groups of
prestored Wiener filtering coefficients according to their
corresponding filtering methods to obtain a plurality of groups of
noise-reduced frequency domain channels;
[0012] perform equalization to received frequency domain signaling
symbols by using a frequency domain channel response provided by
the original noise-containing frequency domain channel, and
demodulate, deinterleave and convolutionally decode the equalized
signaling symbols to obtain effective signaling bits;
[0013] perform encoding, interleaving, modulation and OFDM framing
to the obtained effective signaling bits according to the order at
a transmitting end to obtain OFDM signaling symbols, and divide the
received frequency domain signaling symbols by the OFDM signaling
symbols to obtain a signaling frequency domain channel; and
[0014] select a group of noise-reduced frequency domain channel
that is closest to a response of the signaling frequency domain
channel from the plurality of groups to be an optimal frequency
domain channel.
[0015] To achieve the above objective, the present invention also
provides a low-complexity channel noise reduction apparatus for a
multi-carrier transmission mode in a wireless local area network.
The apparatus includes a first sequence divider, a channel
equalizer, a demodulator/deinterleaver/convolutional-decoder, a
convolutional-encoder/interleaver/modulator, an OFDM framer, a
second sequence divider, a Wiener filtering processer and a channel
analyzer/selector, wherein
[0016] the first sequence divider is configured to receive a
frequency domain long training sequence and divide it by a locally
stored long training sequence to obtain an original
noise-containing frequency domain channel;
[0017] the channel equalizer is configured to receive frequency
domain signaling symbols and perform channel equalization to the
frequency domain signaling symbols with the original
noise-containing frequency domain channel to obtain equalized
signaling symbols;
[0018] the demodulator/deinterleaver/convolutional-decoder is
configured to demodulate, deinterleave and convolutionally decode
the equalized signaling symbols to obtain effective signaling
bits;
[0019] the convolutional-encoder/interleaver/modulator and the OFDM
framer are configured to perform convolutional encoding,
interleaving, modulation and OFDM framing to the effective
signaling bits to obtain OFDM signaling symbols;
[0020] the second sequence divider is configured to receive the
frequency domain signaling symbols and dividing the frequency
domain signaling symbols by the OFDM signaling symbols to obtain a
signaling frequency domain channel;
[0021] the Wiener filtering processer is prestored with a plurality
of groups of Wiener filtering coefficients and is configured to
perform Wiener filtering to the obtained original noise-containing
frequency domain channel by using filtering methods corresponding
to the plurality of groups of Wiener filtering coefficients to
obtain a plurality of groups of noise-reduced frequency domain
channels; and
[0022] the channel analyzer/selector is configured to select a
group of noise-reduced frequency domain channel that is closest to
a response of the signaling frequency domain channel from the
plurality of groups to be an optimal frequency domain channel.
[0023] The present invention provides a low-complexity channel
noise reduction method for multi-carrier transmission mode in
802.11a/g/n wireless LAN systems. The present invention selects the
optimal frequency domain channel impulse response by using a known
long training sequence portion and a highly protected signaling
sequence portion of the multi-carrier mode frame structure to
ensure the receiver to have a good operation threshold in
environments with different time-delay spreads at the cost of a low
complexity. Instead of detecting time domain channel responses, the
method of the present invention directly performs noise reduction
to a noise-containing frequency domain channel by using preset
Wiener filtering coefficients to obtain multiple frequency domain
channel responses, among which there must be a relatively optimal
frequency domain channel response. The relatively optimal frequency
domain channel response can be selected by using the highly
protected signaling sequence to calculate the signaling frequency
domain channel. The rule for selecting the optimal frequency domain
channel response may be the minimization of mean square error
(MSE).
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic diagram of the frame structure of a
multi-carrier mode in 802.11a/g systems of the prior art.
[0025] FIG. 2 is a flowchart illustrating the low-complexity
channel noise reduction method of a preferred embodiment according
to the present invention.
[0026] FIG. 3 is a schematic diagram of the low-complexity channel
noise reduction apparatus of a preferred embodiment according to
the present invention.
[0027] FIG. 4 is a schematic diagram of a time-domain channel
impulse response obtained based on the long training sequence.
[0028] FIGS. 5 to 8 are schematic diagrams of received
constellation points after Wiener filtering process.
DETAILED DESCRIPTION
[0029] The present invention will be described and specified below
in combination with specific exemplary embodiments and accompanying
drawings, so that the technical content of the present invention
could be fully understood.
[0030] As described above, in general, a receiver selects
appropriate Wiener filtering method and coefficients based on
channel power time delay characteristics in a practical
transmission environment. In other words, the receiver should
estimate the time domain channel impulse response to detect
multipath, however, weak multipath components always cannot be
easily detected. In light of this, instead of selecting Wiener
filtering coefficients by performing multipath detection to the
time domain channel, the present invention first performs Wiener
filtering to a noise-containing channel by using Wiener filtering
methods corresponding to a plurality of groups of Wiener filtering
coefficients prestored in the system, and then selects the group of
Wiener filtering coefficients and the corresponding filtering
method which can obtain the smallest mean square error (MSE)
according to certain rule. Specifically, the receiver first
receives a time domain long training sequence and converts it into
the frequency domain by fast Fourier transform (FFT) conversion to
obtain a frequency domain long training sequence. The frequency
domain channel response containing noise is obtained by dividing
the obtained long training sequence by a locally stored frequency
domain long training sequence. As a signaling sequence generally
adopts highly protected transmission mode, while the protection for
data sequence is relative weak to ensure high transmission
efficiency, the method of the present invention includes: 1)
perform equalization, demodulation, deinterleaving and decoding to
the obtained signaling sequence to obtain signaling bits by
directly using the noise-containing frequency domain channel
response; 2) perform encoding, interleaving, modulation and framing
to the obtained signaling bits according to the transmission mode
at the transmitting end to obtain an ideal signaling symbol
sequence; 3) divide the actually received frequency domain
signaling symbols by the ideal signaling symbol sequence to obtain
the frequency domain channel. Further, the noise-containing
frequency domain channel is processed by using a plurality of
preset Wiener filtering methods and coefficients to obtain
noise-reduced frequency domain channel. In fact, due to the
existence of weak paths in the transmission environment, the
distortion of channel response will occur while reducing the noise,
and therefore a channel analysis and selection module is used to
select an optimal Wiener filtering coefficient and method to
demodulate the following data sequence. This result is a good
compromise between the noise reduction and channel distortion,
which can ensure an appropriate operation threshold of the receiver
in different transmission modes and different channel
environments.
[0031] By introducing cyclic prefixes, a received signal in a
multi-carrier orthogonal frequency division multiplexing (OFDM)
system can be expressed as a cyclic convolution of a transmitted
signal and a channel response, and correspondingly in the frequency
domain, a received frequency domain signal can be expressed as the
product of a transmitted frequency domain signal and a frequency
domain channel response, namely, R(k)=H(k)S(k)+N(k), where R(k) is
the received frequency domain sequence; S(k) is a known frequency
domain training sequence; H(k) is the frequency domain channel
response; N(k) is white noise within transmission. Such feature
enables easy implementations of channel estimation and channel
equalization of OFDM systems. In the channel estimation, as the
transmitted sequence S(k) is known, we can get
H ^ ( k ) = R ( k ) S ( k ) - N ( k ) S ( k ) , ##EQU00002##
namely, the noise-containing frequency domain channel response can
be obtained by dividing a received training sequence by a local
training sequence; conversely, data symbols can be calculated by
using the obtained frequency domain channel according to the
formula
S ( k ) = R ( k ) H ^ ( k ) - N ( k ) H ^ ( k ) ##EQU00003##
during data demodulation. For the transmission of multicarrier mode
in 802.11a/g/n systems, as each element of the long training
sequence and the signaling sequence has a value of +1 or -1, the
division operation is just a simple symbol conversion
operation.
[0032] As mentioned above, channel time delay spread is often used
as a parameter to classify Wiener filtering, or in other words, the
channel time delay spread parameters are classified by levels, and
each level is corresponding to a group of Wiener filtering
coefficients and filtering method. In a conventional method, the
noise-containing frequency domain channel is converted into the
time domain through the inverse fast Fourier transform (IFFT)
conversion, and the obtained time domain channel is detected to
obtain channel time delay spread so as to select optimal
coefficients and their corresponding method. However, OFDM
transmission systems employing cyclic prefixes usually include
virtual carriers, which make it easier for spectrum shaping, but
will simultaneously lead to the loss of high frequency portions of
the frequency channel. Thus, the IFFT conversion is no longer a
completely orthogonal conversion, which will lead to energy
leakage. The energy originally concentrates on one subcarrier will
scatter over all subcarriers, so that some weak multipath cannot be
accurately detected.
[0033] Instead of detecting the time domain channel response, the
method of the present invention directly performs noise reduction
to a noise-containing frequency domain channel by using preset
Wiener filtering coefficients to obtain multiple frequency domain
channel responses where there will be a relatively optimal
frequency domain channel response, and then selects the optimal
frequency domain channel response using another frequency domain
channel calculated by using subsequently received highly protected
signaling sequence. The rule for selecting the optimal frequency
domain channel response may be the minimization of mean square
error (MSE).
[0034] Referring to FIG. 2, the present invention provides a
low-complexity channel noise reduction method for a multi-carrier
mode in a wireless LAN, the method is detail described as
follows:
[0035] Step S100: divide a received frequency domain long training
sequence by a locally stored long training sequence to obtain an
original noise-containing frequency domain channel, wherein the
long training sequence is composed of +1's and -1's, therefore the
sequence division operation is just a sequence multiplication
operation.
[0036] Step S200: perform Wiener filtering to the original
noise-containing frequency domain channel by using a plurality of
groups of prestored Wiener filtering coefficients according to
their corresponding filtering methods to obtain a plurality of
groups of noise-reduced frequency domain channels; in the noise
reduction process, some noise may still remain and a portion of
channel distortion may occur.
[0037] Step S300: perform equalization to received frequency domain
signaling symbols by using a frequency domain channel response
provided by the original noise-containing frequency domain channel,
and demodulate, deinterleave and convolutionally decode the
equalized signaling symbols to obtain effective signaling bits;
[0038] Step S400: perform encoding, interleaving, modulation and
OFDM framing to the obtained effective signaling bits according to
the order at a transmitting end to obtain OFDM signaling symbols.
Since the signaling symbols are more highly protected than the data
symbols, it is convinced that the actually received OFDM signaling
symbols are correct. Then, divide the received frequency domain
signaling symbols by the OFDM signaling symbols to obtain a
signaling frequency domain channel, wherein the OFDM signaling
symbols and the received frequency domain signaling symbols are
both composed of +1's and -1's, therefore the sequence division
operation is just a sequence multiplication operation.
[0039] Step S500: select a group of noise-reduced frequency domain
channel that is closest to a response of the signaling frequency
domain channel from the plurality of groups to be an optimal
frequency domain channel.
[0040] Referring to FIG. 3, FIG. 3 is a schematic diagram of the
low-complexity channel noise reduction apparatus according to a
preferred embodiment of the present invention. The embodiment
provides a low-complexity channel noise reduction apparatus for a
multi-carrier transmission mode in a wireless LAN, the apparatus
includes: a first sequence divider 100, a channel equalizer 200, a
demodulator/deinterleaver/convolutional-decoder 300, a
convolutional-coder/interleaver/modulator 400, an OFDM framer 500,
a second sequence divider 600, a Wiener filtering processer 700 and
a channel analyzer/selector 800, wherein,
[0041] the first sequence divider 100 is configured to receive a
frequency domain long training sequence and divide it by a locally
stored long training sequence to obtain an original
noise-containing frequency domain channel;
[0042] the channel equalizer 200 is configured to receive frequency
domain signaling symbols and perform channel equalization to the
frequency domain signaling symbols with the original
noise-containing frequency domain channel to obtain equalized
signaling symbols;
[0043] the demodulator/deinterleaver/convolutional-decoder 300 is
configured to demodulate, deinterleave and convolutionally decode
the equalized signaling symbols to obtain effective signaling
bits;
[0044] the convolutional-encoder/interleaver/modulator 400 and the
OFDM framer 500 are configured to perform convolutional encoding,
interleaving, modulation and OFDM framing to the effective
signaling bits to obtain OFDM signaling symbols;
[0045] the second sequence divider 600 is configured to receive the
frequency domain signaling symbols and dividing the frequency
domain signaling symbols by the OFDM signaling symbols to obtain a
signaling frequency domain channel;
[0046] the Wiener filtering processer 700 is prestored with a
plurality of groups of Wiener filtering coefficients and is
configured to perform Wiener filtering to the obtained original
noise-containing frequency domain channel by using filtering
methods corresponding to the plurality of groups of Wiener
filtering coefficients to obtain a plurality of groups of
noise-reduced frequency domain channels;
[0047] the channel analyzer/selector 800 is configured to select a
group of noise-reduced frequency domain channel that is closest to
a response of the signaling frequency domain channel from the
plurality of groups to be an optimal frequency domain channel.
[0048] A multi-carrier OFDM transmission mode in 802.11a/g systems
is taken for example to describe the whole process of the present
invention. In 802.11a/g systems, each OFDM symbol has a protection
interval of 0.8 .mu.s. Normally, channel time delay spreads are
considered not to exceed 0.8 .mu.s. In this embodiment, a multipath
channel model having three paths is employed, as specified in the
following Table 1.
TABLE-US-00001 TABLE 1 the multipath channel model employed in this
embodiment Parameter of the channel model First path Second Path
Third Path Time delay (us) 0 0.5 0.7 Power (dB) 0 -25 -25
[0049] As shown in the above table, the first path is a major path
of the transmission model; the power of either the second path or
the third path is 25 dB lower than that of the first path; the
delays of the second path and the third path are 0.5 .mu.s and 0.7
.mu.s, respectively. It can be easily found from FIG. 4 that the
second path and the third path cannot be detected based on time
domain channel responses since the weak paths are drowned by the
energy leaked from the major path. The present invention will be
further specified below in combination with two different
transmission environments.
[0050] The present invention first obtains a noise-containing
frequency domain channel response by using two complete long
training symbols included in a long training sequence. In order to
clearly describe the intend of the present invention, Wiener
filtering adopted below is classified into two settings with
respect to channel time delay spread, wherein a first setting of
Wiener filtering is a low-pass filtering for processing the single
major path; a second setting of Wiener filtering is selected to be
a low-pass filtering with a passband of 0.8 .mu.s. The transmission
modes of data sequence are respectively set as 64-QAM mode and
4-QAM mode to describe the effects of the present invention.
[0051] (1) The data sequence is transmitted in the 64-QAM mode, and
the signal to noise ratio (SNR) is 35 dB. If only the major path
instead of weak paths is taken into account, the first setting of
Wiener filtering will be adopted. The corresponding received
constellation points are as shown in FIG. 5. From FIG. 5 we can see
that the 64-QAM constellation diagram is very ambiguous. In fact,
the second setting of Wiener filtering for this case can obtain a
good result as shown in FIG. 6, with which the determination can be
correctly achieved. The method of the present invention can select
the second setting of Wiener filtering automatically in this case.
Therefore, the weak multipath components should be taken into
account when the SNR condition is fairly good.
[0052] (2) The data sequence is transmitted in the 4-QAM mode, and
the signal to noise ratio (SNR) is 6 dB. If all the multipath
components are taken into account, the second setting of Wiener
filtering will be adopted, and the corresponding constellation
points are as shown in FIG. 7. For this case, the method of the
present invention selects the results of the first setting of
Wiener filtering to be the optimal frequency domain channel, and
the received constellation points are as shown in FIG. 8. It can be
easily found that the method of the present invention is capable of
achieving a good noise reduction effect and the constellation
points are clearer. Thus, in a bad SNR condition, the noise
reduction process should reduce the energy of noise as much as
possible, and the ignorance of weak multipath components will lead
to an improved reception performance.
[0053] For the above two conditions, it is clear that the channel
noise reduction method of the present invention is capable of
automatically achieving a compromise between noise reduction and
channel distortion, which is capable of obtaining good results in
different transmission modes and conditions.
[0054] Numerous embodiments with great variations can be made
without departing from the spirit and scope of the invention. It
will be understood that specific embodiments described in the
specification shall not be intended to limit the scope of the
invention which shall solely be limited by the appended claims.
* * * * *