Title: Method and an apparatus for estimating residual noise in a signal and an apparatus utilizing the method
Abstract: The invention relates to: A method for estimating residual noise in the frequency range (271) of a desired part (240) of a signal to a corresponding apparatus and to a mobile telephone utilizing the method.The object of the present invention is to generate a measure for the residual noise in a signal.The problem is solved in that the amplitude of the signal (114) comprising the noise is modified, and the signal (114) is combined with the modified signal (115) to create a noise estimation measure (116).The invention may e.g. be used in digital radio communications systems to contribute to the monitoring of link quality and therefore to improve payload throughput. One of the advantages of the invention is that an indication of link quality at an early stage in the receiver is provided.
Patent Number: 6,871,066 Issued on 03/22/2005 to Khullar, et al.
| Inventors:
|
Khullar; Anders (Bjarred, SE);
Lindoff; Bengt (Lund, SE);
Stenstrom; Niklas (Helsingborg, SE)
|
| Assignee:
|
Telefonaktiebolaget LM Ericsson (Publ) (Stockholm, SE)
|
| Appl. No.:
|
661677 |
| Filed:
|
September 14, 2000 |
Foreign Application Priority Data
| Current U.S. Class: |
455/423; 455/424; 455/425; 455/67.1; 455/226.1; 375/296; 379/406.08 |
| Intern'l Class: |
H04Q 007//20 |
| Field of Search: |
455/423-425,226.1-226.3,67.1,67.3,67.6,570
379/22.08,406.08
375/296
|
References Cited [Referenced By]
U.S. Patent Documents
| 4633411 | Dec., 1986 | Bliss | 702/77.
|
| 5406588 | Apr., 1995 | Birchler et al. | 375/346.
|
| 5506869 | Apr., 1996 | Royer | 375/224.
|
| 5519730 | May., 1996 | Jasper et al. | 375/260.
|
| 5715240 | Feb., 1998 | Borras et al. | 370/252.
|
| 5911124 | Jun., 1999 | Doran | 455/570.
|
| 5918160 | Jun., 1999 | Lysejko et al. | 455/74.
|
| 5953660 | Sep., 1999 | Ryde et al. | 455/423.
|
| 6118982 | Sep., 2000 | Ghisler et al. | 455/67.
|
| 6134220 | Oct., 2000 | Le Strat et al. | 370/252.
|
| 6339698 | Jan., 2002 | Cullen | 455/67.
|
| 6347217 | Feb., 2002 | Bengtsson et al. | 455/67.
|
| 6539214 | Mar., 2003 | Lapaille et al. | 455/226.
|
| Foreign Patent Documents |
| WO 94/28623 | Dec., 1994 | WO.
| |
| WO 94/28637 | Dec., 1994 | WO.
| |
| WO 95/28814 | Oct., 1995 | WO.
| |
Other References
Yang, Y., European Search Report, App. No. EP 99 61 0054, Mar. 28, 2000,
pp. 1-3.
|
Primary Examiner: Trost; William
Assistant Examiner: Rampuria; Sharad
Attorney, Agent or Firm: Jenkens & Gilchrist, P.C.
Claims
What is claimed is:
1. A method for estimating residual noise in a frequency range of a desired
part of a signal received in a mobile receiver via a digital cellular
radio system, the desired part of the received signal representing a
selected channel of the digital cellular radio system, the method
comprising:
modifying the amplitude of the received signal, the received signal
including the residual noise;
combining the received signal with the modified received signal to create a
noise estimation measure; and
transmitting, uplink from the mobile receiver and via the digital cellular
radio system, the noise estimation measure or a post-processed version of
the noise estimation measure to a link quality control system of the
digital cellular radio system.
2. The method according to claim 1, wherein the noise estimation measure is
based on an average power content of the signal and the modified signal
over their frequency spectra.
3. The method according to claim 2, wherein the noise estimation measure is
based on the average power content of the signal and the modified signal
over one or more common ranges of their frequency spectra.
4. The method according to any one of claims 1 to 3, wherein the signal is
attenuated primarily outside a frequency range of the desired part of the
signal.
5. The method according to claim 2 or 3, wherein the noise estimation
measure is based on a difference in average power content between the
signal and the modified signal.
6. The method according to any one of claims 1 to 3, wherein the signal is
a digital signal.
7. The method according to claim 4, wherein the signal is attenuated
primarily outside the frequency range of the desired part of the signal
via a digital filter.
8. The method according to any one of claims 1 to 3, wherein the noise
estimation measure is quantized in a number of different levels each
indicating different levels of noise present.
9. The method according to claim 1, wherein a noise estimation measurement
is performed during each of the basic time units of a channel of the
digital cellular radio system, and the result is communicated to the link
quality control system of the digital cellular radio system as an
estimator of current link quality.
10. The method according to claim 1, wherein several noise estimation
measurements are performed, the results are stored, and the results are
evaluated, and a derived trend is communicated to the link quality control
system of a digital cellular radio system as an estimator of current link
quality.
11. The method according to claim 1, wherein the noise estimation measure
transferred to the link quality control system is used by the digital
cellular radio system to optimize user information channel throughput by
adjusting at least one of the data transmission rate, the error correction
depth, and a type of modulation.
12. The method according to claim 1, wherein the noise estimation measure
is transferred to a digital demodulator and used to adjust a receiver
algorithm.
13. An apparatus for estimating residual noise in a frequency range of a
desired part of a signal received in a mobile receiver via a digital
cellular radio system, the desired part of the received signal
representing a selected channel of the digital cellular radio system, the
apparatus comprising:
means for modifying an amplitude of the received signal, the received
signal comprising the residual noise;
means for combining the received signal with the modified signal to create
a noise estimation measure;
means for transferring the noise estimation measure to a processing unit;
means for storing consecutive values of the noise estimation measure;
means for processing the consecutive values to extract a trend;
means for transmitting, uplink from the mobile receiver and via the digital
cellular radio system, the stored consecutive values, the extracted trend,
a post-processed version of the stored consecutive values, or a
post-processed version of the extracted trend to a quality control system
of the digital cellular radio system.
14. The apparatus according to claim 13, wherein the means modified for
combining the signal with the signal to create a noise estimation measure
comprise a power meter for measuring average power content of the signal
and the modified signal over at least one of a plurality of common ranges
of their frequency spectra.
15. The apparatus according to claim 13 or 14, wherein the means for
modifying the amplitude of the signal comprising the noise include means
for attenuating the signal primarily outside the frequency range of the
desired part of the signal.
16. The apparatus according to claim 14, wherein the means for combining
the signal with the modified signal to create a noise estimation measure
comprise means for computing a difference in average power content between
the signal and the modified signal.
17. The apparatus according to claim 13 or 14, wherein the apparatus is
adapted to handle digital signals.
18. The apparatus according to claim 17, wherein the means for attenuating
the signal primarily outside the frequency range of the desired part of
the signal comprise a digital filter.
19. A mobile telephone comprising:
means for estimating residual noise in a frequency range of a desired part
of a signal received in the mobile telephone via a digital cellular radio
system, the desired part of the received signal representing a selected
channel of the digital cellular radio system
wherein the amplitude of the received signal is modified, the received
signal comprising the residual noise;
wherein the received signal is combined with the modified received signal
to create a noise estimation measure;
means for estimating residual noise in the frequency range of the desired
part of the received signal;
means for storing, evaluating, and transmitting, uplink from the mobile
telephone and via the digital cellular radio system, at least one of
resulting noise estimation measurements and post-processed versions of the
resulting noise estimation measurements to a link quality control system
of the digital cellular radio system.
20. The mobile telephone according to claim 19, wherein the mobile
telephone is adapted to perform a noise estimation measurement during each
of a plurality of basic time units of a channel of the digital cellular
radio system.
Description
THE TECHNICAL FIELD OF THE INVENTION
This invention relates to the estimation of residual noise in a signal. The
invention may e.g. be applied for the estimation of link quality in a
digital radio communications system.
The invention relates specifically to: A method for estimating residual
noise in the frequency range of a desired part of a signal.
The invention furthermore relates to: An apparatus for estimating residual
noise in the frequency range of a desired part of a signal.
The invention moreover relates to: A mobile telephone utilizing the method.
DESCRIPTION OF RELATED ART
The following account of the prior art relates to one of the areas of
application of the present invention, cellular digital radio
communications systems.
Technologies for evolving cellular systems from being dominated by voice
telephony to systems being capable of handling large amounts of data as
well are being developed. One of the requirements of next-generation
cellular systems is the ability to provide mobile access to the Internet
using World Wide Web (WWW) applications, in other words to facilitate
`high speed` packet oriented data transfer.
One such so-called 3.sup.rd generation system, is the EDGE system
(EDGE=Enhanced Data rates for Global TDMA Evolution, where TDMA Time
Division Multiple Access (or alternatively EDGE=Enhanced Data rates for
GSM Evolution)). The EDGE system represents a convergence of the US TDMA
system called TIA/EIA-136 and the European GSM system making possible the
provision of wireless TDMA-based high data rate packet services for mobile
users. A standard based on the EDGE concept, termed UWC-136, has been
developed/approved by the European Telecommunications Standards Institute
(ETSI) and by the Telecommunications Industry Association (TIA) in North
America. Other 3.sup.rd generation systems, based on CDMA (Code Division
Multiple Access), are likewise in the development phase: cdma2000
(specified by the (TIA)) and WCDMA (specified by ETSI). Under the name
IMT-2000 (IMT=International Mobile Telecommunications) these three systems
constitute the technology platforms of the International
Telecommunications Union (ITU) for 3.sup.rd generation mobile
communications systems.
On the air interface, GSM (and EDGE) relies on a combination of TDMA and
Frequency Division Multiple Access (FDMA) in that a multitude of time
slots (8 time slots=1 TDMA frame) is mapped on a multitude of carrier
frequencies (for GSM, 125).
In the first generation GSM system a data rate of 9.6 kbit/s based on
circuit switching was provided using GMSK channel modulation
(GMSK=Gaussian Minimum Shift Keying), each channel having a spacing
between carriers of 200 kHz. The General Packet Radio Service (GPRS) for
GSM constitutes an intermediate step in the development path towards
providing high speed packet mode data transfer for mobile users. GPRS
provides peak user data rates between 9 kbit/s and 170 kbit/s by means of
time slot concatenation (up to 8) and variable channel coding (4
different). The GPRS protocol and service capabilities are reused in the
EDGE system. The EDGE concept improves the data rates, among other things
by means of higher level channel modulation and a higher symbol rate.
Using 8-PSK modulation (PSK=Phase-Shift Keying), a symbol rate of 3 bits
per modulated symbol, and multislot operation, a three-fold increase in
user bit rate compared to GPRS may be achieved. A data rate of e.g. 384
kbit/s may be achieved with the same 200 kHz carrier spacing and TDMA
frame structure and by combining several (up to 8) time slots. The
improvement in data rate comes at the cost of a greater sensibility to
noise, including disturbances from adjacent channels (adjacent channel
interference). The maximum achievable data rate will generally depend on
mobile speed and system load.
A digital radio receiver can be regarded as an analogue radio front-end,
followed by an AD-converter. A cellular, digital radio system such as GSM
or EDGE consists of several frequencies, channels, within the allocated
frequency band. For communication, the selected channel needs to be
filtered out and it is an advantage to do this in the digital domain.
Digital filters are easy to reprogram, inexpensive and do not have filter
parameters varying with temperature and voltage.
For a GSM channel using GMSK modulation it is OK to filter hard (i.e. e.g.
to use a filter with a slightly narrower bandwidth than the 200 kHz
carrier spacing of a GSM channel). This is, however, not the case for a
corresponding EDGE channel using 8-PSK modulation and an EDGE receiver
will consequently have more problems with adjacent channel interference
compared to an ordinary GSM receiver. This type of modulation will also be
more sensitive to noise than for example GMSK. Thus: A compromise must be
made between the wish to have a broadbanded filter to include all
information and the wish to avoid noise from adjacent channels.
Consequently, a system such as EDGE will depend on an LQC system (LQC=Link
Quality Control) to optimise system throughput.
The Quality of Service (QoS) of a digital wireless communications system
may be largely related to three areas, namely call handling quality,
communication quality and coverage quality. The present invention, when
used in a cellular communications system, is primarily related to
communication quality, i.e. to such effects as radio channel impairment,
transmission delay, echo, background noise, etc. For data transmission,
the communication quality may be quantified based on bit error rate and
transmission delay. For packet oriented data transmission, bit error rate
can be kept low by means of retransmission, resulting, however, in
increasing delay and decreasing throughput. To optimize channel
throughput, it is therefore of great importance to be able to
(dynamically) evaluate the channel quality, and, based hereon, to adjust
the essential transmission parameters to the quality of the radio channel.
The measurement and reporting of the current link quality are thus most
critical tasks that need to be carefully addressed. A report of the
presence of a strong interferer would impact the data transmission rate,
the amount of error correction overhead, or ultimately result in the
choice of a more robust modulation or coding scheme (e.g. from 8-PSK to
GMSK channel modulation).
SUMMARY
It is an object of the present invention to generate a measure for the
residual noise in a signal.
This is achieved according to the invention in that the amplitude of the
signal comprising the noise is modified, and the signal is combined with
the modified signal to create a noise estimation measure, as disclosed in
claim 1.
In the present context the term `noise` is taken to mean random noise as
well as disturbances from other sources. When referred to the application
of the invention in a cellular communications system, the term `noise`
includes disturbances from adjacent channels (adjacent channel
interference) or from channels using the same channel frequency as the
`selected` or `desired` one in another cell (co-channel interference).
The residual noise estimate is based on the assumption that the amount of
signal components present in the frequency ranges outside that of the
desired frequency range (e.g. that of the selected channel of a cellular
communications system) is a measure for the amount of `noise` present
inside the desired frequency range. The usefulness of the resulting noise
measure will in general depend on the structure of the signal. For signals
representing the potentially overlapping channels of a communications
system and the like, where disturbances from `adjacent channels` are of
major concern, the measure is obtained in a simple way and very useful as
an indicator of channel quality.
When, as disclosed in claim 2, the noise estimation measure is based on the
average power content of the signal and the modified signal over their
frequency spectra, it is ensured that a simple measure for comparing the
signals based on well known measurement techniques is provided.
Alternatively, the noise estimation measure could be based on the gradient
of average power (i.e. the changes over time) of the signals in question
or on any other appropriate method.
When, as disclosed in claim 3, the noise estimation measure is based on the
average power content of the signal and the modified signal over one or
more common ranges of their frequency spectra, it is ensured that a simple
measure for comparing the signals is provided, and that the measurement
may be concentrated to a relevant part of the frequency range, e.g. the
frequency range outside the desired part of the signal.
When, as disclosed in claim 4, the signal is attenuated primarily outside
the frequency range of the desired part of the signal, it is ensured that
the modification of the signal outside the desired frequency range may be
performed in a simple way e.g. by using a filter. The attenuation may e.g.
be achieved by means of a bandpass filter that passes the desired
frequency range (ideally) unchanged and attenuates all other frequencies.
Or, alternatively, it may consist of a low-pass filter, ir the desired
frequency range is located from a certain frequency and downwards, or of a
high-pass filter, if the desired frequency range is located from a certain
frequency and upwards. The modification may, however, alternatively take
any other appropriate form. It may e.g. consist of an amplification of the
signal outside the desired frequency range e.g. by means of one or more
frequency selective amplifiers tuned to the relevant frequencies.
When, as disclosed in claim 5, the noise estimation measure is based on the
difference in average power content between the signal and the modified
signal, it is ensured that a simple method for comparing the signals is
provided.
When, as disclosed in claim 6, the signal is a digital signal, it is
ensured that the measure and the following processing are more stable over
time and against environmental influences, which further promises better
performance and lower equipment production costs than corresponding
functionality implemented in traditional analogue techniques.
When, as disclosed in claim 7, the signal is attenuated primarily outside
the frequency range of the desired part of the signal by means of a
digital filter, it is ensured that a cost-effective and flexible solution
that is readily adapted to changes is achieved.
When, as disclosed in claim 8, the noise estimation measure is quantized in
a number of different levels each indicating different levels of noise
present, it is ensured that the measure may be used to quantify the amount
of undesired noise in the signal.
When, as disclosed in claim 9, the desired part of the signal represents a
selected channel of a digital cellular radio system, and the noise
estimation measure or a postprocessed version thereof is communicated to a
link quality control system of said digital cellular radio system as an
estimator of current link quality, it is ensured that a simple measure for
the current link quality is provided and fed back to the transmission
system. Other advantages of the present invention are that the
implementation is potentially low-cost and that a link quality estimate is
provided at an `early stage` of the signal path, i.e. before the signal is
demodulated and further processed in the digital demodulator. This allows
a fast response to changing transmission conditions allowing a dynamic
adaptation of system throughput to current link quality.
When, as disclosed in claim 10, a noise estimation measurement is performed
during each of the basic time units (i.e. time slot or burst) of a channel
of the digital cellular radio system, and the result is communicated to a
link quality control system of the digital cellular radio system as an
estimator of current link quality, it is ensured that the link quality may
be continuously monitored and corresponding system actions may be taken
instantaneously.
When, as disclosed in claim 11, several noise estimation measurements are
performed, the results are stored, and the results are evaluated, and a
derived trend is communicated to a link quality control system of a
digital cellular radio system as an estimator of current link quality, it
is ensured that a trend in the link quality may be derived and reported.
When, as disclosed in claim 12, the noise estimation measure transferred to
the link quality control system is used by the digital cellular radio
system to optimize user information channel throughput by adjusting the
data transmission rate, the error correction depth and/or the type of
modulation, it is ensured that an optimal user throughput considering the
current link quality may be achieved.
When, as disclosed in claim 13, the noise estimation measure is transferred
to the digital demodulator and used to adjust the receiver algorithm, it
is ensured that an optimization of receiver processing and thus power
consumption may be achieved.
An apparatus for estimating residual noise in the frequency range of a
desired part of a signal is furthermore provided by the present invention.
When, as disclosed in claim 14, it includes means for modifying the
amplitude of the signal comprising the noise, and means for combining the
signal with the modified signal to create a noise estimation measure, and
means for transferring the measure to a processing unit, it is ensured
that a simple measure for the residual noise in a signal is provided.
When, as disclosed in claim 15, the means for combining the signal with the
modified signal to create a noise estimation measure comprise a power
meter for measuring average power content of the signal and the modified
signal over one or more common ranges of their frequency spectra, it is
ensured that a simple measure for comparing the signals is provided and
that the measurement may be concentrated to a relevant part of the
frequency range, e.g. the frequency range outside the desired part of the
signal.
When, as disclosed in claim 16, the means for modifying the amplitude of
the signal comprising the noise include means for attenuating the signal
primarily outside the frequency range of the desired part of the signal,
it is ensured that the modification of the signal outside the desired
frequency range may be performed in a simple way, e.g. by using a filter.
When, as disclosed in claim 17, the means for combining the signal with the
modified signal to create a noise estimation measure comprise means for
computing the difference in average power content between the signal and
the modified signal, it is ensured that a simple way of providing the
noise estimation measure is achieved.
When, as disclosed in claim 18, it is adapted to handle digital signals, it
is ensured that the measure and the following processing steps are more
stable over time and against environmental influences, which further
promises better performance and lower equipment production costs than
corresponding functionality implemented in traditional analogue
techniques.
When, as disclosed in claim 19, the means for attenuating the signal
primarily outside the frequency range of the desired part of the signal
comprise a digital filter, it is ensured that a cost-effective and
flexible solution that is readily adapted to changes is achieved.
When, as disclosed in claim 20, it includes means for storing consecutive
values of the noise estimation measure, and means for processing the
values to extract a trend, and means for communicating the individual
values or the extracted trend or postprocessed versions hereof to a
quality control system, it is ensured that an appropriate postprocessing
of the noise estimation measure may be performed readily. The processing
means may e.g. include means for quantizing the noise estimation measure
in a number of different levels, i.e. to grade a given measurement value
on an appropriate scale according to the expected dynamic range of the
values.
A mobile telephone is moreover provided by the present invention. When, as
disclosed in claim 21, it contains an apparatus, comprising means for
performing the steps of claim 1, for estimating residual noise in the
frequency range of a desired part of a signal, and means for storing,
evaluating and transmitting resulting noise estimation measurements or
postprocessed versions thereof to a link quality control system of a
cellular radio system, it is ensured that the cellular radio system is
provided with a link quality estimate that may be used to optimize the
user throughput.
When, as disclosed in claim 20, it is adapted to perform the noise
estimation measurement during each of the basic time units (i.e. time slot
or burst) of a channel of the digital cellular radio system, it is ensured
that the link quality may be monitored continuously and corresponding
system actions may be taken instantaneously.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained more fully below in connection with a
preferred embodiment and with reference to the drawings in which:
FIG. 1 shows the front-end of a receiver for a mobile telephone of a
cellular digital radio communications system according to the invention,
FIG. 2 shows an idealized sketch of the frequency dependence of the
received power of the signal before and after the digital filter, and
FIG. 3 shows a part of a front-end of a receiver for a mobile telephone of
a cellular digital radio communications system according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
FIG. 1 shows the front-end of a receiver for a mobile telephone of a
cellular digital radio communications system according to the invention.
In FIG. 1, 101 denotes the receiver antenna (optionally used together with
the transmitter part via a duplexer), and 110 denotes the received radio
frequency signal. The received signal 101 is fed to an RX-band filter 102
that filters out the relevant received frequency band, delivering the
signal 111 containing the frequency band allocated to the cellular system
in question, e.g. the GSM frequency band around 900 MHz or the IMT-2000
frequency band allocated around 1950 MHz. The signal 111 is fed to an
analogue demodulator 103 that mixes the relevant part of the frequency
band down in the baseband, using a system-given channel frequency 109 for
the down-conversion.
The output signal 112 from the analogue demodulator 103 is fed to an
analogue filter 104 that filters out the frequencies around the relevant
channel, resulting in the signal 113. The signal 113 contains noise and
disturbances from adjacent channels. An AD-converter 105 (AD=Analogue to
Digital) converts the signal 113 to the digital domain in the form of the
signal 114. A digital filter 108 filters out the relevant channel 115 from
the input signal 114. Using the digital signals before and after the
digital filter 108, i.e. signals 114 and 115 as inputs, the Link Quality
Estimator 106 measures the difference in average power content between the
two input signals and outputs a signal 116 to the signal processing unit
107. The measure 116 represents an indicator of residual noise in the
frequency range of the desired signal 115.
The measure 116 may be further processed by the signal processing unit 107.
As a result, a message 117 representing the current link quality (or a
trend herein) is forwarded to the transmission part 118 of the mobile unit
and transmitted to the Link Quality Control system of the digital radio
communications system, i.e. to the base station presently allocated to the
mobile telephone.
FIG. 2 shows an idealized sketch of the frequency dependence of the
received power of the signal before and after the digital filter. The
frequency axis is denoted by 260. Although the relevant signals 114 and
115 of FIG. 1 are digital, a frequency dependence of the power content
corresponding to their analogue representations is shown for illustrative
purposes.
The input signal to the digital filter 114 of FIG. 1 is schematically
depicted in FIG. 2.a. The signal contains noise (not indicated) and signal
components 230, 250 from adjacent channels, whose signal power is
primarily located in the frequency ranges 270 and 272, in addition to the
desired signal components 240 of the selected channel 271, i.e. there is
undesired amplitude at frequencies outside as well as inside the frequency
range of the desired/selected channel 271. The sketch represents a
situation with a strongly disturbing neighbouring signal 230 and a
moderately disturbing neighbouring signal 250. The grey shaded areas in
FIG. 2.a represent the frequency ranges where an overlap with the adjacent
channels is present. After (non-ideal) filtering of the signal 114 of FIG.
1, the resulting signal 115 of FIG. 1 primarily has signal amplitude 241
in the frequency range 271 of the desired signal, as schematically
illustrated in FIG. 2.b (where the signal amplitudes outside the range 271
(mainly components 231 and 251) are somewhat exaggerated for illustrative
purposes). However, a part of the amplitude of the signal in the desired
frequency range 271 originates from noise and/or adjacent channels, as
indicated by the shaded area 280. By measuring the average power of signal
115 of FIG. 1 and subtracting it from the average power of signal 114 of
FIG. 1 (both taken over the entire frequency range of signal 114) we get a
measure for the `noise` (i.e. undesired signal components) in the desired
frequency range 271. This is illustrated in FIG. 2.c, where the sum of the
shaded areas 232 and 252 represents the measure.
I.e. we assume that the amount of signal components present in the
frequency ranges 270, 272 outside that of the selected channel is a
measure for the amount of `noise` (i.e. primarily undesired signal
components from adjacent channels) present inside the frequency range 271
of the selected channel. The usefulness of the resulting noise measure
will in general depend on the structure of the signal 114. For signals
representing the potentially overlapping channels of a communications
system and the like, the measure is certainly very useful.
After filtering, the received signal consists of the signal from the
selected channel and additional noise from adjacent channels, not removed
by the non-ideal filters (cf. FIG. 2.b). The relation in strength between
the selected channel and adjacent channels will determine the amount of
residual adjacent channel noise. This relation will influence the receiver
performance.
An estimation of the adjacent channel residual noise will therefore also be
useful in the following digital signal processing (block 107 of FIG. 1).
Receiver algorithms may be tailored to match the interference presence in
each single burst.
The measurements of the average power of the signals 114 and 115 of FIG. 1
are performed over the frequency range where the signal 114 has a
significant amplitude, i.e. ranges 270, 271 and 272 in FIG. 2. In a
special embodiment of the invention, the frequency range of measurement is
restricted to that outside the selected channel (i.e. to the ranges 270,
272). This has the advantage of eliminating the attenuation of the filter
in the desired frequency range 271 from the measurement.
The output signal 116 of FIG. 1 is quantized in the Link Quality Estimator
block 106 and takes on one of a relevant number of levels, e.g. 8, in
which the range of the possible results is subdivided. The `scale` of the
levels may be linear or logarithmic dependent on the dynamic range of the
measurement results. The interpretation of the measurements is as follows:
A relatively large absolute difference in average power content represents
a relatively large residual noise and hence a relatively low link quality
estimate, whereas a relatively small absolute difference in average power
content represents a relatively small residual noise and hence a
relatively high link quality estimate. This may be illustrated by FIG.
2.c, where the shaded area 232, taken alone, represents a relatively large
absolute difference in average power content corresponding to a relatively
large amount of residual noise in the desired frequency range and
correspondingly a relatively low link quality. Similarly, the shaded area
252, taken alone, represents a relatively small absolute difference in
average power content corresponding to a relatively small amount of
residual noise in the desired frequency range and correspondingly a
relatively high link quality.
In an alternative embodiment, a part of the evaluation of the difference in
average power content between the two input signals 114 and 115 of FIG. 1
to the Link Quality Estimator 106 is performed in the control unit
contained in the processing block 107 (cf. 320 in FIG. 3).
FIG. 3 shows a part of a front-end of a receiver for a mobile telephone of
a cellular digital radio communications system according to the invention.
In FIG. 3, the signal processing unit 107 is subdivided in control unit 320
and digital demodulator 321 blocks.
The signal 115 from the digital filter 108 is fed to the digital
demodulator 321 for further processing and to the control unit 320 for
possible evaluation and extraction of further information concerning link
quality in addition to that of the noise estimation measure signal 116
from the Link Quality Estimator block 106. The control unit 320 includes a
Digital Signal Processor (DSP) and memory (e.g. RAM) for the storage of
the most recent noise estimation measures, and for the generation of the
link quality estimate 117 that is fed to the transmission part 118 of the
mobile telephone for transfer to the Link Quality Control system. In a
preferred embodiment of the invention, the functions of the control unit
and the digital demodulator are integrated in one functional block (cf.
107 of FIG. 1).
One use of the residual noise measure 116 according to the invention is to
adjust the link quality estimate 117 reported and used by the network for
optimising system throughput. This use of the invention would e.g. improve
the performance of the LQC concept proposed for the EDGE system.
The digital demodulator 321 demodulates the digital signal 115 and extracts
the user-specific information and control information. The control unit
320 (e.g. a DSP with internal and possibly extra accessible external
memory) evaluates the control information. One part of the control
information is extracted from the received signal 115 and possibly
transferred from the demodulator 321 via the connection 319, the other is
the noise estimation measure 116 transferred from the Link Quality
Estimator 106. Based thereon, the control unit 320 performs the relevant
actions, e.g. sends a report 117, via the transmit part 118 of the mobile
telephone, to the Link Quality Control system concerning the present
channel quality or the trend in channel quality. The latter is based on
stored values of recent channel quality data. In a preferred embodiment of
the invention, the measured noise estimation values are stored and
evaluated in the Link Quality Estimator 106. It may alternatively be
stored and evaluated in the control unit 320 of the processing unit 107.
Based on a number (e.g. 100) of stored values, the trend may be derived by
any one of a variety of standard methods, e.g. by simply taking the
average value, or by computing the slope of a straight line fitted to the
stored measurements (time, value) and evaluating it together with the most
recent measurement value, etc.
The link quality estimation data may e.g. be transmitted from the mobile
station to its presently allocated base station and hence to the LQC
system via one of the logic control channels of the system that carries
signalling or synchronization data (as opposed to traffic channels that
carry the user speech or data), as symbolized by the signal 117 to the
transmit part 118 of the mobile station. In a GSM system, the logic
control channels are e.g. termed BCCH (Broadcast Control Channel), CCCH
(Common Control Channel), SDCCH (Stand-alone Control Channel), ACCH
(Associated Control Channel), the latter having a Slow (SACCH) and Fast
(FACCH) version. The SACCH and FACCH may both be used to transfer
signalling data, including e.g. a link quality estimate, during a
connection.
The basic unit of a logic control channel in the GSM and EDGE systems is
184 bits long. To ensure protection against transmission errors, the 184
control bits are mapped onto a 456 bit block by adding check bits and a
channel code to provide error correction possibilities.
The contents of the logical channels are mapped onto and transferred via
physical channels. The 456 bit blocks are partitioned in subblocks of 57
bits length, which are interleaved with subblocks of neighbouring 456 bit
blocks and mapped onto logical `bursts` each of 114 bits length. By the
addition of extra bits for administrative purposes (tail bits and guard
period), the logical bursts are mapped onto the physical bursts each of
156.25 bits length (or 577 .mu.s) for the actual transmission. This is the
basic time slot unit for a GSM or EDGE based system onto which all normal
and control information is to be mapped.
A physical channel in a TDMA system is defined as a time slot with a time
slot number in a sequence of TDMA frames. To protect against fading,
frequency hopping may be applied. This means that the physical channel is
partitioned in both time and frequency so that each burst is transmitted
via a different radio frequency (RF) channel. The physical channel is thus
defined as a sequence of RF channels and time slots. Each carrier
frequency supports eight physical channels mapped onto eight time slots
within a TDMA frame.
Details of the GSM logical traffic and control channels and their mapping
onto physical channels are standardized in GSM Recommendation R.05 dealing
with the `Physical layer on the radio path `.
The quality measurements (i.e. the noise detection estimate and its further
processing) may be performed in each burst or with a certain frequency,
e.g. every TDMA frame (=8 time slots) or every multiframe (=26 TDMA frames
in a GSM system), etc. Alternatively, the measurement frequency may be
made dependent on the measurement results following a predefined algorithm
(e.g. relatively high quality measures being associated with relatively
low measurement frequencies, the measurement frequencies being confined to
a certain min-max-range). Similar patterns may be employed for the
reporting of the measure to the Link Quality Control system. The link
quality measure may be reported via one of the signalling channels of the
cellular communications system. Referring to the GSM system: If a
relatively slow frequency of reporting is needed (e.g. in reporting
trends) the SACCH signal may e.g. be used (1 SACCH is sent per multiframe,
each consisting of 26 TDMA frames, i.e. every 120 ms). If an `instant`
reporting is needed (e.g. when reporting per burst) the FACCH signal may
be used. The FACCH is inserted instead of user data using a so-called
`stealing flag` to distinguish signalling data from user data within a
burst.
In a preferred embodiment of the invention, the report of the current link
quality estimates or the trend therein to the Link Quality Control system,
is used by the system to decide whether to change parameters concerning
the link throughput such as data transmission rate, `error correction
depth` and/or type of modulation (e.g. between 8-PSK and GMSK). This
decision may be based on other parameters possibly reported by the mobile
station as well, e.g. the received level of the channel carrier, the
absolute distance between the base and mobile stations, the bit error
ratio before channel decoding, etc.
Another use of the information represented by the residual noise measure is
to introduce ways to compensate each burst by signal processing. Each
burst will have an estimation of the interference. This information may be
used for:
Fast adaptation of for example receiver filtering properties.
Input data to noise cancellation algorithms.
In a preferred embodiment of the invention, the measurements are performed
in each burst and the results are currently reported to the signal
processing unit via the connection 319 in FIG. 3 and used therein to
modify the receiver algorithm to improve receiver performance, e.g. by
adjusting the amount of `data processing` (error correction, etc.)
controlled by the link quality estimate. This will enable the use of
algorithms, designed to optimize performance at different reception
conditions. If e.g. the link quality is good, less complex receiver
algorithms may be employed and thus power may be saved. The latter is of
particular importance in mobile units such as cellular telephones.
Although the invention has been described in relation to a digital cellular
radio communications system using TDMA, it may also be used in all other
situations where a measure for the residual noise in a signal is needed,
e.g. in other communications systems, be it CDMA based systems, frequency
hopping or non frequency hopping systems, analogue systems, etc.
*
