Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2000-02-18
2001-11-27
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S233000, C455S062000
Reexamination Certificate
active
06324220
ABSTRACT:
TECHNICAL FIELD
The present invention from a first aspect relates to a stabilization technique for precoders of the type used in data communications systems. In particular, the present invention is concerned with stabilizing the precoder in a robust manner by operating upon the transfer function of the precoder filter.
Furthermore, from a second aspect the present invention relates to a point-multipoint data communications system wherein data equalization and precoding is performed in the subscriber terminals.
BACKGROUND OF THE INVENTION
In radio communications systems, the transmitted signal travels from transmitter to receiver over a channel consisting of a number of different paths, known as multipaths. The transmitted signal component travelling along a particular path experiences absorption, reflection or scattering by any objects located along that path This causes the signal components arriving at the receiver to have different amplitudes, phases and delays so that they interred with one another. This interference is known as multipath fading, and results in Intersymbol Interference (ISI) at baseband. ISI describes the spreading out of the data symbols so that components of past and future symbols are superimposed on the current symbol, thus increasing the difficulty of symbol detection and causing symbol errors.
An equalizer is often used at the receiver to remove ISI caused by the channel, by effectively applying the inverse of the channel impulse response to the received data symbols. Such an operation removes the effects of the channel from the received data. IN order to operate correctly, the equalizer requires a known preamble (or training sequence) to be transmitted together with each burst (or packet) of data symbols. The channel characteristics are then calculated from the received training data and used to adapt the equalizer to remove the channel effects from the subsequent burst or packet of data symbols. A decision feedback equalizer (DFE) is an example of a type of equalizer commonly used in radio communication systems since it is able to deal with channels having spectral nulls.
The construction of a decision feedback equalizer is shown in our International Patent Application No. PCT/GB99/01801, the necessary elements of which are incorporated herein by reference and to which the reader is referred for. further details.
Equalization while reducing ISI, also has a number of disadvantages. More particularly, equalization using a training sequence or preamble introduces an overhead in data rate, since the preamble uses up symbols which could othcrwise he been used to carry data. Equalization also introduces an overbead in packet delay, since the equalizer requires a certain period of time to train itself to the channel with the help of the preamble. A separate equalizer is often required at both terminals (base station and subscriber terminal), which means that the equalization overhead is present on both the downlink (base station, to subscriber terminal transmission) and on the uplink (subscriber terminal to base station transmission). In a point-multipoint system with many subscriber terminals accessing a single base station and using a Time Division Multiple Access TDMA) channel access scheme for the uplink, the equalizer at the base station will be required to re-train for each burst received form a different subscriber terminal. This means that delays for re-training will be frequent, and will reduce the overall data rate.
PRIOR ART
Point-to-multipoint systems may use time-division duplex (TDD) operation so as to make the most efficient use of the channel under the asymetric data rate requirements of the subscriber terminals and base station. The TDD mode of operation allows the use of precoding of the data to be transmitted, as described in GB 2262867.
More particularly, GB 2262867 discloses a pre-equalization technique wherein an automatic equalizer in the receiver of a digital communications system is adjusted using a training sequence. The equalizer coefficients for optimum adjustment are then transmitted back to the transmitter and supplied to an equalizer provided therein. After the training period is over, this transmitter equalizer, now having adjusted coefficients is activated and the receiver equalizer is disabled CA 2153641 further discloses a variant of the above-described operation wherein the equalization and pre-equalization load is shared asymetrically between the respective transmitter and receiver.
The underlying assumption for the use of precoding is the reciprocity of the channel impulse response, since TDD entais 2-way transmission on the same carrier frequency. Thus the downlink channel and uplink channel are assumed to have the same impulse response, provided that the interval between downlink and uplink transmissions is short. A recent trend in radio systems is the aim to make simpler and cheaper subscriber terminals by moving the equalization load to the base station. A prior art system with the equalizer and precoder located in the base station terminal is shown in FIG.
1
.
The term precoding is used to describe pre-equalization performed at the transmitter prior to transmission of the data. The precoder requires knowledge of the channel impulse response H(z). This may be estimated from the converged taps of the equalizer on the forward link. Ideally, the transfer function of the precoder should be the inverse of the channel transfer function. When the fading channel is not minimum-phase, a non-linear operation is needed to ensure stability of the precoder.
A prior art method of precoding is Tomlinson-Harashima precoding (TH precoding), which uses a modulo operator block at both the transmitter and receiver. ATH precoding system is shown in FIG.
2
. The output of the modulo M block always has a magnitude in the range −M/2 to +M/2, and this ensures stability of the precoder.
However, there are a number of practical difficuties with the modulo block used in TH precoding. In particular, the signal amplitude at the receiver is distorted, and this causes the automatic gain control (AGC) level to vary due to signal level fluctuations. Further, the dynamic range of the received signal is increased, and therefore the number of useful quantizer levels at the receiver is reduced. The anti-alias filters at the receiver will also require increased dynamic range.
In addition, the use of TH precoding introduces the need for synchronization between modulo blocks at the transmitter and receiver. Without synchronisation the receiver has difficulty in distinguishing between ‘spikes’ (due to transmit modulo operations) and noise, so causing modulo errors.
Furthermore, as TH precoding is highly sensitive to noise on the channel, it is highly unsuitable for use with wireless channels. Previous uses of TH precoding have been limited to wired channels only.
SUMMARY OF THE INVENTION
As mentioned earlier, previous point-to-multipoint subscriber systems aimed to make simpler subscriber terminals by moving the equalization load to the base station. In one aspect of the present invention, however, the aim is to transfer as much of the equalization load as possible to the subscriber terminals, so reducing the re-training delays at the base station This will also allow length reduction of training sequences on the uplink, increasing efficiency in the use of the channel. In another aspect, the present invention aims to provide a precoder which allows for stabilization of the inverse channel impulse response by modification of the precoder filter transfer functions as appropriate. By providing such stabilization then at least some of the drawbacks of the Tomlinson-Harashima method of precoding as described above can be overcome.
In addition, the present invention has been developed in the context of a wireless point-to-multipoint networking infrastructure which is capable of supporting multimedia data traffic at high bit rates in local and wide areas, but the invention could also be used with other networking infrastructures, and in particular
Adaptive Broadband Ltd.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pham Chi
Phu Phuong
LandOfFree
Stabilized precoder for data transmission does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Stabilized precoder for data transmission, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Stabilized precoder for data transmission will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2592017