The present invention relates to a method for processing received signals transmitted via a transmission channel and also relates to a corresponding device. In particular, the present invention is directed to a method and corresponding device for channel impulse response improvements in TDMA systems.
Recently, mobile radio telecommunication systems have widely spread. Such mobile radio telecommunication system operate for example according to a commonly agreed standard, like for example the GSM standard. According to GSM standard, data transmission is performed according to a method of time divisional multiple access (TDMA). The TDMA transmission principle specifies that data are transmitted from a transmitter to a receiver and vice versa only during respectively specified time slots of frames.
Data transmission in such telecommunication systems substantially relies on digital data transmission. However, between a mobile radio transceiver device as a subscriber terminal (hereinafter: mobile station MS) and a stationary radio transceiver device as a serving radio network element (hereinafter: base station BS) data have necessarily to be transmitted as analogue data via the air interface Um.
Consequently, data transmitted by the mobile station MS are received via a base station antenna means of a reception device of the base station BS as analogue data. In the course of the further processing of the thus received data by the reception device, the analogue data are analog to digital converted, i.e. passed through an A/D converter means. At the next stage of the processing, the obtained digital data are supplied to an equalizer means for being equalized. The thus obtained output data are then supplied to a channel codec means for coding/decoding the data. At the reception side, decoding is performed to separate received speech data from associated signaling data.
Particular attention in the course of this processing has to be paid to the equalizing of the received data, since the equalizing is required to reconstruct, at the reception side, the transmitted signal from a mixture of received signals.
For example, assuming a situation in a radio telecommunication network with a base station BS and only a single mobile station MS present in the radio coverage area of the base station. Then, a signal s transmitted from the mobile station MS may reach the base station BS directly via line of sight at a time s(t). However, the same signal s may be deflected by, e.g., a building, a mountain or the like present in the environment. Thus, the same signal may reach the base station BS at a later point of time s(t+T), and is thus superposed to the signal s(t). Due to the delay T, both received signals are no longer in phase with each other. Moreover, the delayed signal s(t+T) may even be more attenuated than the signal s(t) due to the longer transmission path. Thus, the signal received by the base station BS and originating from the mobile station MS is distorted. Now, assuming that another mobile station MSxe2x80x2 is additionally present, then signals sxe2x80x2(txe2x80x2), sxe2x80x2(txe2x80x2+Txe2x80x2) are additionally received by the base station BS, which may lead to interference between the respective transmitted data symbols (intersymbol interference).
Therefore, an equalizer means has to reconstruct (detect) the initially transmitted signal s(t) and/or sxe2x80x2(txe2x80x2) from the received mixture of signals s(t), s(t+T), sxe2x80x2(txe2x80x2), sxe2x80x2(txe2x80x2+Txe2x80x2).
The thus reconstructed (or detected) signal is required to be as similar to the originally transmitted signal as possible. This reconstruction is therefore a main concern when designing equalizers, e.g. for use in a reception device of a base station BS.
Hitherto, in equalizers of reception devices used in mobile telecommunication systems, the channel impulse response (CIR) is estimated, and the thus estimated channel impulse response is used to detect, i.e. to reconstruct the transmitted data symbols.
Document WO 94 28661 representing the prior art described in the preambles of claims 1 and 7 discloses a method for estimating a channel state based on a low complexity model. This method is used by a received in Rayleigh fading environments, and the receiver contains at least one channel stated estimator which uses an auto regressive model for channel variations.
Document U.S. Pat. No. 5,329,547 discloses a method and an apparatus for coherent communication. According to this method, reference symbols are inserted into a data stream. The channel response is estimated by utilizing the stream of reference samples. An estimated data symbol is detected from the stream of data samples by utilizing the estimated channel response.
Document WO 97 23089 discloses a method and apparatus for channel identification utilizing two Least-Squares (LS) estimators. Each LS estimator is used for calculating a sequence of channel values, and further for determining and estimated channel impulse response, over an entire frequency band thereof.
The estimated channel impulse response is usually based on the received samples of the stream of data symbols. Therefore, it is an estimate of the actual (observed) channel impulse response. However, an error in the channel impulse response leads to a degraded performance when detecting/reconstructing transmitted data symbols from received data symbols.
In a previous solution, the above described estimated channel impulse response is assumed to be ideal, thereby disregarding errors, and it is used as it is for data symbol reconstruction. However, this solution was unsatisfactory due to the error in the observed channel impulse response being not taken into account.
Another known solution for improving the performance of an equalizer is disclosed in document U.S. Pat. No. 5,251,233 by Labedz et. al., assigned to Motorola Inc. The basic idea described therein is to estimate the energy of taps in the impulse response. Those taps having an energy below a predetermined threshold level are zeroed in order to reduce noise in the estimated channel impulse response. This previously proposed method may be applied to complex taps or real taps, i.e. may be performed separately for real and imaginary taps. The method disclosed in the Labedz patent improves (i.e. reduces) the bit error rate BER in the reproduced detected signal (which is an indication for the quality of a receiver) in situations where the channel interference ratio C/I or C/(I+N), respectively, with N being a noise signal superposed to an interfering signal, is low and the channel has a poor quality. The method is also beneficial in channels having a short impulse response. (The term taps refers to coefficients of each respective delay element of the corresponding FIR model for the used circuitry. Thus, based on the tap values, poles and zeroes of the transmission function may be calculated.)
However, under good channel conditions, the method as proposed by Labedz et. al. even degrades the receiver performance which may be attributable to the zeroing of amplitude coefficients of certain taps.
Consequently, it is an object of the present invention to provide a method for processing received signals transmitted via a transmission channel, and to provide a corresponding device which improve the receiver performance in all channels irrespective of actual channel conditions.
According to the present invention, this object is achieved by a method for processing received signals transmitted via a transmission channel, the method comprising the step of obtaining an observed response function of said transmission channel, based on said received signals; characterized by the steps of deriving an estimated variance of said received signals; and modifying said observed response function by applying said estimated variance to said observed response function, thereby obtaining a modified impulse response; wherein said step of modifying is effected by multiplying said estimated response function with a vector, the components of which being determined on the basis of parameters of the estimated response function and said estimated variance.
Furthermore, the present invention proposes a device for processing received signals transmitted via a transmission channel, said device comprising obtaining means adapted to obtain an observed response function of said transmission channel, based on said received signals; characterized by deriving means adapted to derive an estimated variance of said received signals; and modifying means adapted to modify said observed response function by applying said estimated variance to said observed response function, thereby obtaining a modified impulse response; said modifying means further comprises a multiplying means adapted to multiply said estimated response function with a vector, the components of which being determined on the basis of parameters of the estimated response function of said estimated variance.
Advantageous further developments of the present invention are as set out in the respective dependent claims.
According to the present invention, the proposed method and/or device are very easy to implement. Moreover, the present invention improves the receiver performance in all channel types as specified in the GSM 5.05 recommendation (e.g. channel models xe2x80x9cHT100xe2x80x9d and xe2x80x9cRA250xe2x80x9d). Additionally, the improvement may be achieved not only independent of the channel type as such, but also independent of the specific channel condition, i.e. in situations of different channel interference ratios.
Preferred embodiments of the present invention are described herein below in detail by way of example with reference to the accompanying drawings.