DE10341108A1 - Method for generating logarithmic probability ratios with rake demodulators e.g. for mobile RF system, requires multiplying signals from rake demodulator with given factor - Google Patents

Abstract

A method for generating logarithmic probability ratios (LLR) during signal demodulation using a rake demodulator (RF1...RFn, MRC) involves multiplying the signals processed by the rake demodulator(s) with a factor (L), which is proportional to the reciprocal spectral interference power density. Independent claims are included for the following: (A) A device for generating logarithmic probability ratios.(B) A device for estimating the transmission quality in rake receivers.

Description

The The invention relates to a method and a device for generating of logarithmic probability ratios in demodulation a signal by means of a rake demodulator. Further The invention aims to provide a method and an apparatus for estimation the transmission quality in Rake receivers.

One typical receiver concept, this is common in mobile radio systems is the rake receiver. The functioning of the Rake receiver based on the fact that signal contributions, over different transmission paths of the radio channel the receiver reach, path-weighted and dubbed synchronized become. For this purpose, the rake demodulator several "fingers" whose outputs with connected to a combiner. The fingers become the individual in operation Assigned propagation paths and take the path-specific demodulation (Delay, Despreading, symbol formation, multiplication by the path weight) in front. The combiner overlays those Signal components over different propagation paths have been transmitted and the same Signal are assigned.

in the Signal path behind the rake demodulator are other signal processing Components arranged. Of particular importance for the achievement a good signal quality (low bit or block error ratio) is a channel decoder, which undoes the transmitter-side channel coding. For decoding are MAP (Maximum a Posteriori Probability) algorithms or SOVA (Soft-Output Viterbi Algorithm) method used, the typical output format of these stages logarithmic probability ratios - so-called LLR's (Log-Likelihood Ratio) - are.

One Another important aspect of mobile receiver systems is that that's an estimate the current transmission quality must become. Cause for this is that different data processing processes in the receiver in dependence controlled by the estimated transmission quality, be activated or deactivated. For example, during a transmission data packets or even the entire data packet at not sufficient transmission quality discarded. A common one used measure of the transmission quality is that Bit error ratio.

So far is usually used to determine the transmission quality the mean received power measured at the receiver input. The middle Receiving power at the receiver input however, is an inaccurate measure of the transmission quality since it is e.g. also by strong signal interferers elevated can be. Other approaches based on the bit error ratio after channel decoding by a subsequent re-coding of the signal and a Comparison of the decoded and re-encoded signal with the input signal to determine the decoder. A disadvantage of this much more accurate Procedure is their high computational effort.

Of the Invention is based on the object, a method and an apparatus to generate a for the further signal processing favorable Data format in the signal path in or behind a rake demodulator to accomplish. Another task is a low-cost Method or a low-cost Device for estimating one for the reception quality characteristic size in one radio receiver with a rake demodulator.

The The invention is based task by the Characteristics of the independent claims solved. Advantageous embodiments and further developments of the invention are in the subclaims specified.

According to the invention or are those of the rake demodulator processed signal or processed signals with a factor L multiplied, which is proportional to the reciprocal spectral Störleistungsdichte is. In this way it is achieved that the signals in the form of approximated logarithmic probability ratios available. LLR's are for the further signal processing in receivers advantageous, because after demodulation in the rake demodulator no further estimate of Channel parameters is required. In particular, for channel decoding an estimate no longer required by channel parameters.

Basically multiplication by the factor L in each of the rake fingers become. An advantageous variant of the method is however characterized in that the combined output from the rake demodulator Signal is multiplied by the factor L. In this case will only a single calculation unit for the signal formatting according to the invention needed in LLR's.

A particular advantage of the generated LLRs is that they allow a simple estimation of the transmission quality in the rake demodulator. In this respect, a particularly advantageous method variant is characterized by that the variance of the LLR's is determined. The variance of the LLR's is clearly related to the bit error ratio at the output of the rake demodulator.

The Determination of the bit error ratio expediently by comparing the determined variance with predetermined reference values, for example, stored in a read-only memory in the receiver could be. Based on the comparison result, the variance is determined Bit error ratio assigned.

The Invention will be described below with reference to an embodiment closer by a single figure explains which is a schematic diagram representation of a rake demodulator with a calculation unit according to the invention for generating the LLR format at the output of the rake demodulator as well a unit of appreciation the bit error ratio shows.

In The figure is a simplified schematic representation of a rake demodulator with a unit according to the invention for calculating output values in LLR format. Of the Construction of a rake demodulator is known and will be described below only explained cursorily. A rake demodulator has a plurality of rake fingers RF1, RF2, ..., RFn, which are parallel to each other and one each delay stage RAM, a despreading stage DS, an integrator I & D and a multiplier M have. The exits the rake finger RF1, RF2, ..., RFn are assigned to a maximum weight combiner MRC (MRC: maximum ratio combining) fed, which adds the path demodulated signal contributions and reconstruct the transmitted signal in this way.

The operation of the rake demodulator is as follows:
The receiver side is the rake demodulator via a data connection 1 a total signal supplied, which results from the superposition of all received signals. By means of the delay stages, for example each consisting of a memory with random access RAM (Random Access Memory), a synchronization of the rake fingers RF1, RF2, ..., RFn is made. For this purpose, a searcher SE (Searcher) determines the channel profile, which indicates the delays of the propagation paths. Each of the memory RAM is controlled by the search device SE with one of the determined time delays, ie it is ensured that the reading of a sample from the memory RAM is retarded relative to the Einleszeitpunkt to the corresponding pfadspezi fischer time delay. Thereby, a specific propagation path of the transmission channel is assigned to each rake finger RF1, RF2,..., RFn. At the output of the memory RAM there are synchronous samples based on the time accuracy given by the sampling frequency (eg double chip ratio).

In the despreading stages DS are the incoming samples with the channel-specific channelization code and the base station individual Scrambling code multiplied. These two codes are from one Unit SCG (Spreading Code Generation) provided. The despreading reverses the transmitter-side band spread in the receiver. By despreading the participant separation is carried out as well as in the case of receipt a signal from several base stations selected one of the transmitting base station. Provided no CDMA (Code Division Multiple Access) system of data transmission underlying the despreading stages DS.

The integrators I & D (Integrate & Dump) perform an integration of the samples (chips) over the length of a symbol. In CDMA systems, a symbol consists of SF chips, where SF denotes the spreading factor. By integration over each SF ₁ , SF ₂ , ..., SF _n chips thus symbols are formed and output at the outputs of the integrator I & D. If the mobile radio system considered is not a CDMA system, the integrators I & D are dispensed with.

In the multipliers M, the path-specific signal contributions generated in this way are multiplied by the path-specific path weights (conjugate complex amplitude values of the corresponding propagation path). To determine the path weights, a channel estimator KS is used in the usual way. At the exit 2 of the channel estimator KS, the estimated path-specific path weights are available. These are multiplied in the multipliers M by the symbols output by the integrators I & D.

In the maximum weight combiner MRC is channel-wise addition the path-specific demodulated signal components. This is also called Maximum Ratio Combining (MRC).

Let f be the despread signal of the finger with the number f, f = 1, ..., n, and h * / f be the conjugate complex amplitude value of the corresponding propagation path. Then, the output of the rake finger f y f = h * f e f

For the output signal of the maximum weight combiner MRC is obtained for the binary Transmission, as is the case, for example, with UTRA FDD (UMTS-Terrestrial Radio Access Frequency Division Duplex), y = Re {Σ f H * f e f }

By weighting with a suitably chosen factor L and adding a sum term E, one obtains from y an LLR format according to λ = Ly + E.

For a MAP detector, with the spectral interference power density N ₀ of the noise present after the despreading (noise and interference) for the quantity L, the value is obtained L = 4 / N 0 ,

The approximation E ≈ 0 yields λ ≈ 4 Re {Σ f H * f e f } / N 0 ,

The spectral interference power density N ₀ is in the signal path behind the despreading means of an estimator (not shown) at the output 3 of the maximum weight combiner MRC is estimated at regular intervals in the receiver. The multiplication of the output variable y of the maximum weight combiner MRC with the quantity L is carried out by means of a multiplier MUL in dedicated hardware or by means of a processor in software. For the further signal processing in the receiver, only the logarithmic probability ratios λ are used. A subsequent estimation of channel parameters (usually signal attenuation and noise variance) for further signal processing is not necessary due to the inventive conversion of the output values of the rake demodulator to the LLR format.

It is particularly advantageous to use the LLR approximation values λ to calculate the bit error ratio at the output 3 of the rake demodulator. For this purpose, the output signal λ is fed to a statistical stage VAR, which calculates the variance σ ² (λ) with respect to a specific number of LLR approximation values λ. The variance σ ² (λ) is passed to a comparator COM, which also has access to a read-only memory MEM. Reference values for the variance σ ² (λ) are stored in the read-only memory MEM, wherein each interval between two reference values is assigned a specific bit error ratio. The comparator COM determines in which reference value interval the measured variance σ ² (λ) lies. The assigned bit error ratio is also read from the read-only memory MEM and at the output 4 provided by the comparator COM. The particular advantage of this method is that a relatively accurate estimate of the transmission quality is already present directly behind the rake demodulator. This makes it possible to make a decision early on the further signal processing as a function of the transmission quality. In particular, in the packet transmission too poorly received data can be discarded immediately.

Claims (12)

Method for generating logarithmic probability ratios (LLR) in the demodulation of a signal by means of a rake demodulator (RF1, ..., RFn, MRC), with the steps: Multiply the or that processed by the rake demodulator (RF1, ..., RFn, MRC) Signals or signals with a factor L, which is proportional to the reciprocal spectral interference power density is. A method according to claim 1, characterized in that L = 4 / N ₀ , where N _{0 is} the spectral Störleistungsdichte. Method according to claim 1 or 2, characterized that the output from the rake demodulator (RF1, ..., RFn, MRC), combined signal multiplied by the factor L. Procedure for estimation transmission quality in Rake receivers through the steps - Produce of logarithmic probability ratios in demodulation a signal by means of a rake demodulator (RF1, ..., RFn, MRC) according to the method according to any one of the preceding claims; - Determine the variance of logarithmic probability ratios. Method according to claim 4, characterized by step - To compare the determined variance with given reference values; - Determine a bit error ratio dependent on from the comparison result. Method according to one of the preceding claims, characterized in that the method is used in CDMA receivers, in particular in UMTS receivers becomes. A device for generating logarithmic probability ratios in the demodulation of a signal by means of a rake demodulator (RF1, ..., RFn, MRC), comprising - a calculating means (M), which or processed by the Rake demodulator signal or processed Signals with a factor L, wel is proportional to the reciprocal spectral noise power density multiplied. Apparatus according to claim 7, characterized in that L = 4 / N ₀ , where N _{0 is} the spectral Störleistungsdichte. Apparatus according to claim 7 or 8, characterized that the output from the rake demodulator (RF1, ..., RFn, MRC), combined signal multiplied by the factor L. Apparatus for estimating the transmission quality in rake receivers, with - one Calculating means (M) for generating logarithmic probability ratios the demodulation of a signal by means of a rake demodulator (RF1, ..., RFn, MRC) according to any one of claims 7 to 9, and - one further calculation means (VAR) for determining the variance of the logarithmic Likelihood ratios. Apparatus according to claim 10, characterized by - a comparison means (COM) for comparing the determined variance with predetermined reference values, and - one Means (MEM) for determining a bit error ratio in dependence from the comparison result. Device according to one of claims 7 to 11, characterized that the device is used in a CDMA receiver, in particular in a UMTS receiver becomes.