Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/7103—Interference-related aspects the interference being multiple access interference
  • H04B1/7105—Joint detection techniques, e.g. linear detectors
  • H04B1/71057—Joint detection techniques, e.g. linear detectors using maximum-likelihood sequence estimation [MLSE]
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/14—Relay systems
  • H04B7/15—Active relay systems
  • H04B7/204—Multiple access
  • H04B7/216—Code division or spread-spectrum multiple access [CDMA, SSMA]

Definitions

• the invention relates to a CDMA communication system comprising at least a transmitter in which a data sequence is spread by a code sequence and comprising at least a receiver in which the data sequences are recovered by a detector, sequences of estimates for the transmitted data sequences being computed for the purpose of the detection while a description of the communication channel situated between the transmitter or transmitters and a receiver is incorporated.
• the invention likewise relates to a receiver for a CDMA communication system and, more specifically, to a detector for such a receiver.
• Code-Division Multiple Access In a communication system based on Code-Division Multiple Access (CDMA) the signals of various users are simultaneously transmitted in a common frequency band and with a common carrier frequency.
• Code-division multiple access systems are based on a spread band technique, that is to say, the signal to be transmitted is spread out over a frequency band which is substantially wider than the minimum frequency band necessary for transmitting the signal. As a result of the spreading of the band, code-division multiple access systems are generally highly insensitive to interference.
• each bit to be transmitted is multiplied by a codeword mutually agreed upon by transmitter and receiver.
• mutually orthogonal codewords mutual interference by the signals from the individual users is ruled out in principle. But realistic requirements as to wave propagation over the surface of the earth such as multipath propagation lead to the fact that this orthogonality is no longer adhered to. If, in addition, a synchronization of the accesses by the individual users is dispensed with to realise a highly simple access to the common frequency band, or if different bit rates are permitted, the signal of a user can be detected only with more circuitry and cost or with a degraded quality, because the signals can no longer be mutually orthogonal.
• joint probability distributions for the transmitted data sequences are computed by computing the associated moments and deriving therefrom the sequences of estimates for the transmitted data sequences.
• new data are continuously transmitted and the receiver makes decisions about the transmitted data values.
• the joint probability distribution changes continuously, so that a sequence of joint probability distributions is to be computed.
• the joint probability distribution can be constantly improved by the incoming receive signal.
• probability distributions automatically provide quality information about the estimates. This quality information can be evaluated beneficially in a decoder downstream in the circuit.
• the use of moments to describe probability distributions is advantageous in that a highly simple description of the probability distribution can be obtained with the moments.
• the joint probability distribution is computed only with a restricted set of moments.
• a restricted set of moments will provide only an approximate description of the joint probability distributions.
• the use of a restricted set of moments provides a reduction of cost of the detector, without having to accept significant quality losses on detection.
• first and second order moments of the joint probability distribution are computed to compute an approximate joint probability distribution.
• a Kalman filter is provided as a detector.
• the computed estimate vector and associated error covariance matrix are not assigned to a probability distribution.
• the estimate vector can be considered a mean vector of the joint probability distribution (first moment of the joint probability distribution) and the error covariance matrix can be considered a covariance matrix of the joint probability distribution (second moment of the joint probability distribution.
• decisions about the values of the data sequence are made on the basis of the estimates and these decisions are fed back inside the detector.
• the quality of the detection process can generally be improved drastically, because the discrete nature of the transmitted data sequences is taken into account. Especially with a Kalman filter such a feedback does not take place.
• the quality of the estimate, when the values are fed back, is taken into account for the value that has been fed back.
• the quality of the detection process can be improved even more. If the detector is followed by a decoder in the circuit, which decoder can process decisions with quality grading (soft decision), the communication reliability is enhanced distinctly.
• Fig. 1 shows a CDMA communication system comprising N mobile stations and one base station
• Fig. 2 shows a receiver for such a CDMA communication system
• Fig. 3 shows function blocks of a detector
• Fig. 4 shows a time diagram representing transmitted data sequences of a plurality of users of a CDMA communication system
• Fig. 5 shows a state diagram of a modified Kalman filter for estimating transmitted data signals
• Fig. 6 shows a block circuit diagram of a feedback step in the state diagram shown in Fig. 5,
• Fig. 7 shows a state diagram for the estimation of the covariance matrix
• Fig. 8 shows a block circuit diagram of a feedback step in the state diagram shown in Fig. 7.
• the transmit data are available as binary data sequences b ; .
• the binary data sequences b j are recovered from speech signals by appropriate coding.
• These binary data sequences b- are exchanged between individual mobile radio sets and their base station BS in whose coverage area the mobile radio sets happen to be located.
• each data sequence b- is multiplied by a code sequence s-. In the following a bit of the code sequence is referenced a chip to distinguish it from a bit of the transmitted data sequence.
• the individual code sequences are to be selected discriminatively.
• the mobile station randomly selects a code sequence and transmits the selection to the base station when a connection is established between mobile station and base station.
• the different locations of the individual mobile stations provides that the transmitted signals arrive at the base station by different radio channels.
• the transmitted signals are exposed to distortions caused by, for example, reflection and multipath propagation.
• These distorted signals are superimposed on each other at the antenna of the base station BS to form a continuous receive signal y(t) which receive signal contains noise signal portions n(t).
• a receive data sequence b- is estimated for each transmitted data sequence b- in the received signal y(t). For example, the recovery of speech signals from these input data and the distribution of the received data to the individual receive ports, for example, by transfer to a communication network, are not further shown here.
• Fig. 2 shows the basic structure of a receiver according to the invention.
• the received signal y(t) is first preamplified by a HF preamplifier stage 21 and bandpass filtered.
• the bandpass filtered received signal is mixed in mixers 23a, 23b with the HF signal itself and with a signal that has a quadrature phase relative to the HF signal.
• the real input signal y(t) is directly converted to the baseband during which operation a complex baseband signal containing a real portion and an imaginary portion is produced.
• sampling frequency is to be twice as high as the limit frequency of the baseband signal. In the embodiment this is achieved in that the sampling is carried out at twice the chip rate.
• the samples are subsequently converted in analog-digital converters 25a, 25b into a digital value sequence y(k).
• This digital value sequence y(k) is applied to a digital signal processor 26 which contains, for example in a read-only memory 27, a program for estimating the transmitted binary data sequences. Intermediate results arising from the estimation are buffered by the signal processor 26 in a read write memory 28.
• the computational capacity of a single signal processor is no longer sufficient under specific circumstances. In that case more signal processors over which the computational capacity is to be spread are to be provided.
• Fig. 3 shows in a circuit diagram the functions realised by this or by these signal processor(s) respectively, as long as they are necessary for the signal estimation.
• a description of the communication channel is necessary for each user, which description is produced by a channel estimator 31.
• a so- termed channel impulse response is often used for the channel description.
• To determine the channel description it is possible, for example, to insert a training data sequence into the transmit signal, from which sequence the channel impulse response can be computed in the receiver by correlators.
• a code sequence generator 32 is necessary which produces the value of one chip for each individual user at the sampling instant k.
• a channel impulse response formed by a radio channel impulse response and the code sequences will be advantageously made available to the detector 30.
• the data of the individual users which have an effect on the discrete time received signal y(k) at the sampling instant k are combined in a vector b(k).
• the vector b(k) comprises not only one bit per user but, depending on the time dispersion, various bits per user.
• the vector h(k) describes how the transmitted bits cause interference to one another.
• the vector takes the actual radio channel, filter in the communication circuit and the spread sequences into account.
• Fig. 4 shows a very simple embodiment.
• the CDMA communication system has three users.
• the modulation method is Phase Shift Keying and the radio channel for each user comprises a direct link (no multipath propagation, no fading, no attenuation), which link is only disturbed by Additive White Gaussian Noise (AWGN).
• AWGN Additive White Gaussian Noise
• the impulse response vector h(k ⁇ ) is in this simple case determined only by the code sequences used for the spreading. From Fig.
• h(ko) [+ 1 , -1 , + l] ⁇ .
• h(ko) [+ 1 , -1 , + l] ⁇ .
• the detector may have a highly flexible structure: For example, code sequences can be permitted of which the period does not correspond to the data period used. Furthermore, the users can be permitted to have different and variable data rates.
• the embodiment shown in Fig. 4 shows that integrating binary code sequences into the impulse response vector h(k) is very simple, because in the impulse vector h(k) the signs may only be changed depending on the chips.
• the observation equation can be completed by a state transition equation which describes how the composition of the vector b(k) is changed at the transition from sampling instant k to sampling instant k+ 1
• b(k+ l) A(k) * b(k) + b ⁇ (k+ l)
• A(k) * b(k) + b ⁇ (k+ l) With the aid of matrix A(k) there is formally described which data are omitted from the vector b(k) or b(k+ 1) as they no longer contribute to the received signal y(k+ l) at the instant k+ 1.
• the vector b ⁇ (k+ l) the data are added to the vector b(k) or b(k+ l) respectively, which data have just been transmitted at the instant k+ 1 and thus influence the received signal for the first time.
• the state equation in this form is only used for describing in general by way of a formula how data are added to the vector b(k+ 1) or read from this vector.
• b(k+ 1) is computed in a signal processor, such operations are preferably not carried out by matrix multiplications and matrix additions, but by specific storing operations.
• the vector b(k) is changed to b(k+ l) at the transition.
• Fig. 5 shows a section of a state diagram by which the computation steps to be carried out by the signal processor 26 are shown in diagrammatic form.
• the detector itself may be considered a modified Kalman filter, modified by a feedback of decisions having information about quality (soft decision).
• the computation of the covariance matrix necessary for estimating the data sequences is given in diagrammatic form in the state diagram shown in Fig. 7.
• the computations which are to be carried out by the signal processor 26 for the feedback are shown in Fig.
• the power ⁇ n of the receiving noise n(k) is to be taken into account.
• the power can easily be estimated in the receiver, for example, within the scope of the channel identification. An accurate estimate is not necessary because Kalman filters are, as is known, robust to errors of the noise power.
• A. distribution has the moments b * (k) and P * (k). It is especially advantageous that in the embodiment the improved moments are computed by equations which are similar to the filter equations of the Kalman filter. As a result, essentially the same algorithm can be used for the computation.
• b * (k) For the mean value vector b * (k) is obtained:
• the vector u- is a unit vector in which only the i **1 element is different from zero i.e. is equal to one. It was assumed that the i m element of the state vector b(k) is to be decided on and sampled from the vector.
• moments b ⁇ k) and P * (k) occur, which take into account that the i 1*1 element of the state vector b(k) is a bit.
• the bit decision is made on the basis of the associated element of the estimate vector or mean value vector b * (k).
• the bit error probability can be approximated because the moments b * (k) and P * (k) are assigned to a probability distribution. For the bit error probability there is obtained: 1 -
• P(bit errors) quality criterion
• the modified filter equations can be evaluated accordingly often, and instead of b(k) and
• h (k+1) ⁇ (k) £ (k) .
• the matrix Q b ⁇ (k) is a diagonal matrix which contains the variances of the bits that have just been transmitted at instant k+ 1 i.e. a 1 value (otherwise 0) in the row in which the associated bit appears in the unknown vector b(k+ l).
• b * (k+ l) there appears a zero because there is assumed that the bits are uniformly distributed i.e. without a mean value.
• the detector described produces estimates for the bit error probabilities. This may be used in a decoder to enhance the reliability of transmission.
• the detector also uses multipath propagation to advantage. For the code sequences no limitations with respect to the period durations used have to be taken into consideration. Different and variable data rates of the users are permitted. The users need not be synchronized either. All these are advantages which are generally absent in sub-optimum detectors.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)
• Radio Relay Systems (AREA)
• Noise Elimination (AREA)

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Citations (3)

• 1993
  • 1993-05-21 DE DE4316939A patent/DE4316939A1/de not_active Withdrawn
• 1994
  • 1994-05-20 WO PCT/IB1994/000116 patent/WO1994028642A1/en not_active Application Discontinuation
  • 1994-05-20 CZ CZ1995148A patent/CZ286408B6/cs not_active IP Right Cessation
  • 1994-05-20 EP EP94914530A patent/EP0656162A1/en not_active Withdrawn
  • 1994-05-20 KR KR1019950700284A patent/KR100336544B1/ko not_active IP Right Cessation
  • 1994-05-20 HU HU9500161A patent/HU215623B/hu not_active IP Right Cessation
  • 1994-05-20 JP JP7500442A patent/JPH07509356A/ja not_active Ceased
  • 1994-05-20 AU AU66869/94A patent/AU682689B2/en not_active Ceased
  • 1994-05-20 CN CN94190310A patent/CN1063600C/zh not_active Expired - Fee Related
  • 1994-07-05 TW TW083106145A patent/TW241419B/zh active

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