WO2002078205A1 - Demodulation et desetalement dans des recepteurs amcr - Google Patents

Demodulation et desetalement dans des recepteurs amcr Download PDF

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Publication number
WO2002078205A1
WO2002078205A1 PCT/GB2002/001438 GB0201438W WO02078205A1 WO 2002078205 A1 WO2002078205 A1 WO 2002078205A1 GB 0201438 W GB0201438 W GB 0201438W WO 02078205 A1 WO02078205 A1 WO 02078205A1
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Prior art keywords
receiver
channel
lmmse
rake
mode
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PCT/GB2002/001438
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English (en)
Inventor
Andrew Robert Nix
Yan Qint Bian
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The University Of Bristol
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Publication of WO2002078205A1 publication Critical patent/WO2002078205A1/fr

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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
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • 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
    • 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
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • H04B1/712—Weighting of fingers for combining, e.g. amplitude control or phase rotation using an inner loop
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
    • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
    • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
    • H04B2201/70707—Efficiency-related aspects
    • H04B2201/7071—Efficiency-related aspects with dynamic control of receiver resources

Definitions

  • the circuit block diagram for an LMMSE receiver is shown in Figure 1.
  • the adaptive weights are calculated blindly based on a data derived symbol sequence (see equation 5).
  • the role of the LMMSE filter is to despread the desired CDMA signal on each rake finger and to adapt in a manner to reduce MAI.
  • the LMMSE operates at the far lower symbol rate, unlike chip level equalisers that operate at the chip rate.
  • the LMMSE is therefore a low complexity solution for the removal of MAI and the realisation of high capacity networks.
  • An aim of the invention is to enhance the reception of signals transmitted using a CDMA scheme.
  • the invention also consists in a method of receiving CDMA-type signals, wherein the reception mode is configured to a demodulating and despreading mode most suited for signals received on a channel in accordance with measured properties of said channel.
  • the available modes include one or more of a correlation mode, a rake mode and a LMMSE mode.
  • a number of received components which differ from one another by relative path delays, are used in the demodulation and despreading of the received signal.
  • the components may be obtained by comparing the received signal to a threshold. Where several components exceed the threshold, the number of components used may be limited, for example by choosing a predetermined number of the strongest components. Where only one component is obtained, the demodulation and despreading mode selected becomes a correlation mode.
  • the invention provides apparatus for receiving CDMA-type signals, the apparatus being capable of despreading and demodulating signals received on a channel by using either a rake process or a LMMSE process, the apparatus being capable of selecting one of said processes for use with received signals on the basis of measured properties of the channel.
  • h k (t), L and r / ,* represent the complex channel gain, the number of resolvable paths and the excess delay respectively.
  • the channel model consists of several path clusters (resulting from multiple reflection and diffraction).
  • the fading processes on each path are statistically independent and Rayleigh distributed [10]. It is assumed that the channel multipath delay spread, T tribe, & L/W, is comparable or greater than the chip period T c , where W represents the signal bandwidth.
  • the multipath will introduce ISI (Inter Symbol Interference) and the propagation channel is affected by frequency selective fading. For a DS-CDMA system, this ISI will cause serious MAI.
  • the received signal can be mathematically expressed as:
  • a reconfigurable TDD mode detector is now developed to suppress MAI and ISI over noisy, frequency selective, time varying channels. More specifically, a reconfigurable blind adaptive LMMSE circuit is proposed with dynamic parameter settings and path selection. The architecture of this reconfigurable adaptive receiver is shown in Figure 2, where the LMMSE blocks take the form shown in Figure 1.
  • radio channel parameters such as rms delay spread and Eb No must be estimated to determine the value of the adaptive step-size, ⁇ s If the rms delay spread value is low (below 75ns in the present embodiment), the value of ⁇ IM s is set to zero for all paths (i.e. the LMMSE adaptation circuit is placed in a power saving mode). This implies that there is no need to support the adaptive LMMSE (mode 1) and the resulting receiver is reconfigured as a conventional Rake structure (mode 2). The channel estimation data is analysed to dynamically determine the number of rake taps, L, required and their path delays.
  • MRC Maximal Ratio Combining
  • EGC Equal Gain Combining
  • the operating modes of the reconfigurable receiver are summarised in Figure 4.
  • the threshold value of Eb/No between mode 1 and mode 2 is determined by system simulation and is different for UL and DL operation.
  • the rms delay spread thresholds between modes 1, 2 and 3 are also determined via simulation.
  • Optimum values for a UMTS TDD-CDMA embodiment are given in [1][2].
  • Table 1 summarises the values of the adaptive parameters in each mode. It should also be noted the path delay for each chosen finger is also an adaptive set of parameters that are passed to the receiving circuit.
  • the receiver reconfigures and generates an output whose SNR (signal to noise ratio) is close to optimal.
  • SNR signal to noise ratio
  • the receiver architecture now operates only on the selected branches. Assuming the delay spread is sufficient to require LMMSE operation, the adaptive coefficient update algorithm for the LMMSE filter aims to minimise the following function:
  • the final detection is made according to:
  • the receiver is able to overcome prevailing interference and noise enhancement, even in deep fading channels. Since an adaptive algorithm is used and interference is suppressed before the multipath is combined, knowledge of the exact fading gain on each path is not required, however explicit information regarding multipath delay is necessary.
  • the adaptive receiver does not require interference information and no training sequence is necessary, hence the method is considered suitable for use in both the UL and DL sections of the UMTS TDD standard.
  • this reconfigurable adaptive LMMSE structure is attractive since it embodies both the fixed Rake and correlation architectures in a compact and simple receiver structure.
  • Figure 5 presents a BER performance comparison between the proposed channel reconfigurable adaptive receiver and a number of fixed structures in multi-user time varying multipath channel.
  • Figure 5 demonstrates the dramatic performance enhancement offered using the Reconfigurable Adaptive Receiver (RAR) of the embodiment relative to the standard LMMSE receiver, rake receiver and chip level equaliser in a time varying multipath channel.
  • the performance of the proposed reconfigurable receiver can be seen to be more stable than the LMS chip level equaliser and very close to that of the RLS chip-level equaliser. It should be noted that the RLS equaliser is considered too complex for practical implementation - generally being several orders of magnitude more complex than the newly proposed RAR.
  • key channel parameters ⁇ M s, h k , L
  • the complexity of the proposed reconfigurable receiver is far less than that of a chip level equaliser and the technique can also be applied successfully to the UL.
  • the minimum required SNR grows as the number of users increases.
  • the required minimum Eb/N 0 for the LMMSE is less than that of the MF for small numbers of users (four or less based on Figure 5).
  • the value of Eb/(No+I 0 ) reduces, where I 0 represents the interference power spectral density.
  • the PS and NPS-LMMSE are very sensitive to low values of SNR. This sensitivity arises from the iterative use of decision directed errors.
  • the use of reconfigurability prevents the use of the LMMSE in channel with low S/N or low rms delay spread.
  • the UMTS DS-CDMA Direct Sequence Code Division Multiple Access
  • the system is primarily intended to offer 3G services in micro and pico cellular environments. In particular, the system intends to support traffic hot spots in campus style environments.
  • the capacity of a TDD-CDMA system is divided asymmetrically between the uplink (UL) and downlink (DL). This asymmetry can be used to support multimedia services by adjusting the UL and DL requirements of the TDD-CDMA cell.
  • the task of the receiver is to separate data belonging to the user of interest from data belonging to interfering users.
  • MUD Multi-User Detectors
  • MUD Multi-User Detectors
  • MLSE Maximum Likelihood Sequence Estimator
  • the filter weights are adaptively updated to achieve the minimum mean square error (MSE) during one symbol interval and this can be written as:
  • the receiver is able to overcome prevailing interference and noise enhancement, even in deep fading channels. Since an adaptive algorithm is used and interference is suppressed before the multipath is combined, knowledge of the exact fading gain on each path is not required, however explicit information regarding multipath delay is necessary.
  • the adaptive receiver does not require interference information and no training sequence is necessary, hence the method is considered suitable for use in both the UL and DL sections of the UMTS TDD standard.
  • this reconfigurable adaptive LMMSE structure is attractive since it embodies both the fixed Rake and correlation architectures in a relatively compact and simple receiver structure.
  • FIG. 8(i) shows the convergence of a chip level DFE with 6 feedforward and 5 feedback taps.
  • Two convergence algorithms have been simulated, the Least Mean Square (LMS) and the Recursive Least Square (RLS).
  • LMS Least Mean Square
  • RLS Recursive Least Square
  • Figure 8(ii) shows the various BER performance results for the DL analysis. Once again, the Rake outperforms the correlation receiver, however both the RLS chip level equaliser and LMMSE offer significantly superior performance.
  • Figure 9 shows the BER behaviour versus E o /N 0 under low rms delay spreads conditions on the UL.
  • the main signal energy arrives on the first path, with other weaker paths having much lower SNR.
  • Figure 11 presents a BER performance comparison between the proposed channel reconfigurable adaptive receiver (see section 3) and a number of fixed structures.
  • the performance of the proposed reconfigurable receiver can be seen to be more stable than the LMS chip level equaliser and very close to that of the RLS chip-level equaliser.
  • the RLS equaliser is considered too complex for practical implementation.
  • key channel parameters ⁇ LMS, hk, L
  • the complexity of the proposed reconfigurable receiver is far less than that of a chip level equaliser and the technique can also be applied successfully to the UL.
  • This part of the document presents receiver architectures and performance results relating to the UMTS (Universal mobile telecommunication system) TDD (time division duplex) mode.
  • System performance in terms of uncoded BER (Bit Error Rate) and overall capacity are compared for MF (Matched Filter), Rake and blind adaptive LMMSE receivers. Uplink comparisons are performed in a multi-user scenario over time varying frequency selective channels.
  • MF Matched Filter
  • MF Matched Filter
  • Rake Rake
  • blind adaptive LMMSE receivers Uplink comparisons are performed in a multi-user scenario over time varying frequency selective channels.
  • a modified blind adaptive LMMSE receiver is proposed. Factors such as channel variation, interference suppression and low implementation complexity are considered.
  • the results demonstrate that the proposed receiver architecture can greatly reduce the interference floor at the basestation and thus significantly improve performance and capacity in the UMTS TDD mode.
  • Multipath fading is common to both dense urban (microcell) and indoor (picocell) environments.
  • the multipath channel degrades signal quality by introducing additional interference, thus limiting the capacity and performance of a DS (Direct Sequence) - CDMA system.
  • the Rake receiver is believed to be the optimum DS-CDMA receiver for a single user in a multipath channel.
  • reception of the desired user code suffers interference from the presence of other user codes in the multipath channel.
  • the cross correlation noise introduces an interference floor, and the resulting system becomes interference limited, rather than noise-limited [5].
  • the various mobile users are not perfectly time synchronous since each user communicates to the basestation (BS) through an independent, imperfectly time aligned channel.
  • the adaptive LMMSE [15] requires a predefined symbol level training sequence to be sent from the desired user to minimise the mean square error (MSE).
  • MSE mean square error
  • the TDD-CDMA standard does not support such a sequence.
  • the LMMSE is known to suffer from phase error problems in channels suffering deep multipath fades [16].
  • a pre-Rake structure can be applied to either the UL or DL to enhance the performance of both FDD and TDD operation [8].
  • PS- LMMSE blind adaptive LMMSE receiver with dynamic path selection
  • the adaptation is based on time varying channel parameters such as signal to noise ratio (SNR) and RMS delay spread.
  • Blind adaptation operates using decision directed (data-derived) training and aims to subtract Multiple Access Interference (MAI) prior to multipath combining.
  • MAI Multiple Access Interference
  • the simulation assumes the use of QPSK modulation. This modulation is sent over frequency selective fading channels assuming K simultaneously active users. Individual user data is aggregated at the BS. An orthogonal Walsh code of length 16 is used in the spreading process. The wideband channel distortion is modelled using a time variant tapped delay line transformation.
  • the time-variant UL impulse response for the k-th user, h k (f) can be written as:
  • the received waveform is generated as the summation of the signal from each user, which is calculated by the convolution of the user signal with its independent channel.
  • the correlation receiver is implemented as a bank of Matched Filters (MF) that correlate the received signal with the wanted user's code and maximises SNR by the resulting processing gain.
  • MF Matched Filters
  • This structure is only optimum in a single user, non-fading environment.
  • the conventional detector suffers from significant performance degradation in the presence of MAI, resulting in overall inefficiency in the communication system.
  • the following mathematical analysis is based on BPSK (although simulations make use of QPSK).
  • the BER of a conventional receiver for the k-th user, p c can be approximated by [17]:
  • N the processing gain
  • Vk- defines the ratio of the received signal strength of the k'-th user to the received signal strength of the desired k-th user
  • 2 ⁇ f " t - e " 2 / dt (7)
  • the Rake is a powerful technique for combating the effects of multipath fading.
  • the error probability using a Rake at large SNR values can be approximated as [3]
  • r (,) represents the received signal vector over a processing window for the i-th symbol interval and y k , ⁇ (l) represents the i-th symbol, on the 1-th branch, for the k-th user.
  • W k, ⁇ are the complex coefficients of the Q-tap LMMSE detector for the 1-th branch of the k-th user. The coefficients are chosen to minimise the MSE, which depends on several random quantities, such as the cross-correlation, the time offsets, and the power levels of the received signals.
  • the MSE is defined as:
  • ⁇ is the variance of the desired response and Jk, ⁇ is the final steady-state mean squared error for the adaptive algorithm. This equation can be used to estimate the impact of the noise and the interference floor on the receiver performance.
  • Figure 12 demonstrates the UL simulation results for the different receiver structures in a medium RMS delay spread channel.
  • the mean of the first path is marginally stronger than the second path, however both have similar relative strengths.
  • MAI will result from the asynchronous multi-user UL transmissions. This largely results from the independent nature of the multipath propagation channels, which enables the strongest path to occur at different times for different users.
  • Figure 14 demonstrates that the PS-LMMSE can significantly improve the performance and capacity of a multi-user TDD-CDMA system relative to the MF or Rake.
  • the Rake and correlation receiver can support up to six users (37.5% of the processing gain).
  • the PS-LMMSE can achieve up to nine users (56% of processing gain).
  • the PS-LMMSE and NPS-LMMSE achieves a BER of 1.6xl0 "3 , which is worse than the Rake (BER of 10 "3 ).
  • the MF offers the worst performance with a BER of 10 '2 .
  • Figure 16 shows the UL BER behaviour versus Eb/N 0 for low rms delay spread conditions.
  • the main signal energy arrives on the first path, with other weaker paths having much lower SNR.
  • additional taps have no benefit in the Rake receiver.
  • the results of Figure 16 indicate, once again, that blind LMMSE is far more sensitive to low values of SNR than either the correlation or Rake receiver.
  • E b /N 0 increases, the blind LMMSE becomes feasible, offering excellent results for E b /N 0 values of 19.5 dB or higher. This implies that a reconfigurable receiver is required, using the LMMSE at high E b / 0 values and the MF or Rake at lower values [1].
  • Results show that at low delay spreads, the required minimum E b /N 0 at a target BER of 10 " (15.5dB for the correlation receiver) is much lower than for larger rms delay spread channels (19-20 dB from Figure 12). As mentioned above, in these channels the correlation detector offers good overall UL performance.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Noise Elimination (AREA)

Abstract

L'invention concerne un récepteur de signaux du type AMCR, configuré pour choisir le mode de réception des signaux reçus le plus approprié, sur la base d'un ou de plusieurs attributs de la voie de communications. Les modes de réception possibles comprennent un mode de corrélation, un mode de Rake, et un mode LMMSE. Les attributs de la voie mesurés comprennent et la puissance du parcours dans la voie.
PCT/GB2002/001438 2001-03-26 2002-03-26 Demodulation et desetalement dans des recepteurs amcr WO2002078205A1 (fr)

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GB0107612A GB0107612D0 (en) 2001-03-26 2001-03-26 Demodulating and despreading in cdma receivers

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102004042557A1 (de) * 2004-09-02 2006-03-23 Siemens Ag Verfahren zur Auswahl einer Detektionsvariante für ein von einer Funkstation eines Funkkommunikationssystems über einen Funkkanal empfangenes Signal sowie Funkstation und Computerprogramm
WO2006097742A1 (fr) * 2005-03-18 2006-09-21 Orange Sa Emetteur-recepteur de base, equipement utilisateur mobile et procede de selection d'un mode de reception
EP1995895A1 (fr) * 2006-03-14 2008-11-26 NEC Corporation Appareil de communication radio, procede de selection de systeme de reception et programme de selection de systeme de reception
WO2013052877A1 (fr) * 2011-10-05 2013-04-11 Qualcomm Incorporated Appareil et procédé de sélection entre récepteurs dans système de communication sans fil
US20140016677A1 (en) * 2012-07-16 2014-01-16 Qualcomm Incorporated Method and apparatus to dynamically select ue processing capabilities based on channel impulse response estimates
US8867676B2 (en) 2004-09-17 2014-10-21 Telefonaktiebolaget Lm Ericsson (Publ) Method and apparatus for controlling interference suppressing receivers

Citations (1)

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Publication number Priority date Publication date Assignee Title
US6167031A (en) * 1997-08-29 2000-12-26 Telefonaktiebolaget Lm Ericsson (Publ) Method for selecting a combination of modulation and channel coding schemes in a digital communication system

Patent Citations (1)

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US6167031A (en) * 1997-08-29 2000-12-26 Telefonaktiebolaget Lm Ericsson (Publ) Method for selecting a combination of modulation and channel coding schemes in a digital communication system

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
LATVA-AHO M ET AL: "Reconfigurable adaptive RAKE receiver for wideband CDMA systems", VEHICULAR TECHNOLOGY CONFERENCE, 1998. VTC 98. 48TH IEEE OTTAWA, ONT., CANADA 18-21 MAY 1998, NEW YORK, NY, USA,IEEE, US, 18 May 1998 (1998-05-18), pages 1740 - 1744, XP010288219, ISBN: 0-7803-4320-4 *

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102004042557A1 (de) * 2004-09-02 2006-03-23 Siemens Ag Verfahren zur Auswahl einer Detektionsvariante für ein von einer Funkstation eines Funkkommunikationssystems über einen Funkkanal empfangenes Signal sowie Funkstation und Computerprogramm
DE102004042557B4 (de) * 2004-09-02 2007-03-01 Siemens Ag Verfahren zur Auswahl einer Detektionsvariante für ein von einer Funkstation eines Funkkommunikationssystems über einen Funkkanal empfangenes Signal sowie Funkstation und Computerprogramm
DE102004042557B9 (de) * 2004-09-02 2007-10-04 Siemens Ag Verfahren zur Auswahl einer Detektionsvariante für ein von einer Funkstation eines Funkkommunikationssystems über einen Funkkanal empfangenes Signal sowie Funkstation und Computerprogramm
DE102004064043B4 (de) * 2004-09-02 2008-07-24 Siemens Ag Verfahren zur Auswahl einer Detektionsvariante für ein von einer Funkstation eines Funkkommunikationssystems über einen Funkkanal empfangenes Signal sowie Funkstation und Computerprogramm
US8867676B2 (en) 2004-09-17 2014-10-21 Telefonaktiebolaget Lm Ericsson (Publ) Method and apparatus for controlling interference suppressing receivers
WO2006097742A1 (fr) * 2005-03-18 2006-09-21 Orange Sa Emetteur-recepteur de base, equipement utilisateur mobile et procede de selection d'un mode de reception
EP1995895A1 (fr) * 2006-03-14 2008-11-26 NEC Corporation Appareil de communication radio, procede de selection de systeme de reception et programme de selection de systeme de reception
EP1995895A4 (fr) * 2006-03-14 2014-07-16 Nec Corp Appareil de communication radio, procede de selection de systeme de reception et programme de selection de systeme de reception
WO2013052877A1 (fr) * 2011-10-05 2013-04-11 Qualcomm Incorporated Appareil et procédé de sélection entre récepteurs dans système de communication sans fil
US9294937B2 (en) 2011-10-05 2016-03-22 Qualcomm Incorporated Apparatus and method for selection between receivers in a wireless communication system
US20140016677A1 (en) * 2012-07-16 2014-01-16 Qualcomm Incorporated Method and apparatus to dynamically select ue processing capabilities based on channel impulse response estimates
US9178562B2 (en) * 2012-07-16 2015-11-03 Qualcomm Incorporated Method and apparatus to dynamically select UE processing capabilities based on channel impulse response estimates

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