Joachim Wehinger

Fast Link Adaptation for MIMO OFDM

We investigate link-quality metrics (LQMs) based on raw bit-error-rate, effective signal-to-interference-plus-noise ratio, and mutual information (MI) for the purpose of fast link adaptation (LA) in communication systems employing orthogonal frequency-division multiplexing and multiple-input-multiple-output (MIMO) antenna technology. From these LQMs, the packet error rate (PER) can be estimated and exploited to select the modulation and coding scheme (MCS) among a class of candidate MCSs that achieves the maximum throughput for the current channel state under a specified target PER objective. We propose a novel MI-based LQM and compare the PER-estimation accuracy obtained with this LQM with that resulting from using other LQMs by means of comprehensive Monte Carlo simulations. Search methods for the MCS in the class that is most suitable for a given channel state are presented. An algorithm for obtaining a practical upper bound on the throughput of any LA scheme is proposed. The investigated LQMs are applied to the IEEE 802.11n standard with a 2 × 2 MIMO configuration and practical channel estimation. The proposed MI-based LQM yields the highest PER estimation accuracy, and its throughput shows only 1.7 dB signal-to-noise-ratio (SNR) loss with respect to the upper bound but up to 9.5 dB SNR gain, compared to the MCS maximizing the throughput for the current noise variance.

Iterative channel estimation and data detection in frequency‐selective fading MIMO channels

Signals transmitted through multiple-input multiple-output (MIMO) wireless channels buffer from multiple-access interference (MAI), multipath propagation and additive noise. Iterative multiuser algorithms mitigate these signal impairments. while offering a good tradeoff between performance and complexity. The receiver presented in this paper performs channel estimation. multiuser detection and decoding in an iterative manner. The estimation of the frequeucy selective. block-fading channel is initiated with the pilot symbols. In subsequent iterations. soft decisions of all the data symbols are used in an appropriate way to improve the channel estimates. This approach leads, to significant Improvement of the overall receiver performance, compared to other schemes. The bit-error-rate (BER) performance of the receiver is evaluated by simulations for different parameter setups. Copyright (C) 2004 AEI. (Less)

Novel SINR-to-CQI Mapping Maximizing the Throughput in HSDPA

The HSDPA extension of UMTS promises an improved cell capacity and throughput. This enhancement is mainly achieved by three newly introduced techniques, namely adaptive modulation and coding, hybrid automatic repeat request, and fast scheduling. The used coding and modulation scheme depends on the transmitted transport block size, which in turn is signalled via the channel quality indicator (CQI). The CQI values are calculated with the help of SINRs at each User Equipments and fed back to the base station. An appropriate CQI estimation is necessary to transmit data packets with transport block sizes that maximize the throughput. Therefore, in this work we propose a novel SINR-to-CQI mapping which achieves optimum throughput for selected ITU propagation channel models.

Improved channel estimation for iterative receivers

In iterative receiver structures, soft information becomes available after the decoding stage. This information is used to enhance the quality of the channel estimates for the next iteration. We derive a generalized estimator based on the linear minimum mean square error (LMMSE) principle for deterministic pilot information combined with soft information. We present the special case of multi-carrier code division multiple access (MC-CDMA) in detail and provide simulation results. The presented channel estimation algorithm can be also applied to direct sequence (DS)-CDMA and multiple-input multiple-output (MIMO) systems.

Iterative detection and channel estimation for MC-CDMA

Multi-carrier code division multiple access (MC-CDMA) systems are under intense investigation for high bit rate wireless communications systems. Their equalization is based on the fast Fourier transform, allowing for an efficient implementation. Iterative receivers with joint detection and decoding have been shown to achieve very good performance for direct-sequence (DS)-CDMA systems. We apply this concept to MC-CDMA, the multiuser detector is implemented as parallel interference canceller with post-minimum mean squared error filtering. In this contribution a new pilot based channel estimation scheme based on random time sequences is developed. The presented simulation results for a multi path scenario show, that in a fully loaded system with 64 users the single user bound can be approached up to 1 dB. A bit error rate (BER) of 10/sup -3/ is reached already at an E/sub b//N/sub 0/ of 12 dB with a 4 state, rate 1/2 convolutional code.

Mutual Information Metrics for Fast Link Adaptation in IEEE 802.11n

We investigate link quality metrics (LQMs) based on mutual information (MI) for fast link adaptation (FLA) in IEEE 802.11n wireless local area network (WLAN) with convolutional coding and higher order modulations. The LQMs are scalar quantities that map the receivers post-decoding behaviour in a frequency-selective channel into an equivalent behaviour in an AWGN channel. From this metric the expected packet error rate (PER) can be predicted. Two LQMs are presented that are derived from the SINRs of all sub-carriers and streams: the effective SNR and the effective mutual information. The effective mutual information is the mean mutual information including a correction factor which improves the PER estimation accuracy in various highly frequency-selective channels. The FLA algorithm dynamically selects a modulation and coding scheme (MCS) that maximizes the throughput (TP), while keeping the average PER over time below a target value. Furthermore, we present methods of searching for the most suitable MCS. Practical performance bounds are obtained by means of simulations. We show that both investigated LQMs yield accurate PER estimates and that the resulting TP of the FLA algorithm used in a 2 x 2 MIMO bit-interleaved coded modulation (BICM) OFDM system that performs channel estimation is only 1.7 dB from the performance bound of the TP.

Throughput enhancement by cancellation of synchronization and pilot channel for UMTS high speed downlink packet access

In this paper, we show by simulations that the interference generated by the synchronization channel (SCH) and common pilot channel (CPICH) has considerable impact on the throughput of the high-speed physical downlink shared channel (HS-PDSCH) in UMTS. First, the simulation model for high speed downlink packet access (HSDPA) in UMTS is described. The downlink physical layer for the user data on the HS-PDSCH is modelled in detail, whereas the associated signalling is idealised as error-free. The interference from signalling and data channels on the HS-PDSCH is modelled adequately. As a general guideline for system settings in the simulations, we use the test cases specified in 3GPP TS 25.101 whenever applicable. Cancelling the SCH interference at the mobile side leads to an improvement of approx. 1 dB on an additive white Gaussian noise channel. Therefore, it is worthwhile to implement interference cancellation (IC) for the SCH and CPICH in future mobile receivers for HSDPA.

On channel estimators for iterative CDMA multiuser receivers in flat Rayleigh fading

In this work we compare the channel estimation algorithms for use in an iterative CDMA receiver in a block fading environment. The receiver consists of a soft multiuser data estimator, a bank of single user decoders, und a multiuser channel estimator. The multiuser data estimator is implemented as parallel interference canceler with unconditional post-MMSE filtering (PIC-MMSE) and the decoder is a soft-in soft-out MAP decoder. In the channel estimator we make use of dedicated pilot symbols and fed back soft-code symbols which are exploited as additional soft pilot symbols when the iterations proceed. We show that using extrinsic information increases the receiver performance significantly compared to using a posteriori information in the feedback for channel estimation. We introduce a linear MMSE (LMMSE) estimator which takes into account the variances of fed back code symbols and compare it to approximations of the least-squares (ALS) estimator and the linear minimum-mean-square-error (ALMMSE) estimator. Performance results are illustrated in terms of bit error rate (BER) and average normalized square error (ANSE) of the channel estimators. They show that the newly proposed LMMSE algorithm outperforms the ALS and ALMMSE algorithms.

Estimators for Time-Variant Channels Applied to UMTS-HSDPA

In this work we analyze the mean-square error (MSE) of two correlative channel estimators in time-variant channels. Both estimators make use of orthogonal basis functions that approximate the time-variant channel. The first approximation is based on a set of time shifted rectangular basis (RB) functions. The second approximation makes use of Slepian basis (SB) functions. We calculate the number of basis functions that minimize the MSE for the RB-and the SB-approach. Both estimators are designed for the full velocity range using the maximum velocity as design parameter only. Further, we apply the two estimators to the high-speed downlink packet access (HSDPA) mode of UMTS and illustrate the impact of the MSE on the throughput. With the SB-approach we observe a gain of roughly 0.4 dB over the RB-functions in case of an i.i.d. Rayleigh multipath channel with five taps

Two-stage space-time receiver for UMTS frequency division duplex

In this paper we examine a space-time multiuser receiver with two parallel interference cancellation (PIC) stages for the uplink in the UMTS-FDD mode. The structure has a performance which is better than the one of the maximum-ratio combiner (MRC) and can reach the single-user performance under the assumption of perfect channel knowledge. Even with channel estimation the proposed receiver achieves a very good performance while maintaining low complexity