Markku J. Heikkilä

Chip-level channel equalization in WCDMA downlink

The most important third generation (3G) cellular communications standard is based on wideband CDMA (WCDMA). Receivers based on TDMA style channel equalization at the chip level have been proposed for a WCDMA downlink employing long spreading sequences to ensure adequate performance even with a high number of active users. These receivers equalize the channel prior to despreading, thus restoring the orthogonality of users and resulting in multiple-access interference (MAI) suppression. In this paper, an overview of chip-level channel equalizers is delivered with special attention to adaptation methods suitable for the WCDMA downlink. Numerical examples on the equalizers′ performance are given in Rayleigh fading frequency-selective channels.

Decoding and performance of space-time trellis codes in fading channels with intersymbol interference

The design criteria of space-time trellis codes assume a flat fading channel without intersymbol interference (ISI). In this paper, decoding and performance of space-time codes is examined in a system for which the assumptions made for the criteria are not valid. We investigate a suboptimal joint equalization and decoding technique for space-time trellis codes to be used in frequency-selective fading channels and with pulse shapes causing substantial ISI. The performance of space-time codes is evaluated allowing different receiver complexity levels and compared to delay diversity and single-antenna transmission in a GSM EDGE based system. The results show a performance advantage for space-time coded systems in terms of block error probability. The observed bit error rates, however, indicate that successful concatenation of these codes with outer codes may not be straightforward when compared to simpler delay diversity transmission.

Increasing HSDPA throughput by employing space-time equalization

In the downlink of wideband code division multiple-access (WCDMA) systems, users are assigned orthogonal spreading codes. A multipath radio channel distorts the spreading waveforms causing interference between users. This so-called multiple-access interference is especially harmful in case of data transmission which may require both high quality and high transfer speed. Since the interference is caused by the channel, the most straightforward way of suppressing the interference is to apply linear channel equalization. We apply linear chip-level equalization to high-speed downlink packet access (HSDPA) signal. Moreover, we study the performance improvement when dual-antenna receivers are used in the mobile terminal. Data throughput gains are evaluated in a multipath environment both at link level and at system level for which we present a simple simulation technique.

Performance evaluation of space-time coding for EDGE

In this paper, we evaluate the performance of both space-time trellis codes and block codes in EDGE system against delay diversity. We address the issues related to outer channel coding and equalization of ISI due to a multipath channel and pulse shaping at the transmitter. The results show a performance advantage for a space-time coded system in ISI-free channels. However, in realistic channels with outer channel coding, delay diversity seems to be more applicable in terms of performance and complexity.

Higher data rates with space-time block codes and puncturing in WCDMA systems

In this paper, we evaluate the use of a punctured convolutional code with space-time block codes as a way of achieving higher data rates in WCDMA downlink. The use of more than two transmit antennas is also addressed. The achievable data rates through puncturing are evaluated with a complete UTRA FDD downlink model using a conventional RAKE receiver. The results show that with a space-time block code for two transmit antennas combined with two receiving antennas the date rates can be easily more than doubled by puncturing, when compared to single antenna transmission and reception. However, the use of four antenna space-time block codes does not seem to provide much more if any, additional performance gain compared to two antenna space-time block code.

Comparison of multiantenna techniques for high-speed packet communication

We evaluate the use of multiantenna techniques as a way of achieving higher data rates in WCDMA downlink packet transmission. The evaluation is limited to different open loop transmit diversity techniques using two, three and four transmit antennas without any knowledge of the wireless channel at the transmitter. The achievable data rates through multiantenna techniques are evaluated as link level throughputs with a UTRA FDD downlink model for HSDPA, using a conventional RAKE receiver with one and two receiving antennas. The model includes H-ARQ, while the dynamic link adaptation is not modelled explicitly. The results show that most of the performance gain is achieved with additional receiving antenna, while the gain from transmit diversity is rather limited and in some cases even negative.

CDMA equalizer quantization

Our topic is channel equalizer quantization for a cellular handset. We answer questions regarding quantization of variables internal to the equalizer using some estimated bounds and simulation. This includes the catastrophic event of breakdown due to a roundoff-induced singularity in Cholesky factorization. Realistic modeling of the received waveform and channel estimation includes quantization effects, as well as quantization of the taps used in the operation of the equalizer. Modem results are given in terms of frame error rate and throughput for various quantization scenarios.

Space-time trellis coding with turbo equalisation for the EGPRS system

Transmit diversity techniques like delay diversity (DD) or space-time trellis coding (STTC) utilise several transmission antennas and thereby protect mobile radio systems against fast fading. The more complex STTC can recover bursts with a few errors, but on the other hand it increases bit errors for bad bursts. Therefore the concatenation of an outer convolutional code and STTC is not superior to the simple DD scheme. The outer coding is commonly used in mobile packet data applications to detect and correct transmission errors. We are interested in further improving STTC performance by the iterative turbo equalisation (TE) technique, which performs space-time decoding-equalisation and outer decoding in an iterative fashion. Our objective is to analyse whether TE can exploit the more complex STTC structure by comparing the iterative gains of STTC and DD. The performance evaluation is done by simulations in the Enhanced General Packet Radio System (EGPRS) platform.