Rene Habendorf

Hardware-software codesign of a 14.4 MBit - 64 state - Viterbi decoder for an application-specific digital signal processor

Viterbi decoders are employed frequently in wireless radio systems. They often require high computational power which can only be handled by dedicated application specific integrated circuits (ASICs). Because of their flexibility and speed of development, DSP-based software solutions are desirable, however. Currently available DSPs are unable to provide enough computational power to perform as the Viterbi decoder in systems such as digital video broadcasting (DVB) where a DSP needs to perform 64-state Viterbi decoding at 4-30 MBit/s as well as other receiver algorithms. Hence, we strive to provide ways of increasing DSP computational power. One way is to provide multiple, parallel, data paths in a processor. Two different methods of parallel computation of a Viterbi decoder are presented and their DSP architecture requirements are analyzed. These methods are not only applicable for the Viterbi decoder in DVB systems, but for all terminated convolutional codes. We then derive a data path design for a highly parallel DSP. We introduce special instructions which not only speed up the computation of Viterbi decoders, but are also beneficial for computing a fast Fourier transform. This data path design can calculate an add-compare-select butterfly in two cycles and thus allows the DSP to perform the computation of the Viterbi decoder for the current German variant of DVB-T (14.4 Mbit/s) at a modest 70 MIPS. This leaves enough computational power to perform other receiver algorithms at a targeted clock rate of 200 MHz.

Vector precoding with bounded complexity

We investigate nonlinear pre-equalization techniques for the downlink of multiuser systems from a transmitter equipped with multiple antennas to non-cooperative single antenna mobile receivers. In the focus of this paper is the strongly varying complexity of the tree-structured vector precoding problem. Since the maximum latency of signal processing algorithms is of major interest for practical implementations, suboptimum schemes with an upper bounded complexity are discussed. Simulation results reveal a close to optimum performance at the expense of moderate computational efforts for the considered scenario.

Nonlinear multiuser precoding for non-connected decision regions

Spread spectrum CDMA systems suffer from interference in frequency-selective channels. By applying multiuser transmission (MUT) methods in the base station in conjunction with smart antennas, the interference can be mitigated and the downlink (DL) cell capacity can be substantially increased while keeping the complexity of the battery operated mobile devices within a limit. Besides linear multiuser transmission, known as zero forcing joint transmission and transmit Wiener filters, the nonlinear Tomlinson Harashima precoding (THP) was proposed. The contribution of this paper is the extension of THP with a search algorithm exploiting the degrees of freedom given by the equivalent non-connected decision regions. The simulation environment is similar lo the 3GPP TDD CDMA standard extended with the HSDPA mode.

Overloaded TDD-CDMA cells with multiuser transmission

The number of supportable users in the TDD-CDMA downlink is usually limited by multiple-access interference and by the limited number of available orthogonal spreading codes. By using multiuser transmission (MUT) methods in the base station in conjunction with smart antennas, the interference can be mitigated and the downlink cell capacity can be substantially increased. The number of supported users and the necessary transmit power in the CDMA downlink with linear MUT methods, like joint transmission and nonlinear methods like minimum bit error rate multiuser transmission is investigated. By co-locating different scrambling code groups in one base station, the number of supported users is roughly proportional to the number of applied antennas, but the spatio-temporal dimension can also be exceeded in certain circumstances. Furthermore, a simplified phase-only minimum bit error rate multiuser transmission is proposed, including the first and second order analytic BER derivatives. The simulation environment is similar to 3GPP TDD CDMA or the Chinese TD-SCDMA.

On Ordering Optimization for MIMO Systems with Decentralized Receivers

This paper addresses Tomlinson-Harashima precoding (THP) for the downlink of multiuser systems, where the transmitter is equipped with multiple antennas and each decentralized receiver has a single antenna. Although the performance of THP strongly depends on the ordering of the precoded symbols, current THP approaches apply data independent sorting algorithms. The contribution of this paper is the extension of THP with an algorithm exploiting the knowledge of the data symbols to compute the optimum stream ordering for precoding

Pre-Equalization for TDD Systems with Imperfect Transceiver Calibration

We investigate nonlinear pre-equalization techniques for the downlink of multi-user systems from a transmitter equipped with multiple antennas to non-cooperative single antenna mobile receivers. The contribution of this paper is the derivation of minimum mean square error (MMSE) pre- equalization for imperfect transceiver calibration at the base station and the resulting nonreciprocal channels. The performance of the proposed schemes is investigated numerically in both, uncoded and coded transmission.

Multiuser Transmission in Cellular Systems With Different Sector Configurations

This paper considers the downlink for multiuser systems with a multiple antenna base station site and non-cooperative single antenna mobile stations. Traditionally, the goal of the base station site setup is user separation. We compare different deployment setups with respect to their achievable performance for several possible transmission schemes. A shift in strategy from separated sectors to overlapping sectors is proposed. Here, multiuser preceding techniques like Tomlinson-Harashima preceding (THP) allow for the exploitation of the available diversity and therefore offer large gains compared to the traditional separated sectors approach.

Nonlinear Predistortion for OFDM SDMA Systems

This paper investigates nonlinear predistortion techniques for the downlink of OFDM systems from a transmitter equipped with multiple antennas to non-cooperative single antenna mobile stations. Although the performance of nonlinear predistortion schemes like Tomlinson Harashima precoding (THP) strongly depends on the ordering of the precoded symbols, current THP approaches apply data independent sorting algorithms. A tree search algorithm is proposed which combines matrix decomposition and the feedback structure of the precoder to compute a data dependent precoding order

Transmitter-Based Minimization of Error Rates in the Downlink of Wireless Systems

Using CDMA technology, one has to contend with interference in frequency-selective channels caused by multipath propagation and fading. Transmitter-based methods such as zero forcing joint transmission and transmit Wiener filter only reduce the bit error rate indirectly and do not exploit the knowledge of the transmit data symbols. Nonlinear optimization strategies, e.g., the Constrained Minimum BER MUT approach, outperform such methods while the computational complexity prevents their implementation. In this paper, a spread-spectrum TDD-CDMA downlink transmission system with 4-QAM modulation is considered, whereas perfect channel knowledge both in the transmitter and the receiver is assumed. Deriving a convex optimization problem, an alternative solution method with considerable decrease of the complexity is proposed.

SPC09-5: Reduced Complexity Vector Precoding for the Multiuser Downlink

We investigate nonlinear preceding techniques for the downlink of multi-user systems from a transmitter equipped with multiple antennas to non-cooperative single antenna mobile receivers. The contribution of this paper is the extension of the tree-structured vector precoding algorithm with a search radius estimation based on the statistics of the preceded symbols to reduce the number of evaluated nodes. Simulation results reveal a significant reduction of computational complexity at the expense of only slight performance degradations.