Petri Komulainen

Decentralized Minimum Power Multi-Cell Beamforming with Limited Backhaul Signaling

A decentralized solution is proposed for the coordinated multi-cell multi-antenna minimum power beamformer design problem with single-antenna users. The optimal minimum power beamformers can be obtained locally at each base station (BS) relying on limited backhaul information exchange between BSs. The original centralized problem is reformulated such that the BSs are coupled by real-valued inter-cell interference terms. The coupling is handled by taking local copies of the interference terms at each BS and enforcing consistency between them. The consistency constraints are then decoupled by a standard dual decomposition approach leading to a distributed algorithm. The proposed method is able to guarantee feasible solutions even if the interference information is outdated or incomplete. In addition, the proposed approach allows for a number of special cases, where the backhaul information exchange is reduced at the cost of somewhat sub-optimal performance. The performance of the proposed coordinated multi-cell transmission is compared with the coherent multi-cell beamforming and with inter-cell interference nulling in different scenarios with varying interference. A near-optimal performance can be achieved even with significantly reduced backhaul information exchange and with relatively high velocities and/or low backhaul signaling rates.

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.

Performance evaluation of superorthogonal turbo codes in AWGN and flat Rayleigh fading channels

Turbo codes are parallel concatenated codes whose performance in the additive white Gaussian noise (AWGN) channel has been shown to be near the theoretical limit. In this paper, we describe a low-rate superorthogonal turbo code that combines the principles of low-rate convolutional coding and that of parallel concatenation. Due to the bandwidth expansion, this code outperforms the ordinary turbo code both in AWGN and especially in fading channels. Thus, superorthogonal turbo codes are suited mainly for spread-spectrum applications. For the purposes of iterative decoding, we concisely describe the connection between the optimal maximum a posteriori symbol estimation and suboptimal soft-output decoding based on sequence estimation. The suboptimal decoder produces outputs that can directly be used as additive metrics at successive decoding iterations, without the need for estimating channel noise variance. Simulation results in AWGN and flat Rayleigh fading channels are also presented, along with analytical upper bounds of bit- and frame-error probabilities.

Effective CSI Signaling and Decentralized Beam Coordination in TDD Multi-Cell MIMO Systems

We propose decentralized algorithms and corresponding signaling concepts of effective channel state information (CSI) for weighted sum rate (WSR) maximization via linear downlink transmit-receive beamforming in multi-cell multi-user MIMO systems operating in the time-division duplex (TDD) mode. The iterative processing consists of optimization steps that are run locally by base stations (BS), and facilitated by a combination of over-the-air uplink pilot signaling and scalar backhaul information exchange. In the first novel strategy, the coordinating cells update their transmit precoders and receivers by executing a cell-specific optimization loop one cell at a time, which guarantees monotonic convergence of the network-wide problem. The strategy employs separate uplink channel sounding (CS) and busy burst (BB) signaling to reveal the effective channels of the terminals to the neighboring BSs. In the second strategy, we sacrifice the monotonic convergence and devise a faster scheme where the BSs are allowed to optimize their variables in parallel based on just the CS responses and additional backhaul information. We make use of the optimization framework where the WSR maximization is carried out via weighted sum mean-squared-error (MSE) minimization, and generalize the approach by employing antenna-specific transmit power constraints. The numerical results demonstrate that WSR maximization has the desirable property that spatial scheduling, or user and beam selection, is carried out implicitly.

SINR Balancing with Coordinated Multi-Cell Transmission

Coordinated multi-cell processing facilitates multi-user precoding techniques across distributed base station (BS) antenna heads. Hence, it can efficiently exploit the available spatial degrees of freedom in a multi-user multiple-input multiple-output (MIMO) channel. A generalised method for joint design of linear transceivers with coordinated multi-cell processing subject to per-BS/antenna power constraints is proposed. The system optimisation objective is to balance the weighted SINR across all the transmitted data streams. The method can accommodate a variety of scenarios from coherent multi-cell beamforming across a large virtual MIMO channel to single-cell beamforming with inter-cell interference coordination and beam allocation. The performance of different coherent/non-coherent and coordinated/noncoordinated multi-cell transmission methods with optimal and heuristic beam allocation algorithms is numerically compared in different scenarios with varying inter-cell interference. The coherent multi-cell beamforming greatly outperforms the non-coherent cases, especially at the cell edge and with a full spatial load. However, the coordinated single-cell transmission with interference avoidance and dynamic beam allocation performs considerably well with a partial spatial loading.

On the Value of Coherent and Coordinated Multi-Cell Transmission

Coordinated multi-cell processing facilitates multi- user precoding techniques across distributed base station (BS) antenna heads. This paper evaluates the performance of different coordinated multi-cell transmission methods with several channel allocation algorithms in different scenarios with varying inter- cell interference. A generalised method for joint design of linear transceivers with coordinated multi-cell processing subject to per-BS power constraints is provided for single antenna receivers. The method can accommodate a variety of scenarios from coherent multi-cell beamforming across a large virtual multiantenna broadcast channel to single-cell beamforming with inter-cell interference coordination and channel allocation. The system optimisation objectives are weighted SINR maximisation across all the transmitted data streams and weighted sum rate maximisation. The numerical examples demonstrate the value of the coherent multi-cell beamforming, which greatly outperforms the non-coherent cases especially at the cell edge. The coordinated single-cell transmission with interference avoidance and dynamic user allocation is also shown to perform relatively well with a partial spatial loading.

Adaptive channel equalization and interference suppression for CDMA downlink

In the synchronous CDMA downlink using orthogonal codes, the intracell multiple access interference (MAI) is essentially caused by the frequency selective multipath channel. Therefore, intracell MAI may be suppressed by simple linear channel equalization. Similarly, intercell interference can be suppressed by means of linear space-time filtering. We compare two different adaptive receiver algorithms in the context of one or several receive antenna elements. The first one is based on adaptive LMMSE chip estimation, whereas the other one performs simple interchip interference cancellation by adaptive chip separation. The presented receiver structures are suitable for systems with long code scrambling, such as 3rd generation wideband CDMA standard. The results show considerable performance gains when compared to the conventional RAKE receiver.

Distributed Coordinated Multi-Cell Transmission Based on Dual Decomposition

A distributed solution is proposed for the coordinated multi-cell multi-antenna minimum power beamformer design problem with single-antenna users. The minimum power beamformers are obtained locally at each base station (BS) relying on limited backhaul information exchange between adjacent BSs. The original centralised problem is reformulated such that the BSs are coupled by real-valued inter-cell interference terms. The coupled interference terms are handled by taking local copies of the terms at each BS and enforcing consistency between them. The consistency constraints are then decoupled by a standard dual decomposition approach leading to a distributed algorithm. The proposed method is able to guarantee feasible solutions even if the interference information is outdated or incomplete, at the possible cost of increased sum power. In addition, the proposed approach allows for a number of special cases, where the backhaul information exchange is reduced at the cost of somewhat sub-optimal performance. The performance of the proposed coordinated multi-cell transmission is compared with the coherent multi-cell beamforming, as well as, with inter-cell interference nulling in different scenarios with varying interference. The numerical examples demonstrate that a near-optimal performance can be achieved even with significantly reduced backhaul information exchange.

Distributed implementation of coordinated multi-cell beamforming

A distributed implementation of coordinated multi-cell beamforming with single-antenna users in a time-correlated fading scenario is considered. The minimum power beamformers are obtained locally at each base station (BS) relying on limited backhaul information exchange between adjacent BSs. The original centralised problem is reformulated such that the BSs are coupled by real-valued inter-cell interference terms. The problem is then decoupled by a standard dual decomposition approach leading to a distributed algorithm. The method is able to guarantee feasible solutions even if the interference information is incomplete or outdated. Occasional high peaks in the transmitted power due to outdated interference terms are alleviated by switching to interference nulling mode when necessary. The performance of the proposed coordinated multi-cell transmission is compared with the coherent multi-cell beamforming, as well as, with interference nulling in different time-correlated fading scenarios with varying interference. The numerical examples demonstrate that a near-optimal performance can be achieved even with significantly reduced backhaul information exchange and with relatively high velocities and/or low signalling rates.

A low-complexity superorthogonal turbo-code for CDMA applications

Turbo-codes are parallel concatenated convolutional codes (PCCC) whose performance in an AWGN channel has been shown to be near the Shannon limit. We describe a low rate superorthogonal turbo-code that outperforms the ordinary turbo-code at the expense of bandwidth expansion both in AWGN and especially in fading channels. Thus superorthogonal turbo-codes are suited mainly for e.g. CDMA applications. We concisely describe the soft output Viterbi (1995) algorithm using a priori inputs (APRI-SOVA) with connection to optimal maximum a posteriori (MAP) symbol estimation. Simulation results in AWGN and fading channels are also presented.