Eko N. Onggosanusi

Performance analysis of closed-loop transmit diversity in the presence of feedback delay

In this paper, a bit-error-rate (BER) analysis for closed-loop transmit diversity in a time-selective Rayleigh fading channel containing feedback delay is presented. In the absence of feedback delay, closed-loop transmit diversity always outperforms open-loop transmit for a given transmitted signal energy. This is no longer true in the presence of feedback delay. We derive closed-form expressions of the average BER for this case assuming QPSK and BPSK signaling. The results of the analysis are instrumental for comparing closed-loop with open-loop schemes under given operating conditions. In particular, we demonstrate that, for a given transmitted energy and number of transmit antennas, open-loop outperforms closed-loop at sufficiently fast channel fading. We also show that, for a given transmitted signal energy and fading rate, closed-loop outperforms open-loop for sufficiently large numbers of transmit antennas while the total average transmitted signal energy is kept constant. For some special cases, closed-form expressions for the fading rate at which the performance of open-loop is equal to closed-loop are obtained.

High rate space-time block coded scheme: performance and improvement in correlated fading channels

In this paper, we study the performance of space-time block coding, also known as space-time transmit diversity (STTD) scheme for high data rate applications with 4 transmit antennas. The scheme consists of two STTD units at the transmitter and an interference-resistant detector at the receiver, termed the double-STTD (DSTTD) scheme. We compare the performance of DSTTD to general multi-input multi-output (GMIMO) signaling for a given data rate and under different channel correlation profiles. It is demonstrated that DSTTD outperforms GMIMO in all cases. In addition, additional performance gain can be attained for DSTTD in correlated channels by selectively ordering the transmit antennas to minimize the average interference terms.

Hybrid ARQ transmission and combining for MIMO systems

In this paper, we investigate several possibilities to increase the efficiency of hybrid automatic retransmission request (HARQ) for multi-input multi-output (MIMO) systems. At the receiver side, two HARQ combining schemes, namely pre-combining and post-combining, are discussed and analyzed. We show that pre-combining is superior to post-combining. In addition, we propose a transmission technique, termed the basis hopping, which improves the HARQ diversity gain especially in slow fading channels. We demonstrate that the use of pre-combining in conjunction with the proposed basis hopping technique provides dramatic gain over a simplistic application of HARQ scheme for MIMO.

Efficient signaling schemes for wideband space-time wireless channels using channel state information

An orthogonal decomposition of a general wideband space-time frequency selective channel is derived assuming antenna arrays at both the transmitter and receiver. Knowledge of channel state information is assumed at both the transmitter and receiver. The decomposition provides a framework for efficiently managing the degrees of freedom in the space-time channel to optimize any combination of bit-error rate and throughput in single-user or multiuser applications. The decomposition is used to derive efficient signaling schemes and receiver structures for a variety of scenarios. For a fixed throughput system, we investigate a power allocation scheme that minimizes the effective bit-error rate. In addition, a strategy to maximize the throughput under a worst-case bit-error rate constraint is proposed. For multiuser applications, we propose a signaling scheme that achieves orthogonality among users by exploiting the temporal channel modes which are common to all users. The effect of imperfect channel state information at the transmitter is also investigated.

Capacity analysis of frequency-selective MIMO channels with sub-optimal detectors

In this paper, a capacity analysis of multi-input multi-out (MIMO) channels with multipath assuming different types of sub-optimal detectors is presented. The results are instrumental for evaluating the potential of multipath MIMO channels when sub-optimal detectors are utilized. It is demonstrated that for some types of detectors, an increase in the number of paths may result in some performance loss, especially at low transmit power. In addition, it is shown that a type of iterative detector can attain near-capacity performance when highly reliable decision feedback is used.

From homogeneous to heterogeneous networks: A 3GPP Long Term Evolution rel. 8/9 case study

Heterogeneous cellular networks (HetNets)—conventional macro base stations overlaid with low-power nodes—are being deployed for reasons of improved spectral efficiency, economy, and scalable network expansion. To optimally realize the benefits of HetNets, it is crucial to factor deployment aspects such as cell association, transmission power, spectrum allocation, and deployment density. This paper presents comprehensive dynamic system level simulations based on the 3GPP Long Term Evolution (LTE) standard modeling spatial, temporal, and frequency domain channel variations and ray-tracing based penetration losses in urban and indoor environments. The simulator takes into account the dynamic (subframe level) modulation and coding scheme and precoder selection, as well as finite rate channel state feedback and transmission aspects intrinsic to LTE. Considering a Rel. 8/9 LTE HetNet, two access schemes are analyzed: first, open access picocells, where results show that a single picocell can double the area spectral efficiency without any impairment of the macro-layer, and second, closed access femtocells, where results show that in typical scenarios, the user throughput is deteriorated on both the macro-layer and the femto-layer. Consequently, open access low-power nodes can be deployed as is, whilst closed access ones require additional interference avoidance mechanisms. These dynamic system level simulations are important for deriving accurate and practically relevant performance trade-offs, while commensurate with the time and frequency granularity of LTE.

A feedback-based adaptive multi-input multi-output signaling scheme

Achieving MIMO capacity requires knowledge of the channel state at the transmitter, which is impractical in systems with a limited feedback resource and high channel dynamics. The paper presents a feedback-based adaptive scheme utilizing a set of predetermined linear transformations. Given a channel realization, a transformation is chosen at the receiver based on an optimality criterion. The selected transformation is signaled to the transmitter with a minimum amount of feedback. We demonstrate that, with judicious choice of optimality criterion and transformation set, the proposed scheme provides 4.5 dB SNR gain over the open-loop MIMO scheme for a 2/spl times/2 scenario with only 2 bit feedback.

Optimal antenna diversity signaling for wide-band systems utilizing channel side information

Optimal multi-antenna wide-band signaling schemes are derived for multipath channels assuming perfect channel state information at the transmitter. The scheme that minimizes the bit-error probability in the single-user case is a rank-one space-time beamformer which focuses the signal transmission in the direction of the most dominant channel mode. Several suboptimal variations are discussed for multiuser applications. The optimal signaling scheme given channel statistics at the transmitter is also derived. The optimal scheme in this case is a full-rank space-time beamformer that transmits on all channel modes. Analysis and simulation results are used to compare the schemes proposed in this paper. Finally, we discuss the optimal signaling scheme when a delayed version of the channel state is available at the transmitter. It is shown that in this case the optimal scheme is a rank-1 beamformer when the channel variations are sufficiently slow and is a full rank beamformer in a sufficiently fast fading channel.

Nested codebook design for MIMO precoders

In multi-input, multi-output (MIMO) communication systems, significant throughput gains can be obtained by closed-loop operation, where two aspects of the transmission are adapted to channel conditions: the transmission rank (number of independent spatial layers); and the precoding matrix which maps the spatial layers to the transmit antennas. To facilitate feedback and signaling, the precoding matrix for each rank is often restricted to a finite pre-determined codebook. Prior research has studied the problem of designing the codebook independently for each rank, with the aim of maximizing throughput gains for a given codebook size. In this paper, we propose to jointly design the family of codebooks for various transmission ranks. Our nested codebooks greatly simplify both codebook design and the selection of precoding matrix for a specific channel. In addition, we prove by analysis and simulation results that the complexity reduction is obtained without sacrificing throughput gains. Due to these advantages, the proposed structured codeboook design has been accepted for use in the 3GPP Long Term Evolution (LTE) standard.

Single and multi-cell scheduling in coordinated multi-point distributed antenna systems

Multi-cell coordination is a technique for interference mitigation in cellular systems. For joint-transmission (JT), the received signal power is boosted by diversity combining of signals from multiple transmission points. This however results in reduced frequency-reuse ratio and cell-splitting order, which may subsequently degrade the network-level throughput. In order to strike a balance between the diversity combining gain and frequency-reuse ratio, in this paper we propose a dynamic scheduling algorithm that adaptively schedules users in single-cell or multi-cell transmission mode to maximize the system sum throughput. Furthermore, basestation association for each user is meticulously addressed to avoid negatively impacting the frequency-reuse gain. The proposed algorithm is evaluated with system-level evaluation tools fully compliant with 3GPP LTE specification, thereby providing a realistic assessment on the gain of multi-cell coordination in terms of system capacity and coverage.