Patrick Svedman

Opportunistic Beamforming and Scheduling for OFDMA Systems

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity

A simplified opportunistic feedback and scheduling scheme for OFDM

Opportunistic beamforming schedules users when they experience a high instantaneous signal-to-noise ratio. Multiple antennas at the transmitter can he used to induce temporal fading to ensure that all users fade at a rate fast enough to ensure fairness. Because feedback is required, the fading rate must be fast enough to ensure fairness among users, but slow enough so that the feedback information is not out of date. An OFDM system with opportunistic beamforming has the advantage that multiple users can be scheduled at the same time in a frequency-selective channel, thus allowing a slower fading rate. However, the overhead cost of feeding back every sub-carrier for every user is very high. We propose a simplified opportunistic feedback scheme that divides the OFDM symbol into clusters. Each user feeds back a figure-of-merit listing its strongest clusters. This scheme greatly reduces the feedback overhead, without sacrificing performance significantly. In addition, the scheme has an inherent on/off waterfilling property. We compare this simplified feedback scheme in a HIPERLAN/2 scenario to a feeding back of all subcarriers and also to a smart antenna system and show that, when there are many users, it outperforms the smart antenna system.

Cross-layer scheduling for multi-user MIMO systems

The increasing demand of wireless services associated with the scarcity of the radio spectrum and the trend to provide end-to-end QoS in emerging and future applications calls for the design of spectrally efficient systems with QoS support. To fulfill these two requirements of spectral efficiency and QoS provision in the highly dynamic environment of mobile radio requires the collaboration of several layers in the system as well as the use of multiple transmit and receive antennas. In a packet network, one important component to achieve the aforementioned efficiency goals is a properly designed scheduling algorithm. Using an information-theoretic framework, we present an overview of the issues associated with the design of packet scheduling algorithms and review several proposed solutions. The article focuses on the advantages of a cross-layer approach to the resource allocation problem and identifies the trade-offs associated with the increased signaling needs. Also, we thoroughly discuss the additional degrees of freedom that multiple transmit and receive antennas can provide

A QoS-aware proportional fair scheduler for opportunistic OFDM

OFDM is an attractive technique to implement multiuser diversity for the downlink. This paper deals with the problem of combining the increased cell throughput of multiuser diversity schemes with a fair distribution of resources among the users. A proportional fair scheduler for opportunistic OFDM is proposed. Its key features are that it can accommodate several quality-of-service (QoS) classes, that it has a tunable fairness level, and that it can be integrated with an opportunistic beamformer to increase system fairness. Simulation results show that the cell throughput of the opportunistic scheme approaches that of a more complex smart antenna scheme for many users. For a densely populated system, the proposed scheme shows a graceful QoS degradation, possibly leading to a high average user satisfaction. The fairness parameter effectively tunes the scheduler between high cell throughput and high fairness.

Performance of TDMA and SDMA based Opportunistic Beamforming

In this work, we analyze opportunistic beamforming with finite number of single-antenna users under the constraint that the feedback overhead from the mobiles to the base is constant. First, we characterize the impact of the fading variances of the users and the spatial correlation on the sum rate of TDMA based opportunistic beamforming using majorization theory. Further, we describe quantitatively the high-SNR behavior in terms of throughput slope and power offset. Next, the impact of the fading variances of the users on an upper bound of the sum rate for space division multiple access (SDMA) based opportunistic beamforming is derived which is tight for high SNR. We propose to adapt the number of active beams to the SNR and the number of active users in a cell and illustrate the corresponding optimization problem by simulations.

Using Unclaimed Sub-Carriers in Opportunistic OFDMA Systems

In this paper, we consider the unclaimed sub-carriers that no user feeds back in an opportunistic OFDMA system with a per sub-carrier power constraint. Unclaimed sub- carriers appear in OFDMA systems with reduced feedback, where the users concentrate the feedback on the sub-carriers with good channel quality. Here we propose and evaluate two ways to use these sub-carriers to improve the system performance. One approach is to transmit pilots symbols on the unclaimed sub- carriers. A second approach is to schedule users based on their feedback about adjacent sub-carriers. Simulation results show that for low to moderate channel RMS delay spread, it is more advantageous to transmit data on unclaimed sub-carriers, even at a low rate, than to use them for additional pilot symbols. For high delay spreads however, transmitting pilots on the unclaimed sub-carriers gives higher system throughput.

System comparison of smart and dumb antennas

Different approaches have been proposed to utilize multiple antennas at both transmitters and receivers of wireless systems. Herein, we provide a system comparison between covariance feedback schemes where a large amount of information is fed back at a low rate and opportunistic beamforming schemes where a small amount of information is fed back at a high rate.

Reduced feedback SDMA based on subspace packings

Herein, we treat Space Division Multiple Access (SDMA) based on partial channel state information and limited feedback. We propose a novel framework utilizing subspace packings, where beamforming, feedback, and scheduling are integrated. Advantages of the proposed framework are that the fed back supportable rates are based on the post-scheduling SINR and that the feedback implicitly contains information about the spatial compatibility of the users. The feasibility region of packings of different dimensions is indicated by the allocation outage probability which is derived. Grassmannian subspace packings, DFT-based packings, and non-orthogonal Grassmannian packings are formulated and studied as candidates. Numerical simulations show better performance for the proposed scheme compared to conventional channel quantization at the receiver with zero-forcing transmission in i.i.d. Rayleigh fading. Finally, we propose and evaluate a beam-graph method to further reduce the feedback load, that can be used in the context of tracking quantized beamformers.

Implementation of a Smart Antenna Multiuser Algorithm on a DSP-Based Wireless MIMO Test-Bed

This paper describes the implementation and performance of three communication schemes on a DSP-based MIMO test-bed. A multicell scenario with two basestations and two users in the same room is evaluated. Eigenbeamforming with and without interference pre-whitening at both transmitter and receiver is compared with SISO multicell communication. Experimental results show that the gain from using interference pre-whitening is significant. The BER performance of beamforming without intercell interference suppression is only marginally better than single antenna communication. The paper also presents the implementation of synchronization, equalization, frequency offset estimation and decision directed beamforming mismatch compensation

Table based performance evaluation for HIPERLAN/2 systems - a multi-parameter design

This paper studies how to estimate the packet error rate (PER) of a HIPERLAN/2 link without using detailed link-level simulations. This is useful mainly for speeding up system-level simulations. An extension to MIMO is also considered. Two different 2D lookup table methods are evaluated, that take both the empirical mean sub-carrier quality across the frequencies and the frequency variability into account. Numerical validations show that these new methods are able to estimate the average PER more accurately than using a traditional 1D mapping. It is shown that these PER estimation methods are critical when characterizing the performance of MIMO transmission schemes where the frequency properties of the effective channel may deviate significantly from the SISO case.