Tarik Muharemovic

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.

Antenna Packing in Low-Power Systems: Communication Limits and Array Design

In this correspondence, we study design of transceiver antenna arrays and its impact on spectral efficiency of low-power systems. Our primary motivation is construction of practical and portable multi-antenna configurations with a very small and a-priori fixed volume for placing antennas. Using spectral efficiency as a target metric for array optimization, we show that any array configuration, transmit or receive, can be characterized via a parameter that we interpret as ldquoeffective degrees of freedom.rdquo For any array configuration, effective degrees of freedom describes an equivalent uncorrelated array, which results in the same low-power behavior of spectral efficiency. Joint optimization of transmit and receive antenna configurations decouples into maximizing effective degrees of freedom for transmitter and receiver separately. To achieve this goal, we introduce and study a theoretical benchmark of ldquolimiting degrees of freedom, rdquo which is the least upper bound on effective degrees of freedom, evaluated over all configurations with finite number of antennas. Limiting degrees of freedom therefore describes the best possible performance for any transceiver array which confines its elements inside a given space. We compute a closed-form expression for limiting degrees of freedom of a circular geometry. Finally, we present numerical procedure and examples for designing linear and square arrays with nonuniform spacing, which typically exhibit significant spectral efficiency gains over uniform arrays.

Robust slope region for wideband CDMA with multiple antennas

We analyze a low power, wideband CDMA system with multiple antennas. The joint slope region - of transmission rates at minimum energy per bit s derived as a function of proportion between vanishing rates for multiple users. Vertices of the slope region may be achieved by a matched filter linear interface, followed by successive interference cancellation. We introduce and evaluate the robust slope region, the largest region which is inside every other slope region and is therefore independent of the relative proportion between transmission rates. Furthermore, we find the robust slope, the maximum slope which can always be guaranteed to all users.

An approach to capacity analysis of coarsely coordinated low power multiple access systems

We consider multiaccess problem in low power systems, where we allow each user to select its own data rate and transmit power locally and independently from other users. Here, every user has a set of low power codebooks, labeled a policy, which accommodates a range of small spectral efficiencies, while treating instantaneous data rates of other users as an unknown compound parameter. Even with such coarse user coordination, multiuser detection enables a system, which is superior to any classic orthogonal division system. First we fully characterise the set of achievable policies, after which we demonstrate that in multiantenna systems, policies are be viewed as awarding protected receiver spatial dimensions to each user.

On the outage probability of a class of signaling schemes for multiple antennas

The concept of outage probability is appropriate for delay-constrained transmission over quasi-static channels since it serves as a useful lower bound for the frame error probability. In this work, we characterize a rich class of schemes, each member of which exhibits an identical asymptotic (in SNR) outage probability decay, given by maximum spatial diversity available in the system. The class includes several commonly used spatial power allocation and beamforming schemes, derived for varying amount of channel information at the transmitter and the receiver.