We describe the VLSI implementation of MIMO detectors that exhibit close-to optimum error-rate performance, but still achieve high throughput at low silicon area. In particular, algorithms and VLSI architectures for sphere decoding (SD) and K-best detection are considered, and the corresponding trade-offs between uncoded error-rate performance, silicon area, and throughput are explored. We show that SD with a per-block run-time constraint is best suited for practical implementations.

We study the capacity behavior of full-band MIMO-OFDM systems in the absence of channel state information both at the transmitter and the receiver. Based on capacity lower and upper bounds, we quantify the transmission rate penalty due to channel uncertainty as a function of the number of transmit antennas, the number of resolvable taps in the channel, the power delay profile, and the bandwidth. Our analysis reveals that for a given bandwidth and transmit power there is an optimum, capacity-maximizing, number of transmit antennas. Numerical results show that using a large number of transmit antennas in systems employing bandwidths of several GHz (such as in ultrawideband systems) is detrimental from a capacity point of view. Finally, we evaluate the capacity performance of space-frequency unitary codebooks recently introduced in M. Borgmann and H. Bolcskei, (2005)

Recently, the use of coherent space-frequency coding in orthogonal frequency-division multiplexing (OFDM)-based frequency-selective multiple-input multiple-output (MIMO) fading channels has been proposed. Acquiring knowledge of the fading coefficients in a MIMO channel is already very challenging in the frequency-flat (fast) fading case. In the frequency-selective case, this task becomes significantly more difficult due to the presence of multiple paths, which results in an increased number of parameters to be estimated. In this paper, we address code design for noncoherent frequency-selective MIMO-OFDM fading links, where neither the transmitter nor the receiver knows the channel. We derive the code design criteria, quantify the maximum achievable diversity gain, and provide explicit constructions of full-diversity (space and frequency) achieving codes along with an analytical and numerical performance assessment. We also demonstrate that unlike in the coherent case, noncoherent space-frequency codes designed to achieve full spatial diversity in the frequency-flat fading case can fail completely to exploit not only frequency diversity but also spatial diversity when used in frequency-selective fading environments. We term such codes "catastrophic.".

We quantify the impact of coherence on pulse position modulation (PPM) over wideband fading channels by computing achievable rates and an upper bound on uncoded symbol error probability. We study the influence of channel estimation accuracy on the optimum diversity order and furthermore find that a near-optimum receiver typically needs to estimate a few channel taps only

Space-frequency coded orthogonal frequency division multiplexing (OFDM) is capable of realizing both spatial and frequency-diversity gains in multipath multiple-input multiple-output (MIMO) fading channels. This naturally leads to the question of variable allocation of the channel's degrees of freedom to multiplexing and diversity transmission modes. In this paper, we provide a systematic method for the design of space-frequency codes with variable multiplexing-diversity tradeoffs. Simulation results illustrate the performance of the proposed codes.

Previous work on space-frequency coded multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) has been restricted to idealistic propagation conditions. In this paper, using a broadband MIMO channel model taking into account Ricean K-factor, transmit and receive angle spread, and antenna spacing, we study the impact of the propagation environment on the performance of space-frequency coded MIMO-OFDM. For a given space-frequency code, we quantify the achievable diversity order and coding gain as a function of the propagation parameters. We find that while the presence of spatial receive correlation affects all space-frequency codes equally, spatial fading correlation at the transmit array can result in widely varying performance losses. High-rate space-frequency codes such as spatial multiplexing are typically significantly more affected by transmit correlation than low-rate codes such as space-frequency block codes. We show that in the MIMO Ricean case the presence of frequency-selectivity typically results in improved performance compared to the frequency-flat case.