Babak Daneshrad

An Energy-Efficient Water-Filling Algorithm for OFDM Systems

In this paper, we develop an energy-efficient power allocation algorithm for the parallel channels of an OFDM system. This algorithm provides the optimum solution to a nonlinear fractional program involving an objective function called \textit{energy-per- goodbit} (EPG). The EPG objective function models the impact of both transmit power and constant circuit power consumption. The energy minimization problem formulation is quite general and subsumes both maximize rate (MR) and maximize margin (MM) problems as specific cases. As a result, the energy efficiency viewpoint provides a convenient and unified perspective of the various water-filling solutions. Using a numerical example, we show that the energy-efficient solution is quite different from the MM or MR solutions and can provide several dBs of performance improvement. We also study the impact of rounding to discrete constellation sizes.

A performance improvement and error floor avoidance technique for belief propagation decoding of LDPC codes

In this work, we introduce a unique technique that improves the performance of the BP decoding in waterfall and error-floor regions by reversing the decoder failures. Based on the short cycles existing in the bipartite graph, an importance sampling simulation technique is used to identify the bit and check node combinations that are the dominant sources of error events, called trapping sets. Then, the identified trapping sets are used in the decoding process to avoid the pre-known failures and to converge to the transmitted codeword. With a minimal additional decoding complexity, the proposed technique is able to provide performance improvements for short-length LDPC codes and push or avoid error-floor behaviors of longer codes

Multi-antenna testbeds for research and education in wireless communications

Wireless communication systems present unique challenges and trade-offs at various levels of the system design process. Since a variety of performance measures are important in wireless communications, a family of testbeds becomes essential to validate the gains reported by the theory. Wireless testbeds also play a very important role in academia for training students and enabling research. In this article we discuss a classification scheme for wireless testbeds and present an example of the testbeds developed at UCLA for each of these cases. We present the unique capabilities of these testbeds, provide the results of the experiments, and discuss the role they play in an educational environment.

Energy-Constrained Link Adaptation for MIMO OFDM Wireless Communication Systems

We present a link adaptation strategy for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based wireless communications. Our objective is to choose the optimal mode that will maximize energy efficiency or data throughput subject to a given quality of service (QoS) constraint. We formulate the link adaptation problem as a convex optimization problem and expand the set of parameters under the control of the link adaptation protocol to include: number of spatial streams, number of transmit/receive antennas, use of spatial multiplexing or space time block coding (STBC), constellation size, bandwidth, transmit power and choice of maximum likelihood (ML) or zero-forcing (ZF) for MIMO decoding. Additionally, we increase the fidelity of the energy consumption modeling relative to the prior art. The resulting solution allows us to easily and quickly search the space of possible system parameters to deliver on the QoS with minimal energy consumption. Moreover, it provides us insight into where crossovers occur in the choice of the radio parameters. Application of the results to a generic MIMO-OFDM radio shows that the proposed strategy can provide an order of magnitude improvement in energy efficiency or data throughput relative to a static strategy.

An IS Simulation Technique for Very Low BER Performance Evaluation of LDPC Codes

We introduce an Importance Sampling (IS) method that successfully simulates the performance of Low density Parity Check (LDPC) Codes in an AWGN channel at very low bit error rates (BERs). By effectively finding and biasing bit node combinations that are the dominant sources of error events, called trapping sets, the developed technique provokes more frequent decoder failures. Consequently, fewer simulation runs and higher simulation gains are achieved. Regardless of the block size of an LDPC code, only a few dominant trapping set classes cause decoder failures at low BER regions. Therefore, the proposed technique allows the performance evaluation for any size LDPC code at very low BER regions with remarkable simulation gains. For BERs of 10-20, we observed simulation gains on the order of 1014.

Energy-Efficient Power Loading for a MIMO-SVD System and Its Performance in Flat Fading

In this paper we formulate a power loading problem for the spatial subchannels (parallel channels) of a single-carrier MIMO-SVD system. The power loading solution is designed to minimize the energy-per-goodbit (EPG) of the MIMO-SVD system. The optimum power loading obtained by solving a nonlinear fractional program, has a closed-form expression and is found by applying a water-filling procedure. It is observed that the optimum total transmit power (water level) depends not only on the MIMO channel realization, but also on the ratio of circuit power cost (which depends on the number of antennas) to transmit power cost (which depends on path loss and other factors). We study the statistical performance (using simulation) of the solution in Rayleigh and Rician flat-fading channels. Using outage EPG as a measure of performance, we determine the MIMO configuration (from a set of allowed configurations) that yields the minimum outage EPG. It is observed that the average number of spatial subchannels utilized (which indicates preference for diversity or multiplexing) depends on the ratio of circuit power cost to transmit power cost and fading type (Rayleigh or Rician). For both cases, the results show that both multiplexing and diversity obtained by MIMO systems are critical for energy efficiency.

A practical, hardware friendly MMSE detector for MIMO-OFDM-based systems

Design and implementation of a highly optimized MIMO (multiple-input multiple-output) detector requires cooptimization of the algorithm with the underlying hardware architecture. Special attention must be paid to application requirements such as throughput, latency, and resource constraints. In this work, we focus on a highly optimized matrix inversion free MMSE (minimum mean square error) MIMO detector implementation. The work has resulted in a real-time field-programmable gate array-based implementation (FPGA-) on a Xilinx Virtex-2 6000 using only 9003 logic slices, 66 multipliers, and 24 Block RAMs (less than 33% of the overall resources of this part). The design delivers over 420 Mbps sustained throughput with a small 2.77-microsecond latency. The designed linear MMSE MIMO detector is capable of complying with the proposed IEEE 802.11n standard.

I/Q mismatch cancellation for MIMO-OFDM systems

MIMO-OFDM systems are gaining prominence in high data rate applications due to their increased spectral efficiency, in MIMO-OFDM systems, I/Q mismatch causes interference between the frequency mirror subcarriers. This can significantly degrade and limit the performance of the system. We present a mathematical analysis of the effects of I/Q gain, delay and phase mismatches in MIMO-OFDM systems. We show the impact of I/Q mismatch when more antennas are added. We also derive a linear optimal solution to cancel all three I/Q mismatches and an RLS based adaptive filter for the same. We show simulation results and results on an experimental testbed.

A comparative study of MIMO detection algorithms for wideband spatial multiplexing systems

The implementation of wideband MIMO systems poses a major challenge to hardware designers due to the huge processing power required for MIMO detection. To achieve this goal with a complete VLSI solution, channel coding and MIMO detection are preferably separated so that each of them can be fitted into a single chip. In this paper, a comparative study is presented regarding various uncoded adaptive and non-adaptive MIMO detection algorithms. Intended to serve as a reference for system designers, this comparison is performed from several different perspectives including theoretical formulation, simulated BER/PER performance, and hardware complexity. All the simulations are conducted within MIMO-OFDM framework and with a packet structure similar to that of the IEEE 802.11a/g standard. As the comparison results show, the RLS algorithm appears to be an affordable solution for a wideband MIMO system targeted at gigabit wireless transmission. As a direct result of this work, an ASIC for a 25 MHz wideband 8 /spl times/ 8 MIMO-OFDM system using RLS has been designed and fabricated.

Design and development of a 5.25 GHz software defined wireless OFDM communication platform

With growing interest in broadband wireless communication systems, a research platform for evaluating various communication schemes becomes essential. Such a system can be used to validate the assumptions and results of various theoretical and simulation-based studies. This article discusses the design and development of a multicarrier multi-antenna software defined radio, operating at 5.25 GHz in a 25 MHz bandwidth. The calibration process is described in detail along with a discussion of all the issues such as I/Q mismatch and phase noise that degrade and limit the performance of the system. Hardware and software solutions to these problems are described. Results of the calibration process and wireless experiments on the system are presented as well.