Manyuan Shen

Frequency offset and I/Q imbalance compensation for direct-conversion receivers

Two types of wireless system imperfections, namely, frequency offset and in-phase/quadrature (I/Q) imbalance, are addressed in this paper. The I/Q imbalance in radio frequency (RF) direct-conversion not only introduces an unwanted in-band image interference but also degrades the accuracy of carrier estimation. Toward this end, we propose a pilot-based scheme for both frequency offset and I/Q imbalance compensation at the baseband. A low-cost nonlinear least squares (NLS) frequency estimator robust to the I/Q imbalance is developed. Also derived is an I/Q imbalance compensation structure that consists of two stages: a finite impulse response (FIR) filter that compensates the frequency dependent imbalance; and an asymmetric phase compensator that corrects the frequency independent error. The compensation coefficients are optimized by exploiting the phase rotation embedded in the pilot symbols. Both computer simulations and experiment results verify the effectiveness of the proposed scheme in various I/Q imbalance scenarios.

A factor graph approach to iterative channel estimation and LDPC decoding over fading channels

This letter provides a comparative study of joint channel estimation and low-density parity-check decoding algorithms for flat Rayleigh fading channels based on the receiver factor graphs. Two approaches for joint channel estimation and decoding are proposed. Intensive simulation studies are carried out to evaluate the receiver sensitivity to the choice of the factor graph. Results show that when channel statistical information is not available at the receiver, a low-pass filtering approach gives a more robust and simpler solution, and it is only 0.5 dB worse than the optimum Wiener filtering. All methods have about 2 dB improvements over the noniterative receivers.

Frequency offset and I/Q imbalance compensation for OFDM direct-conversion receivers

Two types of RF front-end imperfections in direct-conversion receivers, namely frequency offset and I/Q imbalance, are addressed. The I/Q imbalance not only introduces an unwanted image interference, but also degrades the accuracy of carrier estimation. We propose a pilot-based scheme for both frequency offset and I/Q imbalance compensation. A nonlinear least squares (NLS) frequency estimator, robust to the I/Q imbalance, is developed. Also derived is an I/Q compensation structure that consists of two stages: the frequency dependent imbalance is compensated with an FIR filter whereas the frequency independent part is corrected with an asymmetric phase compensator. The compensation coefficients are optimized by restoring the phase rotation embedded in the pilot symbols.

Implementing an OFDM Receiver on the RaPiD Reconfigurable Architecture

Field-programmable gate arrays (FPGAs) have become an extremely popular implementation technology for custom hardware because they offer a combination of low cost and very fast turnaround. Because of their in-system reconfigurability, FPGAs have also been suggested as an efficient replacement for application-specific integrated circuits (ASICs) and digital signal processors (DSPs) for applications that require a combination of high performance, low cost, and flexibility. Unfortunately, the use of FPGAs in mobile embedded systems platforms is hampered by the very large overhead of FPGA-based architectures. Coarse-grained configurable architectures can reduce this overhead substantially by taking advantage of the application domain to specialize the reconfigurable architecture via coarse-grained components and interconnects. This paper presents the design and implementation of an OFDM receiver in the RaPiD reconfigurable architecture as a case study for comparing the relative cost and performance of ASIC, DSP, FPGA, and coarse-grained reconfigurable architectures. RaPiD is a coarse-grained reconfigurable architecture specialized to the domain of signal and image processing. The RaPiD architecture provides a reconfigurable pipelined datapath controlled by efficient reconfigurable control logic: We have implemented the computationally intensive parts of an OFDM receiver on the RaPiD architecture and have developed careful estimates of corresponding implementations in representative ASIC, DSP and FPGA technology. Our results show that, for this application, RaPiD fills the cost/performance gap between programmable DSP and ASIC architectures, achieving a factor of 6 better than a DSP implementation but a factor of 6 less than an ASIC implementation.

Finite precision implementation of LDPC coded M-ary modulation over wireless channels

This paper studies the finite precision effects on the LDPC decoding performance with high order modulation over Rayleigh fading channels. We compare the performance of different quantization schemes and decoding algorithms using discretized density evolution over code ensembles of various code rates. We find that the min-sum algorithm with a simple correction term using 5 bits for quantization offers a very good performance in all code rate and modulation combinations. The loss is only 0.1 dB compared to floating point implementation. Direct implementation of the sum product algorithm with 6-bit quantization suffers a much larger performance loss when high rate LDPC codes are used. Both software simulations and test results of a hardware decoder implemented on FPGA verify these conclusions.

Iterative receiver design in Rayleigh fading using factor graph

In this paper we develop a novel approach for joint iterative channel estimation and LDPC decoding, based on the factor graph representation and sum-product algorithm. The fast flat fading channel is modeled as an AR process in order to characterize its temporal variation. By approximating the feedback message from the channel decoder as a Gaussian pdf, we show that Kalman smoothing can be employed for channel estimation. We further derive a new bit metric calculation method which consider both channel noise and channel estimation variance. Simulations confirm the effectiveness of our approach.

Performance of clustered OFDM with low density parity check codes over dispersive channels

We present a new low density parity check (LCPC) coded clustered orthogonal frequency division multiplexing (OFDM) system with 16QAM modulation operating over a frequency selective fast fading channel. Various channel estimation methods are studied and pilot-symbol-aided channel estimation with uniquely designed spacing pattern is used to obtain channel side information in fast fading. A BER of 10/sup -6/ is achieved at 12.8dB with Doppler frequency f/sub m/ = 200 Hz and 14.8 dB with f/sub m/ = 100 Hz. The overall capacity of this system is about 1.56 bit/s/Hz. Compared with other wideband systems, our design has much better performance in fading channels than the LDPC coded classical OFDM and is much simpler than TTCM-OFDM system.

An LDPC-based terrestrial multimedia broadcasting (TMB) system: design, implementation and experimental results

The paper describes a multicarrier terrestrial multimedia broadcasting (TMB) system that offers system performance and service versatility suitable for future digital broadcasting networks. The new system utilizes variable rate, variable size LDPC (low-density parity-check) codes to allow simultaneous transmission of different programs with diverse requirements. A prototype system is implemented based on the proposed protocol. Results from both lab tests and field trials demonstrate significant performance advantages of the new design over existing systems such as DVB-T.