Time domain equalization for OFDM systems

In this paper, a time domain equalization algorithm is proposed for single-TX multiple-RX OFDM systems over frequency selective fading channels. The algorithm, which cancels most of the intersymbol interference (ISI), utilizes the orthogonality of the IFFT matrix and the second order statistics of the received signals. The signals are then detected, with the aid of only four pilots, from the equalizer output. The number of pilots required in the proposed algorithm is less than that in existing algorithms and channel length estimation is not needed. In addition, the proposed algorithm is applicable to both the case where the channel length is shorter than or equal to the length of cyclic prefix (CP), and the case where the channel length is longer than the length of cyclic prefix which results in inter-block interference (IBI). Simulation results confirm the effectiveness of the proposed algorithm in both cases and indicate that it is more practical as there is no restriction on the channel and CP lengths. © 2006 IEEE.published_or_final_versio

Quantization Effects in OFDM Systems

The advantage of using orthogonal frequency division multiplexing (OFDM) over the single-carrier modulation is its ability to mitigate interference and fading without complex equalization filters in the receiver. OFDM systems have a high peak-to-average ratio (PAPR) which results in a high requirement for the resolution of AD converters. High-resolution AD converters are therefore widely used in OFDM receivers. However, the power consumption is proportional to the resolution of the AD converters. In this paper we investigate the quantization effects in OFDM systems. Quantization is a nonlinear function which happens in the time domain, so the quantization effect in the frequency domain (important for OFDM) is not simple. Here, we derive a model for the quantization effect in the frequency domain. Further, we investigate whether it is possible to apply low-resolution AD converters in reliable communications based on OFDM. Simulations with an AWGN channel reveal that the proposed model predicts the quantization noise in the frequency domain very well. Difference in $\sigma_\mathrm{q}^2$ between simulation outcomes and our model is less than 0.6\%. Also, simulations show that 5-bits AD resolution is required for OFDM communication over an AWGN channel

Semi-blind time-domain equalization for MIMO-OFDM systems

In this paper, a semi-blind time-domain equalization technique is proposed for general multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. The received OFDM symbols are shifted by more than or equal to the cyclic prefix (CP) length, and a blind equalizer is designed to completely suppress both intercarrier interference (ICI) and intersymbol interference (ISI) using second-order statistics of the shifted received OFDM symbols. Only a one-tap equalizer is needed to detect the time-domain signals from the blind equalizer output, and one pilot OFDM symbol is utilized to estimate the required channel state information for the design of the one-tap equalizer. The technique is applicable irrespective of whether the CP length is longer than, equal to, or shorter than the channel length. Computer simulations show that the proposed technique outperforms the existing techniques, and it is robust against the number of shifts in excess of the CP length. © 2008 IEEE.published_or_final_versio

Time-domain RLS-based channel estimation for MIMO OFDM systems

In this paper, an adaptive channel estimation scheme for MIMO OFDM systems based on time-domain training and recursive least squared (RLS) algorithm is proposed. Time orthogonal as well as simultaneously transmitted training sequences are considered. The channel is assumed to be slowly varying time-dispersive, i.e., constant during one OFDM symbol but changing from symbol to symbol. Channel estimation is performed in time-domain followed by zero-forcing equalization in the frequency-domain. The computational complexity is significantly reduced by applying the matrix inversion lemma. Simulation results show that the proposed estimator with time orthogonal training sequences has better estimation performance over a range of Doppler spreads compared to the case when the training sequences are simultaneously transmitted from the different transmit antennas

Implementation of a Combined OFDM-Demodulation and WCDMA-Equalization Module

For a dual-mode baseband receiver for the OFDMWireless LAN andWCDMA standards, integration of the demodulation and equalization tasks on a dedicated hardware module has been investigated. For OFDM demodulation, an FFT algorithm based on cascaded twiddle factor decomposition has been selected. This type of algorithm combines high spatial and temporal regularity in the FFT data-flow graphs with a minimal number of computations. A frequency-domain algorithm based on a circulant channel approximation has been selected for WCDMA equalization. It has good performance, low hardware complexity and a low number of computations. Its main advantage is the reuse of the FFT kernel, which contributes to the integration of both tasks. The demodulation and equalization module has been described at the register transfer level with the in-house developed Arx language. The core of the module is a pipelined radix-23 butterfly combined with a complex multiplier and complex divider. The module has an area of 0.447 mm2 in 0.18 ¿m technology and a power consumption of 10.6 mW. The proposed module compares favorably with solutions reported in literature

Time Reversal with Post-Equalization for OFDM without CP in Massive MIMO

This paper studies the possibility of eliminating the redundant cyclic prefix (CP) of orthogonal frequency division multiplexing (OFDM) in massive multiple-input multiple-output systems. The absence of CP increases the bandwidth efficiency in expense of intersymbol interference (ISI) and intercarrier interference (ICI). It is known that in massive MIMO, different types of interference fade away as the number of base station (BS) antennas tends to infinity. In this paper, we investigate if the channel distortions in the absence of CP are averaged out in the large antenna regime. To this end, we analytically study the performance of the conventional maximum ratio combining (MRC) and realize that there always remains some residual interference leading to saturation of signal to interference (SIR). This saturation of SIR is quantified through mathematical equations. Moreover, to resolve the saturation problem, we propose a technique based on time-reversal MRC with zero forcing multiuser detection (TR-ZF). Thus, the SIR of our proposed TR-ZF does not saturate and is a linear function of the number of BS antennas. We also show that TR-ZF only needs one OFDM demodulator per user irrespective of the number of BS antennas; reducing the BS signal processing complexity significantly. Finally, we corroborate our claims as well as analytical results through simulations.Comment: 7 pages, 3 figure

Efficient implementation of filter bank multicarrier systems using circular fast convolution

In this paper, filter bank-based multicarrier systems using a fast convolution approach are investigated. We show that exploiting offset quadrature amplitude modulation enables us to perform FFT/IFFT-based convolution without overlapped processing, and the circular distortion can be discarded as a part of orthogonal interference terms. This property has two advantages. First, it leads to spectral efficiency enhancement in the system by removing the prototype filter transients. Second, the complexity of the system is significantly reduced as the result of using efficient FFT algorithms for convolution. The new scheme is compared with the conventional waveforms in terms of out-of-band radiation, orthogonality, spectral efficiency, and complexity. The performance of the receiver and the equalization methods are investigated and compared with other waveforms through simulations. Moreover, based on the time variant nature of the filter response of the proposed scheme, a pilot-based channel estimation technique with controlled transmit power is developed and analyzed through lower-bound derivations. The proposed transceiver is shown to be a competitive solution for future wireless networks