Multi-resolution 64-DAPSK modulation in a hierarchical COFDMtransmission system

Two alternative modulation schemes of 64-QAM (quadrature amplitude modulation) and 64-DAPSK (differential amplitude and phase shift keying) with a coherent and incoherent demodulation have been proposed for digital terrestrial video broadcasting (DTVB) in combination with the coded orthogonal frequency division multiplexing (COFDM) technique (Schafer 1995, and Engels and Rohling 1995). Additionally, a hierarchical transmission system based on a multi-resolution (MR) 64-QAM is described in Engels and Rohling which extends the service area of a TV programme in decoding a stepwise reduced data rate with increasing transmitter distance (graceful degradation). This procedure corresponds to a decreasing picture quality. In this paper MR concepts for the 64-DAPSK are developed. A performance comparison between the hierarchical transmission systems (MR-64-QAM/OFDM and MR-64-DAPSK/OFDM) is described for the uncoded and coded case     

Receiver Design for Single-Carrier Block Transmission Over Doubly Selective Channels

Single-carrier block transmission is an alternative scheme to orthogonal frequency-division multiplexing (OFDM) for wireless broadband communications. In this paper, a receiver is designed for single-carrier block transmission with cyclic prefix for mobile broadband communications. As the wireless transmission is over doubly selective channels, a basis expansion model is used to capture both the time- and frequency-selectivity of the channel and is parameterized for the receiver design. The receiver estimates the channel model coefficients in the time domain and uses these coefficients for equalization in the frequency domain. The channel estimation is assisted by time-domain pilot insertion. The structure of the frequency-domain channel matrix is exploited and a linear minimum mean-square error equalizer is used for the equalization. When the basis expansion model well matches the physical channel, simulation results show superior receiving performance of the proposed system compared with the OFDM system with a similar complexity.     

IQ-Imbalance Compensation for OFDM in the Presence of IBI and Carrier-Frequency Offset

In this paper, we propose a frequency-domain equalization technique for orthogonal frequency division multiplexing (OFDM) transmission over frequency-selective channels. We consider the case where the receiver analog front-end suffers from IQ-imbalance and the local oscillator suffers from carrier-frequency offset (CFO). While the IQ-imbalance results in a mirroring effect, the CFO induces inter-carrier interference (ICI). In addition to ICI, we consider the channel delay spread is larger than the cyclic prefix (CP). This means that inter-block interference (IBI) is present. The frequency-domain equalizer is obtained by transferring a time-domain equalizer (TEQ) to the frequency-domain resulting in a per-tone equalizer (PTEQ). Due to the presence of IQ-imbalance the conventional TEQ (where only one TEQ is applied to the received sequence) is not sufficient to cope with the mirroring effect. A sufficient TEQ consists of two time-domain filters; one applied to the received sequence and another applied to a conjugated version of the received sequence. For the case of IQ-imbalance and CFO, the TEQs are designed according the basis expansion model (BEM) which showed to be able to cope with the ICI problem. Finally, in addition to the frequency-domain PTEQ design procedure, a training-based RLS type initialization scheme for direct per-tone equalization is also proposed     

Multicarrier modulation with blind detection capability using cosine modulated filter banks

The cosine modulated filter bank (CMFB) is introduced as a multicarrier modulation (MCM) technique for wideband data transmission over wireless channels. Under the name discrete wavelet multitone modulation, CMFB has been considered for data transmission over digital subscriber lines. We propose a new receiver structure that is different from those proposed previously. The new structure simplifies the task of channel equalization, by reducing the number of equalizer parameters significantly. We also propose a novel blind equalization algorithm that fits very nicely in the proposed structure. Moreover, we discuss the bandwidth efficiency of the proposed CMFB-MCM system and show that it is superior to the conventional (single carrier) quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM). The CMFB is found to be a signal processing block that stacks a number of vestigial sideband modulated signals in a number of overlapping subchannels in the most efficient way. The proposed CMFB-MCM is also compared to OFDM with respect to bit-error rate performance. Under the conditions that the channel impulse response duration remains less than the length of cyclic prefix, OFDM is found marginally superior to CMFB-MCM. However, OFDM degrades very fast when the channel impulse response duration exceeds the length of the cyclic prefix. CMFB-MCM, on the other hand, is found less sensitive to variations in channel impulse response duration.     

Broad-band OFDM radio transmission for multimedia applications

Future multimedia services will require the transmission of very high data rates over broad-band radio channels. In order to provide these services to mobile users, an appropriate transmission technique has to cope with frequency-selective and time-variant radio channels. The computation complexity for an equalizer increases in a frequency-selective radio channel for high data rate applications. Furthermore, the overhead for channel estimation procedures increases in time-variant channels. To overcome these drawbacks orthogonal frequency division multiplexing (OFDM) has been considered for broadband applications in many publications. The objective of this paper is to describe the large potential, the flexibility and adaptivity of the OFDM transmission technique in frequency-selective and time variant radio channels. Several technical aspects of OFDM transmission systems are discussed, especially the topics of differential modulation for which we compare different demodulation methods and channel coding with soft decision decoding. For higher level differential modulation, multilevel coding is taken into consideration     

Performance analysis of an adaptive receiver structure for digital mobile communications

In this paper we investigate the performance of a combined estimation/equalization technique for the mobile radio channel, assuming a GSM-recommended transmission format (narrowband TDMA with midamble, recommendation 5.04) and MSK modulation scheme. Channel estimation is performed via correlation of the received signal with a suitably modulated replica of the transmitted midamble. Equalization is then obtained by means of a maximum likelihood sequence estimation (MLSE) scheme in the form of a so-called Viterbi equalizer. Our analysis provides theoretical results concerning the bit error rate (BER) attained by the receiver for a given stationary multipath channel model. Simulation results are also presented in order to integrate and validate the theory.     

On the comparison between OFDM and single carrier modulation with aDFE using a frequency-domain feedforward filter

Most comparisons between single carrier and multicarrier modulations assume frequency-domain linear equalization of the channel. We propose a new frequency-domain decision feedback equalizer (FD-DFE) for single carrier modulation, which makes use of a data block transmission format similar to that of the orthogonal frequency-division multiplexing with cyclic prefix (OFDM). The scheme is a nonadaptive DFE where the feedforward part is implemented in the frequency domain, while feedback signal is generated by time-domain filtering. Through simulations in a HIPERLAN-2 scenario, we show that FD-DFE yields a capacity very close to that of OFDM. This result is also confirmed by analytical derivations for a particular case. Furthermore, when no channel loading is considered, FD-DFE performs closely to OFDM for the same averaged frame error rate in a coded transmission. Design methods of the FD-DFE are investigated and a reduced complexity technique is developed, with the result that FD-DFE and OFDM have a similar computational complexity in signal processing     

Single Carrier Modulation With Nonlinear Frequency Domain Equalization: An Idea Whose Time Has Come—Again

In recent years single carrier modulation (SCM) has again become an interesting and complementary alternative to multicarrier modulations such as orthogonal frequency division multiplexing (OFDM). This has been largely due to the use of nonlinear equalizer structures implemented in part in the frequency domain by means of fast Fourier transforms, bringing the complexity close to that of OFDM. Here a nonlinear equalizer is formed with a linear filter to remove part of intersymbol interference, followed by a canceler of remaining interference by using previous detected data. Moreover, the capacity of SCM is similar to that of OFDM in highly dispersive channels only if a nonlinear equalizer is adopted at the receiver. Indeed, the study of efficient nonlinear frequency domain equalization techniques has further pushed the adoption of SCM in various standards. This tutorial paper aims at providing an overview of nonlinear equalization methods as a key ingredient in receivers of SCM for wideband transmission. We review both hybrid (with filters implemented both in time and frequency domain) and all-frequency-domain iterative structures. Application of nonlinear frequency domain equalizers to a multiple input multiple output scenario is also investigated, with a comparison of two architectures for interference reduction. We also present methods for channel estimation and alternatives for pilot insertion. The impact on SCM transmission of impairments such as phase noise, frequency offset and saturation due to high power amplifiers is also assessed. The comparison among the considered frequency domain equalization techniques is based both on complexity and performance, in terms of bit error rate or throughput.     

Equalization for OFDM over doubly selective channels

In this paper, we propose a time-domain as well as a frequency-domain per-tone equalization for orthogonal frequency-division multiplexing (OFDM) over doubly selective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results in interblock interference (IBI). IBI in conjunction with the Doppler effect destroys the orthogonality between subcarriers and, hence, results in severe intercarrier interference (ICI). In this paper, we propose a time-varying finite-impulse-response (TV-FIR) time-domain equalizer (TEQ) to restore the orthogonality between subcarriers, and hence to eliminate ICI/IBI. Due to the fact that the TEQ optimizes the performance over all subcarriers in a joint fashion, it has a poor performance. An optimal frequency-domain per-tone equalizer (PTEQ) is then obtained by transferring the TEQ operation to the frequency domain. Through computer simulations, we demonstrate the performance of the proposed equalization techniques.     

MMSE Equalization of Interference on Fading Diversity Channels

Adaptive equalization is used in digital transmission systems with parallel fading channels. The equalization combines the diversity channels and reduces intersymbol interference due to multipath returns. When interference is present and correlated from channel to channel, the equalizer can also reduce its effect on the quality of information transfer, important applications for interference cancellation occur in diversity troposcatter systems in the presence of jamming, diversity high frequency (HF) systems which must cope with interfering skywaves, and space diversity line-of-sight (LOS) radio systems where adjacent channel interference is a problem. In this paper we develop the general formulation for minimum mean square error (MMSE) equalization of interference in digital transmission diversity systems. The problem formulation includes the use of available receiver decisions to assist in MMSE processing. The effects of intersymhol interference are included in the analysis through a critical approximation which assumes sufficient processor capability to reduce ISI effects to levels small enough for satisfactory communication. The analysis also develops he concept of additional implicit or intrinsic diversity which results from channel multipath dispersion. It shows how the MMSE processor sacrifices diversity to suppress interference even when the interference arrives in the main beams of the receiver antenna patterns. The condition of near synchronous same-path interference is also addressed. Because the spatial angle of arrival of the interference may result in delay differences between interference signals in different antenna channels, interference delay compensation may be required. We show that this effect is compensated for with a small number of appropriately spaced equalizer taps.     

Novel semi-blind ICI equalization algorithm for wireless OFDM systems

Intercarrier interference is deemed as one of the crucial problems in the wireless orthogonal frequency division multiplexing (OFDM) systems. The conventional ICI mitigation schemes involve the frequency-domain channel estimation or the additional coding, both of which require the spectral overhead and hence lead to the significant throughput reduction. Besides, the OFDM receivers using the ICI estimation rely on a large-dimensional matrix inverter with high computational complexity especially for many subcarriers such as digital video broadcasting (DVB) systems and wireless metropolitan-area networks (WMAN). To the best of our knowledge, no semi-blind ICI equalization has been addressed in the existing literature. Thus, in this paper, we propose a novel semi-blind ICI equalization scheme using the joint multiple matrix diagonalization (JMMD) algorithm to greatly reduce the intercarrier interference in OFDM. However, the well-known phase and permutation indeterminacies emerge in all blind equalization schemes. Hence we also design a few OFDM pilot blocks and propose an iterative identification method to determine the corresponding phase and permutation variants in our semi-blind scheme. Our semi-blind ICI equalization algorithm integrating the JMMD with the additional pilot-based iterative identification is very promising for the future high-throughput OFDM systems. Through Monte Carlo simulations, the QPSK-OFDM system with our proposed semi-blind ICI equalizer can achieve significantly better performance with symbol error rate reduction in several orders-of-magnitude. For the 16QAM-OFDM system, our scheme can also improve the performance over the plain OFDM system to some extent.     

Fast-Converging Blind Adaptive Channel-Shortening and Frequency-Domain Equalization

Orthogonal frequency-division multiplexing (OFDM) is a popular transmission format for emerging wireless communication systems, including satellite radio, various wireless local area network (LAN) standards, and digital broadcast television. Single-carrier cyclic-prefixed (SCCP) modulation is similar to OFDM, but with all frequency-domain operations performed at the receiver. Systems employing OFDM and SCCP perform well in the presence of multipath provided that the channel delay spread is shorter than the guard interval between transmitted blocks. If this condition is not met, a channel-shortening equalizer can be used to shorten the channel to the desired length. In modestly time-varying environments, an adaptive channel shortener is of interest. All existing adaptive channel shorteners require renormalization to restrain the channel shortener away from zero. In this paper, we study the use of a unit-tap constraint rather than a unit-norm constraint on the adaptive channel shortener. We use this constraint to manipulate existing algorithms into a framework analogous to the recursive least squares algorithm, and we develop adaptation rules for blind and semiblind frequency domain equalizers for SCCP receivers. Simulations of the proposed algorithms show an order of magnitude improvement in convergence speed, as well as a reduced asymptotic bit error rate     

Estimation and cancellation of transponder distortions in satellite forward links using memory polynomials

In this paper, a practical non‐linear equalizer with iterative distortion cancellation in a satellite receiver is studied. Assuming no prior satellite channel knowledge, distortion estimation and cancellation are performed at the receiver by means of a memory polynomial model used for channel estimation. A data packet transmission scenario over a single‐carrier satellite transponder is simulated, using the not‐linearized amplifier and onboard filter characteristics similar to the direct‐to‐home broadcast DVB‐S2X reference scenario. Varying the memory depth and the non‐linear order of the memory polynomial model trained in the receiver, we compared the packet‐error rate performance of the practical non‐linear equalizer to the standard fractionally spaced linear adaptive equalizer, as well as to an implementation of the non‐linear equalizer with ideal channel knowledge at the receiver. The improved receiver demonstrates superior performance as compared with the standard linear equalizer with up to 5.48‐dB energy efficiency gain for 64‐amplitude and phase‐shift keying for a practical memory polynomial set‐up, and it approaches consistently the packet‐error rate performance of the implementation with ideal channel knowledge when increasing the memory depth and the non‐linear order. Furthermore, it enables the use of high‐order modulation up to 256‐amplitude and phase‐shift keying in the studied scenario, improving significantly the spectral efficiency of the air interface.     

MLSE and MAP Equalization for Transmission Over Doubly Selective Channels

In this paper, equalization for transmission over doubly selective channels is discussed. The symbol-by-symbol maximum a posteriori probability (MAP) equalizer and the maximum-likelihood sequence estimation (MLSE) are discussed. The doubly selective channel is modeled using the basis expansion model (BEM). Using the BEM allows for an easy and low-complexity mechanism for constructing the channel trellis to implement the MLSE and the MAP equalizer. The MLSE and the MAP equalizer are implemented for single-carrier transmission and for multicarrier transmission implemented using orthogonal frequency-division multiplexing (OFDM). In this scenario, a complexity-diversity tradeoff can be observed. In addition, we propose a joint estimation and equalization technique for doubly selective channels. In this joint estimation and equalization technique, the channel state information (CSI) is obtained in an iterative manner. Simulation results show that the performance of the joint channel estimation and equalization approaches the performance when perfect CSI is available at the receiver.     

Low-Complexity Block Turbo Equalization for OFDM Systems in Time-Varying Channels

We propose low-complexity block turbo equalizers for orthogonal frequency-division multiplexing (OFDM) systems in time-varying channels. The presented work is based on a soft minimum mean-squared error (MMSE) block linear equalizer (BLE) that exploits the banded structure of the frequency-domain channel matrix, as well as a receiver window that enforces this banded structure. This equalization approach allows us to implement the proposed designs with a complexity that is only linear in the number of subcarriers. Three block turbo equalizers are discussed: two are based on a biased MMSE criterion, while the third is based on the unbiased MMSE criterion. Simulation results show that the proposed iterative MMSE BLE achieves a better bit error rate (BER) performance than a previously proposed iterative MMSE serial linear equalizer (SLE). The proposed equalization algorithms are also tested in the presence of channel estimation errors.      

Frequency-Domain Channel Estimation and Equalization for Continuous-Phase Modulations With Superimposed Pilot Sequences

This paper proposes a new frequency-domain channel-estimation and equalization method for continuous-phase modulation (CPM) block transmissions with superimposed pilot signals. Our method provides spectral and power-efficient broadband CPM wireless communications with less complexity than previous methods. The proposed frequency-domain channel estimation uses the superimposed pilot sequence as a reference signal to reduce the throughput loss caused by traditionally multiplexed pilots. The proposed CPM frequency-domain decision feedback equalizer (DFE) eliminates the complexity overhead of conventional decomposition-based CPM receivers.      

OFDM系统中低复杂度的时变信道迭代均衡算法

针对正交频分复用系统在时变信道中的均衡问题,提出了一种低复杂度的时变信道均衡算法。该算法首先运用一阶多项式基扩展模型对时变信道进行建模,利用频域信道矩阵能量主要集中在对角线附近的特点,将频域信道矩阵按梳状导频的位置沿对角线分块,然后运用高斯置信传播算法分别进行线性迫零均衡。算法避免了矩阵求逆运算,降低了计算复杂度,同时有效补偿了多普勒频移引起的载波间干扰,提高了系统性能。计算机仿真结果和算法复杂度分析表明,提出的分块迭代均衡算法有效降低了时变信道中系统的误码率,并且具有复杂度低,可分布式计算的特点,因此适用于专用集成电路等硬件实现。      

OFDM系统均衡器研究与设计

无线OFDM通信系统中信道的时变特性非常明显,载波间正交性遭到破坏,出现载波间相互干扰（ICI）,给系统均衡带来了极大困难。基于等效数字基带模型下ICI分析,在现有的OFDM协议标准中,存在一些空闲载波。利用这些空闲子载波信息,研究了一种适用于时变信道OFDM系统的稀疏矩阵均衡器结构。该算法具有ICI消除与均衡效果良好,计算量适中,频谱利用率高的特点。     

Per-tone equalization for MIMO OFDM systems

This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with channel order larger than the cyclic prefix (CP) length. Writing the demodulating fast Fourier transform (FFT) as a sliding FFT followed by a downsampling operation, we show in this paper that by swapping the filtering operations of the MIMO channel and the sliding FFT, the data model for the temporally smoothened received signal of each individual tone of the MIMO OFDM system is very similar to the data model for the temporally smoothened received signal of a MIMO single-carrier (SC) system. As a result, to recover the data symbol vectors, the conventional equalization approach for MIMO SC systems can be applied to each individual tone of the MIMO OFDM system. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative to the recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. In the second part of this paper, we focus on direct per-tone equalizer design and adapt an existing semi-blind equalizer design method for space-time block coding (STBC) SC systems to the corresponding semi-blind per-tone equalizer design method for STBC OFDM systems.     

Iterative MAP Equalization and Decoding in Wireless Mobile Coded OFDM

Orthogonal frequency division multiplexing (OFDM) system suffers extra performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. Realizing that ICI channels are intrinsically ISI channels, this paper proposes a soft-in soft-out (SISO) maximum a posteriori (MAP) equalizer by extending Ungerboeck's maximum likelihood sequence estimator (MLSE) formulation to ICI channels. The SISO MAP equalizer employs BCJR algorithm and computes the bit log-likelihood ratios (LLR) for the entire received sequence by efficiently constructing a trellis that takes into account of the ICI channel structure. A reduced state (RS) formulation of the SISO MAP equalizer which provides good performance/complexity tradeoff is also described. Utilizing the fact that ICI energy is clustered in adjacent subcarriers, frequency domain equalization is made localized. This paper further proposes two computational efficient linear minimum mean square error (LMMSE) based equalization methods: recursive q-tap SIC-LMMSE equalizer and recursive Sliding-Window (SW) SIC-LMMSE equalizer respectively. Simulations results demonstrate that the iterative SISO RS-MAP equalizer achieves the performance of no ICI with normalized Doppler frequency f_dT_s up to 20.46% in realistic mobile WiMAX environment.