This paper details on the design of OFDM receivers. Special attention is paid to the OFDM-specific receiver functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. In part I of the paper, the effects of nonideal transmission conditions have been thoroughly analyzed. To show the impact of the synchronization algorithms-which are most critical in OFDM-on system performance and complexity we consider the design of a complete receiver consisting of symbol synchronization, carrier/sampling clock synchronization and channel estimation. The performance of the algorithms is analyzed and a qualitative estimate of the resulting complexity is given. This allows one to draw conclusions concerning the achievable system performance under realistic complexity assumptions.

Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies.

In digital TDMA-based packet radio channel access schemes operating over slowly time-variant frequency-selective multipath radio channels, it is necessary to achieve fast initial acquisition of receiver synchronization. The authors are concerned with improving the frame sync performance of the ML-algorithm [Fechtel and Meyr, 1993] for feedforward joint frame sync and frequency offset estimation. Emphasis is placed upon improved "non-data-dependent" (NDD) reduced-complexity variants of the ML-estimator. This is achieved by abandoning the test for signal periodicity in favor of a different preamble structure that yields a more pronounced peak of the autocorrelation at the expense of a larger preamble length. The average frame error rate and frequency estimation performance of the new frame/frequency synchronizers is assessed via simulation. For typical examples, the new algorithms are shown to yield frame sync performance gains of several decibels so that asynchronous packet transmission in the lower SNR regions becomes feasible.

Estimation of time-varying dispersive radio channels is one of the most important tasks of receiver synchronisation. State-of-the-art adaptive estimators employing decision feedback suffer from error propagation and limited robustness against faster fading. In this paper, the optimal feedforward channel estimator using only known training symbols is systematically derived. Statistical channel information is assumed to be available. For moderately rapid fading channels (snapshot assumption), the optimal estimator is shown to be divided into the two subtasks of maximum-likelihood acquisition and subsequent Wiener filtering of the acquired quantities. If perfect training sequences resulting in optimal performance are used, the Wiener filtering operation collapses into independent filtering of the individual acquired tap estimates, and the resulting channel estimator becomes efficient and flexible. The performance of the optimal estimator is evaluated for important cases. A design example of a near-optimal HF channel estimator/receiver is discussed. Linear interpolation is used to reduce the computational complexity of receiver coefficient adjustment. Simulation results confirm the superiority and robustness of feedforward synchronization having near-optimal performance at reduced complexity.>

A multipath radio channel concept based on channel orthogonalization techniques in two inner product spaces is presented and compared to the conventional approach of modeling each individual Rayleigh or Rice fading multipath ray individually. The proposed simulator can be applied to linear amplitude/phase modulation and linear fading channels including Nyquist filtering. It is shown to be a good approximation to the conventional model in the case of tight rolloff factors. Channels having quasi-or truly continuous delay profiles can now be properly represented at significantly reduced computational complexity. The proposed simulation concept leads to the lowest level of complexity being achieved for the prevailing channel and noise conditions on a particular channel.>

For land-mobile digital radio links over time-variant frequency-selective multipath channels, power and bandwidth efficient signaling and time diversity, as provided by interleaved trellis-coded modulation, are of great importance in a fading environment. A novel digital radio transceiver concept is presented that encompasses interleaved trellis coded modulation with large time diversity factor, a combined equalization/decoding scheme, and a quasi-optimal receiver sync strategy based on purely feedforward channel estimation. The resulting fully coherent and robust receiver is of remarkably low computational complexity. The average BER performance of such a transceiver operating over flat and selective 2-ray Rayleigh as well as typical land-mobile GSM channels is assessed via simulation. The results reveal the receiver's robustness and good BER performance and also shed light on the effects of insufficient interleaving at low vehicle speeds.>