Luciano Tomba

On the effect of Wiener phase noise in OFDM systems

Multicarrier modulation exhibits a significant sensitivity to the phase noise of the oscillator used for frequency down-conversion at the portable receiver. For this reason, it is important to evaluate the impact of the phase noise on the system performance. We present an accurate method to determine the error probability of an orthogonal frequency-division multiplexing (OFDM) system in the presence of phase noise. In particular, four modulation schemes are analyzed and their performances are compared.

Sensitivity of the MC-CDMA access scheme to carrier phase noise and frequency offset

The combination of multicarrier (MC) modulation and code-division multiple access (CDMA) is seen as a very promising technique for the development of high-capacity wireless local communications. On the other hand, this scheme is very sensitive to the signal distortion generated by the imperfect frequency down conversion at the receiver due to the local oscillator phase noise and frequency offset. In this work, we consider the MC-CDMA architecture that was independently proposed in Yee et al. (1993) and Falsafi et al. (1996), and, in particular, we evaluate the impact of both the carrier frequency offset and the phase noise on the system performance. The results are given in terms of symbol error rate (SER) obtained by means of an analytical approach for the additive white Gaussian noise (AWGN) channel. However, the procedure can be applied to the performance evaluation in the presence of a multipath fading channel as part of a semianalytical approach. This approach offers a realistic means of performance evaluation when the target error probability values are of the order of 10/sup -6/ or lower.

Equalization methods in OFDM and FMT systems for broadband wireless communications

Multicarrier systems are adopted in several standards for their ability to achieve optimal performance in very dispersive channels. In particular, orthogonal-frequency division multiplexing (OFDM) and filtered multitone (FMT) systems are two examples where the modulation filter has an ideal rectangular amplitude characteristic in time and frequency domains, respectively. In this letter, we propose new equalization schemes for FMT and compare their performances with OFDM. In general, FMT has a greater spectral efficiency than OFDM, due to the absence of the cyclic prefix and a reduced number of virtual carriers. However, it exhibits a higher distortion per subchannel, due to the imperfect equalization of the transmit filters. As a performance comparison, we considered both the achievable bit rate (ABR) and the bit error rate (BER) in a multipath Rayleigh fading channel. We note that while ABR gives a theoretical bound on the system bit rate, assuming the knowledge of the channel at the transmit side, the BER refers to an uncoiled system with a fixed modulation. Although FMT requires a fixed structure with a higher computational complexity than OFDM, it turns out that FMT, even with the simplest one tap per subchannel adaptive equalizer, yields a better performance than OFDM, both in terms of ABR and BER. Hence, FMT can be a valid alternative to OFDM for broadband wireless applications, also.

Effect of carrier phase noise and frequency offset on the performance of multicarrier CDMA systems

Kaiser (1995) showed that OFDM-CDMA outperforms DS-CDMA in radio fading channels in terms of spectral efficiency, under the assumption of ideal carrier recovery and chip synchronization. However, residual carrier frequency offset and phase noise degrade the system performance significantly if suitable countermeasures are not taken. In this work we consider the downlink of a cellular multicarrier CDMA system. In particular, we evaluate the impact of the carrier frequency offset and Wiener phase noise on the system performance considering typical indoor radio channels. Moreover, a frequency detector is used to reduce the impairment produced by the carrier frequency offset. Results are given in terms of BER obtained by means of simulations and analytical computations.

A model for the analysis of timing jitter in OFDM systems

Multicarrier systems are gaining much attention for their possible application to the design of wideband wireless networks for both indoor and outdoor environments. We present a method for the evaluation of the bit error rate (BER) degradation in orthogonal frequency division multiplexing (OFDM) systems when the sampling instant is affected by timing jitter. We model the timing jitter by a stationary random process with a known statistic and we describe the error caused by it as additive noise. This decomposition is useful because it allows for the analytical determination of the BER. The method may be applied for both uncoded and coded input data sequences.

Achievable bit rates of DMT and FMT systems in the presence of phase noise and multipath

We examine a modulation technique related to orthogonal frequency division multiplexing (OFDM), called filtered multitone (FMT) modulation, which exhibits significantly lower spectral overlapping between adjacent subchannels than other OFDM schemes like discrete multitone (DMT). In particular, we compare FMT and DMT systems in the presence of phase noise due to the frequency down-conversion circuit, as well as multipath fading of the radio channel. As a measure of system performance we consider the achievable bit rate, which is given by the sum of the bit rates each subchannel is eligible to deliver with a certain bit error probability.

Receiver architectures for FMT broadband wireless systems

Orthogonal frequency division multiplexing (OFDM) and in particular discrete multitone (DMT) modulation has been proposed for the physical layer of wideband wireless local area networks. As an alternative, filtered multitone (FMT) modulation can be also considered, since it exhibits significantly lower spectral overlapping between adjacent subchannels, so providing higher transmission efficiency than DMT. In this contribution, we investigate the possible advantages of FMT over DMT modulation for broadband wireless applications in very dispersive channels and, in particular, we present some simple and effective equalization algorithms.

Performance comparison of space diversity and equalization techniques for indoor radio systems

By means of analytical and numerical methods, we derive the bit error rate (BER) of /spl pi//4-DQPSK systems in frequency-selective fading channels. For a theoretical analysis of the system, a simplified two-ray channel model has been used. However, both Rayleigh and lognormal distributions for the ray envelope have been considered. The system performance in the presence of antenna diversity and in combination with a new nonlinear equalizer has been evaluated. In particular, it is seen that in flat fading environments, space diversity may improve the performance by more than 10 dB at a BER=10/sup -3/. However, for channels with a large delay spread, nonlinear intersymbol interference (ISI) is the predominant disturbance, and the performance can only be enhanced by the nonlinear equalizer.

Performance of digital DECT radio links based on semianalytical methods

We report a very efficient semianalytical approach for the performance evaluation of differential detection schemes for GMSK signals of the DECT standard. Precisely, for a given channel, the performance is determined by means of an analytical procedure which includes the saddlepoint approximation. We consider both static channels (with impulse response generated by the simulation program SIRCIM) and two-ray Rayleigh and log-normal fading channels. As a departure from previous works, our receiver includes an all-digital part after the analog differential detection scheme. The digital part includes: (1) a block for the estimation of both the optimum sampling phase and the nonlinear channel coefficients (by making use of the DECT training sequence), (2) a one-tap decision feedback (DF) equalizer, and (3) a block for the evaluation of the approximate optimum bias level (/spl gamma//sub e/) in the threshold detector. Both the DF equalizer coefficient and /spl gamma//sub e/ are based on the nonlinear channel coefficients estimate. For channels with a normalized delay spread up to 0.2, the use of the optimum threshold together with the DF equalizer permits a gain of about 2 dB at BER=10/sup -6/ with respect to a receiver without equalization and a zero-level decision threshold. In addition, we discover that, in indoor environments, the 2-bit GMSK detector performs roughly the same as the 1-bit detector. The threshold optimization is also effective in the presence of channels affected by fading. To support this statement, we report the performance of the 1-bit differential detection scheme combined with antenna selection diversity in the presence of a two-ray log-normal and Rayleigh fading channel.

Worst case equalizer for noncoherent HIPERLAN receivers

Coherent detection of HIPERLAN Gaussian minimum-shift keying signals calls for complex and expensive receivers. However, when the channel delay spread is limited to at the most 50% of the symbol time and a reliable line-of-sight component of the radiated signal exists (Rician fading model), noncoherent detectors are capable of achieving a good performance. Based on the above motivations, we compare four different demodulation techniques, namely the following: (1) one-bit differential detector; (2) discriminator detector; (3) limiter discriminator detector; and (4) limiter discriminator integrator detector (LDID). The intersymbol interference introduced by these demodulators is nonlinear (with respect to the data symbols) and a decision-feedback equalizer (DFE) based on a mean square-error criterion may not be appropriate. Moreover, at this high speed, a DFE may be very complex to implement. Hence, we propose a new DFE design method that increases the eye-diagram aperture by removing the worst case interference. Performance of the above demodulators in the presence of a simple nonlinear DFE (with feedback part only) is computed in terms of the bit-error rate (BER) by means of the saddle-point approximation. This procedure, for static channels, turns out to be a very general tool with a simple and robust implementation. The same method can be applied, for multipath fading channels, to the BER evaluation as part of a semianalytic approach. The main conclusion from this work is that for LDID demodulators and in the presence of Rician fading channels with an average normalized root mean square delay spread of 0.3 and dual antenna diversity, the new equalizer lowers the outage probability from 60% to 10% at a BER of 10/sup -4/.