Giovanni Santella

A frequency and symbol synchronization system for OFDM signals: architecture and simulation results

A data-aided frequency and symbol synchronization scheme for M-QAM OFDM signals is suggested. At first, the phase discriminator (PD) and frequency discriminator (FD) for, respectively, the symbol and frequency synchronization loop are described. Second, the transfer function and design parameters of the loop filters are provided. The acquisition and tracking performance of the synchronization system are evaluated using simulation for a 4-PSK signal constellation. The cases of additive white Gaussian noise (AWGN) and frequency and time selective multipath Rayleigh channels are separately tested.

Bit error rate performances of M-QAM orthogonal multicarrier modulation in presence of time-selective multipath fading

Orthogonal frequency division multiplexing (OFDM) is considered as an effective technique for high speed digital transmission under multipath fading. We theoretically analyze the behaviour of a coherent pilot-symbol aided M-QAM-OFDM system in the presence of a time selective multipath channel. In the analyzed scheme, pilot symbols are periodically inserted to monitor the channel characteristics. The fading process experienced by the pilot symbols is used to estimate the fade characteristics at the symbol-rate through interpolation. The estimated channel is used to compensate for the random phase variation caused by the Doppler spread, so that the signal can be coherently demodulated. The bit error rate (BER) performance of this coherent OFDM modulation scheme is derived. The results obtained show that, with a reasonable amount of redundancy (frequency of the pilot symbols) introduced, the system performance approaches that of the ideal coherent demodulation, as long as a suitable channel estimate procedure is adopted.

OFDM with guard interval and sub-channel equalization in a 2-resolution transmission scheme for digital television broadcasting

The interest in a new digital terrestrial TV system, that allows improved quality and receiver portability, leads to the choice of a modulation system suitable for operation in a very hostile environment (interferences, multipath). A very effective solution, already experimented with digital audio broadcasting, is the orthogonal frequency division multiplexing (OFDM) technique with a guard interval. Unluckily such an interval grows from a few ten microseconds, sufficient for typical urban propagation environment, up to 600 microseconds, when the emerging single frequency network (SFN) is considered. A long guard interval (as required in a SFN) results in a excessive prolongation of the symbol period (T) and thus in bandwidth efficiency loss. In addition, the use of a too long symbol period is limited by the effects of time selectivity of the channel and by technological limitations. In this light, the use is suggested of a shorter guard interval and a low complexity equaliser at the output of each subcarrier to reduce the error produced by the echoes exceeding the guard interval duration. A multiresolution (MR) transmission scheme has been adopted in order to eliminate the sharp threshold effect in the fringes of the broadcast area. The system performances of a 2-resolution OFDM system have been evaluated by computer simulation on a frequency-selective channel for Rice and Rayleigh propagation environment.<<ETX>>

Adaptive linearization of power amplifier in orthogonal multicarrier schemes

This paper describes an adaptive predistortion system to linearize a memoryless high power amplifier (HPA). The described technique is independent of the modulation format. In the present paper the behaviour of such a predistorter in the presence of orthogonal frequency division multiplexing (OFDM) modulation schemes is analysed. The predistortion scheme uses a digital signal processing microprocessor and memory look-up tables to predistort the baseband drive signal. Simulation results are given where stability of the adaptation algorithm, rate of convergence and effect of quantization are assessed.

Fractional Frequency Reuse Planning for WiMAX over Frequency Selective Channels

Fourth generation broadband wireless access systems based on OFDMA/OFDM techniques operate in point-to-multipoint (PMP) configuration so that conventional cellular planning methods can be used for network design. As an alternative planning method fractional frequency reuse (FFR) strategy has been recently proposed for OFDMA/OFDM cellular systems such as WiMAX. For what concerns the determination of the system outage prediction, planning procedures for multi-carrier systems are usually based on bit error rate curves as a function of the average signal-to-interference plus noise ratio (SINR). Such curves are obtained after simulating the entire transmitter-receiver chain including multipath channel. However these approaches do not explicitly evidence the role of the number of M sub-carriers allocated to each user and are computationally intensive. In order to avoid such inconveniences we express the outage probability as a function of the effective SINR which is a function of the M SINRs (one for each sub-carrier). Such function already includes the decoding effects thus avoiding the re-simulation of the decoder behavior. Furthermore, this approach allows to explicitly evidence the role on planning of the number of sub- carriers allocated for one user. For evaluation purposes we apply the procedure in a simple two cell FFR interference scenario. It is shown that the number of sub-carriers can play a significant role in the entire planning process i.e. the reuse distance can be lowered at the expense of an increased number of sub-carriers to be allocated per block.

Analysis of frequency domain wide-band measurements of the indoor radio channel at 1, 5.5, 10 and 18 GHz

Using a network analyzer, several experiments for the frequency-domain characterization of the indoor radio channel in the 1, 5.5, 10, 18 GHz bands are performed. In the experiments, the frequency response measurements are taken at spatially distributed locations throughout the test area by keeping the receiver in a fixed position and moving the transmitter to different locations. The approximation to the channel impulse response is obtained from the inverse Fourier transform of the frequency response. The transmitter height is varied in different experiments. The measurements are performed in an office environment with mostly metallic walls and inner separations. For each experiment the distance power decay exponent is derived by fitting of the experimental results. Cumulative distribution functions (CDFs) of RMS delay spread (DS) and 3-dB frequency correlation bandwidth are determined.

Bit error rate performances of M-QAM orthogonal multicarrier modulation in presence of time-selective multipath fading†

Orthogonal Frequency Division Multiplexing (OFDM) is considered as an effective technique in high speed digital transmission under multipath fading. In this paper we theoretically analyze the behaviour of a coherent pilot-symbol aided M-QAM-OFDM system in presence of time selective multipath channel. In the analyzed scheme, pilot symbols are periodically inserted to monitor the channel characteristics. The fading process experienced by the pilot symbols is used to estimate the fade characteristics at symbol-rate through interpolation. The estimated channel is used to compensate for the random phase variation caused by the Doppler spread, so that the signal can be coherently demodulated. The bit error rate (BER) performance of this coherent OFDM modulation scheme is derived. The obtained results show that, with a reasonable amount of redundancy (frequency of the pilot symbols) introduced, the system performance approaches that of the ideal coherent demodulation, as long as a suitable channel estimate procedure is adopted.

Performance of a baseband adaptive predistorter in presence of orthogonal multicarrier modulation

The performance of a baseband adaptive predistortion system to linearize memoryless high power amplifier (HPA) are analyzed in the presence of an Orthogonal Frequency Division Multiplexing (OFDM) signal. The predistortion scheme adopted is based on a digital signal processing microprocessor and a memory look-up table to predistort the baseband drive signal. Stability of the adaptation algorithm, rate of convergence and effect of quantization are assessed through simulation. The curves of Total Degradation versus the Output Back-off have been calculated for QPSK, 16QAM and 64QAM constellations, Starting from these results, the improvement brought by the use of the predistorter and the degradation caused by the quantization has been assessed.

Analysis of antenna impact on wide-band indoor radio channel and measurement results at 1 GHz, 5.5 GHz, 10 GHz and 18 GHz

The object of this paper is to investigate the influence of antenna pattern on indoor radio channel characteristics. Different from previous works where this analysis was carried out at a fixed frequency using different antennas, in the present paper (where measurements were taken in a wide frequency range) the variation of the radiation pattern was caused by two factors: the change of the radiation pattern when the same antenna was used at different frequencies and the use of different type of antennas. To carry out this analysis, frequency domain measurements of the indoor radio channel at 1 GHz, 5.5 GHz, 10 GHz and 18 GHz were collected. Measurements were taken using a network analyzer. Several realizations of the channel transfer function were obtained varying, for each measurement, the position of the transmitter and keeping the receiver fixed. Estimate of the channel impulse response was obtained from the Inverse Fourier Transform (IFT) of the frequency response. The measurements were performed in an office environment with mostly metallic walls and inner separations. The obtained data were elaborated to obtain the power versus distance relationship, the Cumulative Distribution Functions (CDFs) of rms Delay Spread (DS) and of the 3 dB frequency correlation bandwidth. Finally, the 3 dB width of the frequency correlation function has been empirically related to the inverse of the rms DS of the impulse response.

On the correlation between transmission quality-of-service (QOS) parameters and image quality of digitally transmitted video in radio terrestrial broadcasting

In this paper the correlation between user-oriented perception based objective measures of digital video quality and signal impairments due to a broadcast radio channel is investigated using simulation. The simulation chain encompasses an MPEG-2 codec, channel coding and OFDM modulation to transmit the signal, then the modulated signal is corrupted by the radio channel (which is assumed frequency selective) and by the receiver AWGN. The amount of degradation introduced is quantified by comparing the same video quality measures at the source and at the receiver end. The amount of degradation is finally related to radio channel characteristics, bit error rate (BER), signal-to-noise ratio (SNR) and error burst distribution.