Essam Sourour

Performance of orthogonal multi-carrier CDMA in a multipath fading channel

A DS-CDMA multi-carrier system is proposed. The transmitted data bits are serial to parallel converted to a number of parallel branches. Each bit on each branch is DS-SS modulated and transmitted with a number of orthogonal and overlapping carriers. Given that sufficient interleaving is provided, this procedure provides many advantages. The system performance is compared to that of the conventional RAKE receiver. It is shown that the multi-carrier system over-performs the RAKE receiver when the system parameters are selected properly. The system utilizes a small number of carriers to solve the intersymbol interference (ISI). The data on each carrier is spread by a lower rate PN code. This reduces the effect of inter-chip interference (ICI). This technique is shown to provide the following advantages: the spread spectrum processing gain is increased, the effect of multipath interference is removed, and frequency/time diversity is achieved.

PreRAKE diversity combining in time-division duplex CDMA mobile communications

A preRAKE system is proposed for the time-division duplex (TDD) code-division multiple-access (CDMA) systems for portable communications. Since the up and down links are time slots on the same carrier frequency, they have the same channel impulse response during a short period of time. Instead of building a RAKE receiver in the portable unit, the base station (BS) can preRAKE the signal before transmission in the down link using the channel impulse response estimated from the up link. When the preRAKED signal is convolved with the channel impulse response, the function of the RAKE receiver is automatically performed. The mobile or portable unit uses a conventional receiver and still achieves the diversity gain of a RAKE receiver.

Direct-sequence spread-spectrum parallel acquisition in nonselective and frequency-selective Rician fading channels

This work presents the performance of the direct-sequence spread-spectrum (DS-SS) parallel acquisition system, previously proposed by the authors (1989, 1990), for nonselective and frequency-selective Rician (i.e. specular plus Rayleigh) fading channels. The acquisition system utilizes a bank of parallel I-Q noncoherent matched filters for the search mode, and a coincidence detector for the verification mode. The probabilities of detection and false alarm are derived and the mean and variance of the acquisition time are evaluated as a measure of the system performance. The nonselective channel is a Rayleigh fast fading channel, while the frequency-selective channel model is the so-called wide sense stationary uncorrelated scattering (WSSUS), selective only on frequency. These channels are typical for aircraft-satellite and line-of-sight (LOS) communications. >

Time-division duplex CDMA communications

The authors examine the time-division duplex (TDD) mode of code-division multiple access (CDMA) communications. The differences from frequency-division duplex (FDD) are shown, and the advantages of using a TDD system are discussed. The TDD mode facilitates the implementation of several functions in the areas of power control, RAKE combining, and selection diversity combining, which can reduce the complexity of mobile units and improve system capacity.

Pre-RAKE diversity combining in time division duplex CDMA mobile communications

A pre-RAKE system is proposed for the time division duplex code division multiple access systems for portable communications. Since the up and down links are time slots on the same carrier, they have the same channel impulse response during a short period of time. Instead of building a RAKE receiver in the portable unit the base station can pre-RAKE the signal before transmission in the down link using the channel impulse response estimated from the up link. When the pre-RAKE signal is convolved with the channel impulse response, the function of the RAKE receiver is automatically performed. The mobile or portable unit can use a conventional matched filter and still achieve the diversity gain of a RAKE receiver.

Mutual decentralized synchronization for intervehicle communications

Data exchange among vehicles can improve road safety and capacity. Most of the proposed intervehicle data communication systems require intervehicle synchronization. Synchronization must be done in a decentralized manner. We propose a new mutual decentralized synchronization system. Using a devoted carrier frequency, each vehicle transmits a continuous periodic train of pulses. The aim of the synchronization system is to make these periodic pulses synchronous to indicate the start of data slots in slotted ALOHA types of media access protocol. Each vehicle measures the power of pulses of other vehicles as well as the time difference between other pulses and its own pulse. Using this information, each vehicle shifts its own pulse transmission time toward a weighted average of other pulse transmission times. Eventually, all periodic pulse trains are synchronized. The system performance is evaluated in nonfading and fading channels.

Delay tracking for direct sequence spread spectrum systems in multipath fading channels

This paper considers the problem of multipath delay tracking in direct sequence spread spectrum systems operating in multipath fading channels. Due to the pulse shaping of the PN code chips, tracking multipath delays is a challenging problem, for which we introduce three novel techniques. In the first technique, maximum likelihood estimation, we search all possible combinations of delays and select the set of delays that minimize a metric derived from the error between the received signal and an estimated signal based on these postulated delays. To reduce complexity, we introduce the ordered maximum likelihood technique, in which the above mentioned metric is minimized iteratively assuming that the channel has one path, then two paths, etc. At each iteration, the delay estimates derived from previous iterations are fixed. Therefore, in each iteration only one delay estimate is produced. Another technique presented in this paper is envelope tracking with subtraction. In this technique, we select peaks of the correlation function between the received direct sequence spread spectrum signal and the local replica. After selecting each peak, the contribution due to the corresponding channel path is subtracted from the correlation function.

Optimizing the performance of limited complexity RAKE receivers

Channel estimation is critical to coherent RAKE receiver performance. We investigate maximum likelihood (ML) channel estimation with side information, using knowledge of the transmit and receive filter responses. Such an approach accounts for the interaction between multipath components. For delay estimation, the ML approach optimizes the RAKE finger placement, even when there are an insufficient number of RAKE fingers. For channel coefficient estimation, the ML approach optimizes combining, regardless of the delay estimation approach used. Suboptimal approaches are also developed, which reduce the complexity at a slight cost in performance.

Frequency offset estimation and correction in the IEEE 802.11a WLAN

The paper shows how to utilize the short training sequence, long training sequence and pilot subcarriers of the IEEE 802.11a frame, to estimate and equalize the effect of both carrier and sampling frequency offset. To reduce cost, the equalization process is performed digitally. Using computer simulation, we show that the presented scheme nearly eliminates the degradation due to frequency offset for all IEEE 802.11a modulation schemes. The performance measures are the root-mean-squared error (RMSE) and BER in a fading channel.

Mutual decentralized synchronization for inter-vehicle communications

Data exchange among vehicles improves road safety and capacity. Most of the proposed inter-vehicle communication systems require synchronization to be done in a decentralized manner. We propose a new mutual decentralized synchronization system. Using a devoted carrier frequency each vehicle transmits a continuous periodic train of pulses. The aim of the synchronization system is to synchronize these periodic pulses to be pointers to the start of data slots in slotted ALOHA. Each vehicle measures the power of pulses of other vehicles as well as the time difference between other pulses and its own pulse. Each vehicle shifts its own pulse transmission time towards a weighted average of other pulse transmission times. Eventually all periodic pulse trains are synchronized. The system performance is evaluated in a non-fading and multi-path fading channels.