F. Said

Performance comparison of space-time block coded and cyclic delay diversity MC-CDMA systems

Spatial diversity is a widely applied technique for enhancing wireless system performance since it greatly reduces the detrimental effects of multipath fading. Space-time block codes have been considered the best choice for transmit diversity in narrowband environments, but their use in broadband channels is questionable due to their inability to pick up multipath diversity. However, when used in conjunction with an MC-CDMA system, they achieve not only full spatial but also variable multipath diversity depending on the employed spreading. In comparison, cyclic delay diversity is an attractive approach to achieve spatial and multipath diversity. Its simplicity and conformability with current standards makes it desirable for multicarrier systems. Previous studies suggest that CDD is only advantageous with an outer channel code for OFDM systems. In this article, we compare STBCs and CDD applied to an MC-CDMA system in terms of complexity and performance. It is shown that for an MC-CDMA system, CDD benefits from spreading and channel coding that makes it very competitive with STBCs, particularly since it is applicable to any number of transmit antennas with no loss in rate.

Dynamic simulator for studying WCDMA based hierarchical cell structures

A dynamic radio network simulator is implemented for studying WCDMA based hierarchical cell structures. The simulator allows estimation of capacity and quality of service related issues in a two-layer network (microcells and macrocells). The input to the simulator is base station and mobile station information and its output is presented as the blocking and dropping probabilities, handoff rate and capacity of the assumed network. Both uplink and downlink are considered. As an example, the impact of between-layer handover on the capacity is investigated. The whole simulator is based entirely on visual C++ software.

Closed-Form Symbol Error Probabilities of STBC and CDD MC-CDMA With Frequency-Correlated Subcarriers Over Nakagami-

The performances of cyclic delay diversity and space-time block-coded multicarrier code-division multiple-access (MC-CDMA) systems operating in a Nakagami-m fading channel with correlated subcarriers are investigated. By using the moment-generating function of the correlated Nakagami-m random variable, explicit closed-form formulas for the exact average symbol error rate (SER) of M-ary signals for an MC-CDMA system employing the aforementioned diversity techniques are derived with a maximum ratio combining detection technique. The average SERs are expressed in terms of higher transcendental functions, such as the Gauss and Appell hypergeometric functions. Both numerical and simulation results are presented, showing excellent agreement.

m

Virtual antenna arrays were recently introduced that allow the application of multiple-input-multiple-output (MIMO) capacity enhancement techniques to mobile terminals with a limited number of antenna elements. This paper investigates the performance improvements of deployed higher order space-time block codes (STBCs) to such virtual antenna arrays. The main idea is to emulate (nT/sub x/ transmit antennas, m/sub Rx/ receive antennas) diversity scheme. This approach successfully deals with the problem of just one receiving antenna in a handset. Simulations results indicate a drastic improvement in error performance over (n/sub Tx/,1) schemes.

Fading Channels

A multiple number of subcarrier frequency offsets is estimated in the frequency domain. This is achieved by adjusting the power of the reference symbol transmitted and utilizing a guard-band power detection method in the receiver. The power of the start symbol of two repetitive symbols, which also acts as a reference symbol in differential modulation, is varied linearly in the frequency domain to increase the power difference between modulated (valid) subcarriers and noisy non-modulated (virtual) carriers. The performance of the proposed frame is confirmed by simulation for a DQPSK system in a multipath-fading environment.

Higher order space-time block codes for virtual antenna arrays

Sensor networks as cooperative spatial diversity (CSD) is implemented and analyzed for cyclic delay diversity (CDD) distributed MC-CDMA system. In such as cooperative system, it is proposed a distributed SISO analysis employing CDD to simplify the system evaluation in terms of probability of errors and maximum achievable throughput [bits/s/Hz]. Furthermore, the moment generating function (MGF) of the correlated Nakagami-m random variable is used and considered for the realistic case of frequency-correlated MC-CDMA subcarries. To this end, an explicit allocation strategy for the distributed diversity is deducted. Simulation results are also presented which show excellent agreement with the analytical framework.

On the guard band-based coarse frequency offset estimation technique for burst OFDM systems

Differential transmit diversity is an attractive alternative to its coherent counterpart, especially for multiple antenna systems where channel estimation is more difficult to attain compared to that of single antenna systems. In this paper we compare two different types of differential transmit diversity techniques for OFDM based transmissions. The first technique uses differential space-time block codes (DSTBC) from orthogonal designs and the second uses the differential cyclic delay diversity (DCDD). The results compare the bit error performance for several transmit antenna configurations. The results show that DCDD offers a very close performance to that of DSTBC, with the advantage of a simplified receiver structure.

Performance Analysis of Distributed CDD MC-CDMA Sensor Networks with Frequency-Correlated Subcarriers over Nakagami-m Fading Channels

In this paper, the performance of a cooperative relaying scheme with cyclic delay diversity (CDD) for an orthogonal frequency division multiplexing (OFDM) system is investigated. The system under consideration consists of multiple single-antenna terminals acting as relays and cooperating to provide spatial diversity. Cyclic shift is introduced at each relay and thus increase the frequency selectivity of relay channels. To exploit a highly selective channel, convolutional code is employed together with OFDM. Simulations demonstrate the bit error performance of the proposed scheme with different numbers of relays as well as performance in flat and frequency selective fading channels. The performance of the system is also compared to conventional multiple-input multiple-output (MIMO) with CDD and a conventional amplify-and-forward (AF) relay system. The results indicate that the proposed scheme provides enhanced performance under various considerations.

Performance comparison of differential space-time signalling schemes for OFDM systems

The interaction between layers in multilayer cellular systems can be in terms of directing the traffic overflow from the lower layer to the overlaying layer (overflow sensitive cell-layer selection strategy), and/or of classifying the users based on their speed (speed sensitive cell-layer selection strategy). In this paper, we study a hybrid speed/overflow sensitive micro/macro cell-layer selection strategy for two-layer CDMA cellular systems. It is shown that the speed threshold has a major impact on the capacity and the handover rate in such systems. Therefore, here we propose a handover rate control scheme where the speed threshold can be dynamically adjusted according to the handover and blocking rate feedbacks. Simulation results show that the proposed scheme can control the handover rate even when the speed distribution of mobile stations changes with time.

Performance of relay cyclic delay diversity in multicarrier system

The performance of multi-hop multiple-input, multiple-output (MIMO) communication scenarios is evaluated for space time block coded (STBC) and the proposed distributed cyclic delay diversity (CDD) technique. By increasing the frequency selectivity, multi-stage CDD with low complexity implementation yields significant performance improvement as relaying network. Both diversity techniques based on frequency-correlated MC-CDMA subcarriers networks are investigated by using the moment generating function (MGF) over correlated Nakagami-m fading channels .The system performance evaluation is provided for the exact average symbol error rate (SER) and the end-to-end bit error rate (BER). In order to maximize the end-to-end throughput [bits/s/Hz], explicit allocations strategies are derived and analyzed. Simulation results are also presented which show excellent agreement with the analytical framework