Zoran Veljovic

A solution for performance improvements of OFDM-CDMA system with pilot tone

In this paper, MIMO OFDM–CDMA downlink scheme is proposed as a solution for improving performance of the OFDM–CDMA downlink system with pilot tone and threshold detection combining (optimum TDC). The new presented system with MIMO included uses space–time block coding applied to two, three and four transmit antennas and it has an arbitrary number of receive antennas. Bit error rate performance in the case of Ricean frequency selective fading is evaluated for the original system as well as for the one with MIMO included. For that reason an adequate simulation model is developed. The results show that the proposed system significantly outperforms the OFDM–CDMA downlink system with pilot tone and optimum TDC.

Performance Analysis of a New OFDM-CDMA System with Pilot Tone for Multimedia Communications

In this letter, we analyze a solution for multiple access technique based on OFDM-CDMA with pilot tone. The proposed system is designed to provide higher data rates in multimedia wireless communications. Primarily, it is achieved by introducing a serial to parallel converter with a pilot tone implemented in each of its output branches. The bit error rates (BER) performance of the proposed scheme are evaluated using computer simulations. The results show that the presented OFDM-CDMA system provides high data rates and low BER and, for the same bandwidth, it performs better than its competitive solution.

BER performance of MIMO OFDM-CDMA system in ricean fading channel

In this paper, bit error rate (BER) performance of the MIMO OFDM-CDMA system, in case of Ricean frequency selective fading, is evaluated using originally developed computer simulation. The assumed system uses space-time block coding applied to two, three and four transmit antennas and has arbitrary number of receive antennas. Channel estimation is performed by implementing pilot sequences at each transmit antenna. The results are compared with BER results of the competitive OFDM-CDMA down link system with threshold detection combining (optimum TDC). The results show that the presented system provides lower BER, even when less subcarriers are used for transmitting single data symbol, than its competitive solution.

On the optimal subcarrier mapping scheme in OFDM decode-and-forward relay systems

BER performance of the DPSK (Differentially Phase Shift Keying) modulated dual-hop OFDM based DF (Decode and Forward) relay system, implementing subcarrier mapping (SCM) at the relay (R) station, is examined. Closed form BER expressions are derived for the two SCM schemes, denoted as BTB SCM (Best-to-Best Subcarrier Mapping) and BTW SCM (Best-to-Worst Subcarrier Mapping) in the scenario with Rayleigh fading on both hops. The accuracy of the obtained analytical results is verified through comparison with the simulation results. It is shown that BTB SCM scheme achieves the lowest BER results for all SNRs (Signal-to-Noise Ratios) of interest, which approves it to be the optimal SCM scheme in dual-hop OFDM DF relay systems, as it has already been known that the same SCM scheme maximizes the capacity.

A New Solution for Simple Cooperative Relaying

In this paper we present a new simple cooperative relaying scheme. The proposed scheme is compared with simple cooperative schemes with virtual orthogonal space time block coding (OSTBC) and quasi orthogonal space time block coding (QOSTBC). Every of these three schemes includes one base station with two antennas, two relay stations each with a single antenna and one mobile station with a single antenna. The aim of the proposed scheme is to preserve, as much as it is possible bit error rate (BER) performance of the cooperative scheme with virtual OSTBC, but with increased code rate. It is shown that the presented scheme obviously outperforms simple cooperative scheme with virtual QOSTBC, while in comparison with simple cooperative scheme with virtual OSTBC has very similar BER performance and increased code rate.

PErformance analysis of LPF based VOQ crossbar switches

In this paper, LPF based schedulers for VOQ crossbar switch implementation through FPGA circuits are analysed. Switch performance (average cell latency, queue lengths and loss probability) are evaluated under different traffic conditions. Results of behavioral simulations on implemented circuits are compared with simulations on modeled circuits that are not limited as implemented ones. Particularly consideration is given to required queue lengths and impact of its limitation to scheduler behavior. It is shown that queue length of 127 cells is enough to manage almost any offered load under various traffic conditions that we considered.

Ergodic capacity of OFDM AF fixed gain relay system with subcarrier mapping

In this paper ergodic capacity of dual-hop OFDM amplify-and-forward (AF) fixed gain (FG) relay system, implementing capacity maximizing best-to-best subcarrier mapping (BTB SCM) scheme at the relay (R) station, is examined. Scenario without direct communication between the source of information (S) and destination terminal (D) is considered, with Rayleigh fading statistics on both hops. A closed form relation for the tight lower bound of the system ergodic capacity is derived. Comparisons of the obtained ergodic capacity results with the simulation results have confirmed the accuracy of the undertaken analytical approach. Thus, the derived analytical expression presents very useful and most accurate available tool for the performance assessment of the OFDM AF FG relay system with BTB SCM, considered to be interesting solution for the next generation of mobile cellular systems.

Exhaustive limited service round robin matching algorithm for buffered crossbar switches

In this paper a high performance and simple scheduling algorithm for combined input-crosspoint crossbar switches, called exhaustive round-robin with limited service (ELRR) is presented and analyzed. We propose using of this scheduling system for arbitration at inputs and crosspoints. If the virtual output queue (crosspoint buffer) becomes empty, the input (crosspoint) arbiter updates its pointer to the next location in a fixed order. Otherwise, the pointer remains at the current virtual output queue (crosspoint buffer). To avoid unfairness under some extremely unbalanced traffic pattern limited service matching is presented. It is shown that this new solution offers high performance for several admissible traffic patterns using only one-cell crosspoint buffers.

Error rates for trellis coded PSK modulation with pilot tone in time-selective fading channel

In this paper trellis coded modulation effects for the PSK scheme with a pilot tone have been evaluated. Closed form analytical expressions for error rates in the radio channel with TCM are derived. The obtained relations are suitable for any fading characteristics as well as for all PSK and QAM modulation methods. Error rates are presented first for the standard land mobile radio channel with flat Rayleigh fading. Then, taking into account the time-selectivity of the fading process, the necessary mathematical transformations are performed leading to error rate determination for the case of TCM PSK with a pilot tone.

Evaluation of trellis coded modulation effects in time-selective fading mobile radio channel

In this paper effects of trellis coded modulation in mobile radio systems are evaluated. The specific channel with timeselective fading process is considered. The complete descriptions of such channel and an appropriate receiver is given. General model for bit error rate calculations is presented. That model enables error determination for every type of fading encountered in mobile radio systems as well as for different TCM schemes. Here we chose to analyse trellis encoding in combination with PSK, PSK with pilot tone and DPSK.