Vimal K. Dubey

Detection schemes for space-time block code and spatial multiplexing combined system

Space-time block code is combined with spatial multiplexing technique over multiple-input multiple-output system to take advantages of both schemes. The transmit antennas are divided into groups and each group transmits space-time coded blocks in parallel. At receiver side, three types of group receivers are proposed to separate the filtered version of the multiplexed space-time coded symbol blocks followed by space-time decoder. Error rate performances of the detection schemes are evaluated in correlated channels. The diversity order of the combined system is compared with that of the SM system and the STBC system.

Accuracy of geometric channel-modeling methods

When constructing a propagation channel model, a substitute is often created by giving an arbitrary shape or form to scatterer distributions based on its intuitive appeal for a certain radio environment. However, such models do not necessarily represent the actual propagation process and may yield inaccurate results. The main objective of this paper is to provide an insight into the underlying relationship between geometric models and the particular physical propagation process they represent. The workhorse is the semi-geometrically based statistical (SGBS) model and the two heuristic rules. The SGBS model defines the distribution of dominant scatterers contributing to the last reradiation of multipath components to the base station. The earlier multiple-reflection process is modeled using the composite Nakagami/log-normal probability density function. Two parameters are then introduced; namely, the effective path length and the normalized space-dependent intensity measure. Using these two metrics, two heuristic rules are subsequently proposed to provide the missing link between the canonical models and the physical channel. The rules are then applied to revisit several widely used geometric models in macro- and microcellular environments. As a working example, the Gaussian scatterer density model is further extended using such an approach. Important channel parameters such as power azimuthal spectrum, power delay spectrum, and azimuthal and delay spreads are then calculated and compared with simulation results.

Ka-band land mobile satellite channel model incorporating weather effects

A Ka-band land mobile satellite (LMS) channel model which is able to take into account weather impairments is proposed. The statistics of the received signal and the BER performance of the system under the proposed channel model are obtained and compared with the results generated using Loos weather-affected LMS channel model. The proposed Ka-band (20/30 GHz) LMS channel model is shown to be more reasonable as it gives realistic results.

Extended orthogonal polyphase codes for multicarrier CDMA system

The Complete Complementary (CC) codes have been able to improve the performance of multicarrier CDMA system for both downlink and uplink transmission. However, the number of users supported by CC code design is limited as its family size is relatively small. In this letter, an extension to CC codes using Zadoff-Chu sequence is proposed. The proposed codes are able to support large number of users and have perfect auto- and cross-correlation function just like CC codes.

BER performance of OFDM system over frequency nonselective fast Ricean fading channels

The fast changing frequency nonselective Ricean fading channel introduces a complicated multiple distortion and an extra additive noise component for an OFDM system. The multiple distortion is the average of the sum of N(N/spl ges/3) correlated Ricean random variables. We propose an approximate technique for calculating the probability density function (PDF) of the multiple distortion under the assumption that the channel response changes in a linear fashion during one OFDM symbol. As a result, the bit error rate (BER) formula of a BPSK modulated OFDM system is derived. The results obtained using the derived formula describe well the OFDM performance under the time variant channel and match very well with the simulation results.

Effect of employing directional antennas on mobile OFDM system with time-varying channel

In this letter, we analyze the inter-carrier-interference (ICI) caused by Doppler spreading in an orthogonal frequency-division multiplexing system when the angle-of-arrival (AOA) of the received signal is not uniformly distributed. An expression of the ICI power is derived by taking into account the probability density function of the AOA. We also present examples using directional antennas at the base station, at the mobile or at both mobile and base station. The ICI power changes marginally when a directional antenna is placed at the base station. Finally, we show that the ICI power can be reduced substantially when a directional antenna, along with frequency tracking, is employed at the mobile.

Exact analysis for downlink MC-CDMA in Rayleigh fading channels

In this letter, downlink multicarrier code-division multiple access (MC-CDMA) systems with maximal ratio combining (MRC) in frequency-selective Rayleigh fading channels is considered. An exact bit error rate (BER) is provided based on the characteristic function (CF) and residue calculation method. The BER performance is evaluated in the time domain instead of the frequency domain. This method does not require any assumption regarding the statistical or spectral behavior of multiple access interference (MAI).

Dynamic simulation model of indoor wideband directional channels

Culled from existing channel measurements, a dynamic indoor wideband directional-channel simulator is constructed by incorporating the spatial-temporal properties as well as the time-varying nature of propagation environments. A two-state semi-Markov model is used to account for the births and deaths of scattering clusters, which represents an improvement over the conventional Markov model where the duration in each state is forced to follow an exponential distribution. The spatial-temporal variations of clusters within their lifespan are modeled based on the real-life measurement results. The ranges of angle of arrival (AOA), angle of departure (AOD), and time of arrival (TOA) for each cluster are partitioned into finite number of intervals using the deterministic simulation methods. Subsequently, the cluster properties at different intervals can be derived from the empirical findings in previous channel-measurement campaigns. The efficacy of the proposed simulator is demonstrated by applying it to characterize a time-varying multiple-input multiple-output (MIMO) channel.

Performance of a Complete Complementary Code-Based Spread-Time CDMA System in a Fading Channel

In this paper, we study the performance of complete complementary code-based spread-time code-division multiple access (CC-ST-CDMA) in frequency-nonselective and frequency-selective fading channels. The theoretical bit-error-rate (BER) expressions conditioned on the number of users and the sequence length (and the number of resolvable paths for multipath frequency-selective fading) are obtained. The theoretical approximation of the BER for the system using long-sequence-length complete complementary codes is investigated as well. The simulations are conducted over flat-fading and two-path frequency-selective fading channels and have been used to verify the theoretical analysis. The results show that the proposed CC-ST-CDMA system has a comparable BER performance with complete complementary code-based multicarrier direct-sequence code-division multiple access (CC-MC-DSCDMA) for a flat-fading channel. However, for a frequency-selective fading channel, the CC-ST-CDMA is found to be more resistant, resulting in about 2-3 dB better performance than the CC-MC-DSCDMA.

Transmit diversity and combining scheme for spatial multiplexing over correlated channels

Space-time block codes (STBC) are combined with a spatial multiplexing (SM) technique to improve SM system performance in the presence of fading correlation. The data rate is compromised for diversity gain in the combined system. Using Alamoutis scheme at the transmitter, a general combining scheme is presented for a 2/spl times/2 multiple-input multiple-output (MIMO) system and extended, with modification, to MIMO systems with more antennas at both ends.