Showing 5 results for Cdma
V. Vakil, H. Aghaeinia,
Volume 5, Issue 1 (3-2009)
Abstract
The throughput enhancement of Space-Time Spreading (STS)-based Multicarrier
Direct Sequence Code Division Multiple Access (MC DS-CDMA) system is investigated in
this paper. Variable Spreading Factor (VSF) is utilized to improve the data throughput of
the system. In this contribution, an analytical approach is proposed to compute a new
expression for the Bit Error Rate (BER) performance of the STS-based MC DS-CDMA
system against pre-dispreading Signal-to-Noise Ratio (SNR) for different values of
spreading factor (SF). The other contribution of the paper is deriving a new closed form
expression for computing the throughput enhancement and the BER performance of the
VSF STS-based MC DS-CDMA system over Rayleigh fading channel. It is demonstrated
that using VSF method in STS-based MC DS-CDMA system improves the throughput of
the system by keeping the BER performance at the target level.
S. Ghavami, B. Abolhassani,
Volume 6, Issue 3 (9-2010)
Abstract
Wide band code division multiple access (WCDMA) signals, transmitted by the
base station high power amplifiers (HPAs), show high peak to average power ratios
(PAPR), which results in nonlinear distortions. In this paper, using computer simulations
effect of using a predistorted HPA on the symbol error rate (SER) of multi-user detectors in
terms of output back-off (OBO) in the transmit power is analyzed. As well, using
polynomials for modeling predistorters to remove nonlinear distortions of traveling wave
tube amplifiers (TWTAs) and of solid state power amplifiers (SSPAs), effect of different
degrees of polynomials on the SER is investigated. Simulation results show that a
polynomial of degree 4 is a sufficient degree polynomial, which fits to the AM/AM
characteristic of the predistorter for TWTAs. As well, for solid state power amplifiers
(SSPAs) with different p values, different approximations are considered and sufficient
degree polynomials are found.
M. Dosaranian Moghadam, H. Bakhshi, G. Dadashzadeh,
Volume 6, Issue 3 (9-2010)
Abstract
In this paper, we propose smart step closed-loop power control (SSPC)
algorithm and base station assignment based on minimizing the transmitter power (BSAMTP)
technique in a direct sequence-code division multiple access (DS-CDMA) receiver in
the presence of frequency-selective Rayleigh fading. This receiver consists of three stages.
In the first stage, with conjugate gradient (CG) adaptive beamforming algorithm, the
desired users’ signal in an arbitrary path is passed and the inter-path interference is
canceled in other paths in each RAKE finger. Also in this stage, the multiple access
interference (MAI) from other users is reduced. Thus, the matched filter (MF) can be used
for the MAI reduction in each RAKE finger in the second stage. Also in the third stage, the
output signals from the matched filters are combined according to the conventional
maximal ratio combining (MRC) principle and then are fed into the decision circuit of the
desired user. The simulation results indicate that the SSPC algorithm and the BSA-MTP
technique can significantly improve the network bit error rate (BER) in comparison with
other algorithms. Also, we observe that significant savings in total transmit power (TTP)
are possible with our proposed methods.
S. Shaerbaf, S. A. R. Seyedin,
Volume 7, Issue 3 (9-2011)
Abstract
Chaos based communications have drawn increasing attention over the past years. Chaotic signals are derived from non-linear dynamic systems. They are aperiodic, broadband and deterministic signals that appear random in the time domain. Because of these properties, chaotic signals have been proposed to generate spreading sequences for wide-band secure communication recently. Like conventional DS-CDMA systems, chaos-based CDMA systems suffer from multi-user interference (MUI) due to other users transmitting in the cell. In this paper, we propose a novel method based on radial basis function (RBF) for both blind and non-blind multiuser detection in chaos-based DS-CDMA systems. We also propose a new method for optimizing generation of binary chaotic sequences using Genetic Algorithm. Simulation results show that our proposed nonlinear receiver with optimized chaotic sequences outperforms in comparison to other conventional detectors such as a single-user detector, decorrelating detector and minimum mean square error detector, particularly for under-loaded CDMA condition, which the number of active users is less than processing gain.
S. Ghavami, B. Abolhassani,
Volume 9, Issue 1 (3-2013)
Abstract
In the down link scenario of code division multiple access (CDMA) systems, multi user detectors (MUDs) such as linear de-correlating detector (LDDs) provide satisfactory symbol error rates (SERs) at the expense of much increased complexity, they require all active users’ spreading sequences, which is impractical from privacy point of view. To overcome this impracticality, a simple matched filter receiver is considered in this paper, which requires no knowledge of co-users’ spreading sequences. However, this simple receiver degrades the SER due to multiple access interference (MAI). To overcome this SER degradation, a zero force (ZF) pre-coder is employed in the transmitter traditionally. Moreover, a composite of CDMA signals has a large peak to average power ratio (PAPR), which causes nonlinear distortion (NLD) at the output of the base station high power amplifier (HPA). This also results in degrading the SER. We analyze the down link scenario of CDMA system to derive an equation for the SER of system with ZF-pre-coder plus HPA in the transmitter and matched filter in the receiver over two cases: additive white Gaussian noise (AWGN) and AWGN plus flat fading channels. Theoretical analysis and numerical results show that the ZF pre-coder increases the total degradation of the link significantly compared with that of the LDD. So, as a solution, rather than using ZF pre-coder, we propose a new method which is called extended joint channel estimation method, it is based on joint estimation of channel gains and LDD operator by the mobile station (MS). In that base station (BS) transmits the row k of LDD operator to the MS k. Simulation results show that the SER of this new proposed method is matched to that of LDD in AWGN channel when the number of pilot symbol repetition is equal to 8. Moreover, this method has the two added advantages of no need for providing the spreading sequences of all co-users and meeting a satisfactory total degradation. Furthermore, our analysis shows that loss in spectral efficiency due to transmitting the pilot symbols in the proposed method is negligible for the practical values of traffic variations.
