S. Ghavami, B. Abolhassani,
Volume 6, Issue 3 (September 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.
S. Ghavami, B. Abolhassani,
Volume 9, Issue 1 (March 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.
