25-08-2017, 09:32 PM
Interference Management for DS-CDMA Systems through Closed-Loop Power Control, Base Station Assignment, and Beamforming
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Abstract
In this paper, we propose a smart step closed-loop power control (SSPC) algorithm and a base station assignment
method based on minimizing the transmitter power (BSA-MTP) technique in a direct sequence-code
division multiple access (DS-CDMA) receiver with frequency-selective Rayleigh fading. This
receiver consists of three stages. In the first stage, with constrained least mean squared (CLMS) algorithm,
the desired users’ signal in an arbitrary path is passed and the inter-path interference (IPI) is reduced 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 use for more reduction of the IPI and MAI 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 reduce the network bit error rate (BER) compared to the other methods. Also, we observe that
significant savings in total transmit power (TTP) are possible with our methods.
Introduction
Code-Division Multiple Access (CDMA) for cellular
communication networks requires the implementation
of some forms of adaptive power control. In uplink of
CDMA systems, the maximum number of supportable
users per cell is limited by multipath fading, shadowing,
and near-far effects that cause fluctuations of the received
power at the base station (BS). Two types of power control
are often considered: closed-loop power control and
open-loop power control [1,2]. In a closed-loop power
control, according to the received signal power at a base
station, the base station sends a command to a mobile set
to adjust the transmit power of the mobile. Also, closedloop
power control is employed to combat fast channel
fluctuations due to fading. Closed-loop algorithms can
effectively compensate fading variations when the power
control updating time is smaller than the correlation time
of the channel. However, in an open-loop power control, a
mobile user adjusts its transmit power according to its received power in downlink [1-5]. In this paper, an adaptive
closed-loop power control algorithm is proposed to
compensate for near-far effects.
Diversity and power control are two effective techniques
for enhancing the signal-to-interference-plus-noise
ratio (SINR) for wireless networks. Diversity exploits the
random nature of radio propagation by finding independent
(or, at least, highly uncorrelated) signal paths for
communication. If one radio path undergoes a deep fade,
another independent path may have a strong signal. By
having more than one path to select from, the SINR at
the receiver can be improved. The diversity scheme can
be divided into three methods: 1) The space diversity; 2)
The time diversity; 3) The frequency diversity. In these
schemes, the same information is first received (or
transmitted) at different locations (or time slots/frequency
bands). After that, these signals are combined to increase
the received SINR. The antenna array is an example
of the space diversity, which uses a beamformer to increase
the SINR for a particular direction
The first goal of this paper is to extend the works in [9]
and [10] by considering multiple-cell system and closedloop
power control. In these works, a RAKE receiver in
single-cell system with conjugate gradient adaptive beamforming
was proposed in the presence of frequencyselective
Rayleigh fading channel, and perfect power
control (PPC) was considered.
In this work, the performance analysis of direct
sequence (DS)-CDMA system in frequency-selective
Rayleigh fading channel has been studied. If the delay
spread in a multipath channel is larger than a fraction of
a symbol, the delayed components will cause inter-symbol
interference (ISI). Adaptive receiver beamforming schemes
have been widely used to reduce both co-channel
interference (CCI) and ISI and to decrease the bit error
rate (BER) by adjusting the beam pattern such that the
effective SINR at the output of the beamformer is optimally
increased [11].
In this paper a RAKE receiver in DS-CDMA system is
analyzed in three stages according to Figure 1 [9]. In the
first stage, this receiver uses constrained least mean
squared (CLMS) adaptive beamforming algorithm to find
optimum antenna weights assuming perfect estimation of
the channel parameters (direction, delay, and power) for
the desired user. The desired user resolvable paths’
directions are fed to the beamformer to reduce the inter-path
interference (IPI) from other directions. Also, the RAKE
receiver uses conventional demodulation in the second
stage and conventional maximal ratio combining (MRC)
in the third stage to reduce multiple access interference
(MAI) and the other interferences. Reducing the MAI
and CCI will further decrease the system BER.
To improve the performance of cellular systems, base
station assignment (BSA) technique can be used. In the
joint power control and base station assignment, a number
of base stations are potential receivers of a mobile transmitter. Here, the objective is to determine the assignment
of users to base stations which minimizes the
allocated mobile powers [12-15]. In simple mode and in
multiple-cell systems, the user is connected to the nearest
base station. This way is not optimal in cellular systems
under the shadowing and multipath fading channels and
can increase the system BER.
Accordingly, the second goal of this paper is to use
base station assignment technique. In [14], the combined
the base station assignment and power control was used
to increase uplink capacity in cellular communication
networks. In that work, it was shown that if there exists
at least one feasible base station assignment, the proposed
algorithm will find the jointly optimal base station
assignment and minimal transmitter power level for all
users. In this paper, we present the base station assignment
method based on minimizing the transmitter power
(BSA-MTP) for decreasing the BER in all cells.
The organization of the remainder of this paper is as
follows. The system model is presented in Section 2. The
RAKE receiver structure is described in Section 3. In
Section 4, we propose smart step closed-loop power control
(SSPC) algorithm. In Section 5, the BSA-MTP technique
is presented. Section 6 describes switched-beam
(SB) technique and equal sectoring (ES) method. Finally,
simulation results and conclusions are given in Section 7
and Section 8, respectively.