Essay: MC CDMA (telecommunications)

Peak to Average Power Ratio (PAPR) is one of the most
important drawback faced by Multi Carrier Code Division
Multiple Access (MC CDMA). In this paper a new technique
is introduced to reduce the PAPR, it is based on combining
Discrete Hartley Transform and Mu-law Companding. The
performance of PAPR is calculated using the Complementary
Cumulative distribution (CCDF) function. The simulation
results of the new technique shows a better reduction in
PAPR compared to other methods.
1. Introduction
MC CDMA is a multiple access scheme used in telecommunication
systems, allowing multiple users to transmit
their data at the same time over same frequency band. It is a
combination of Orthogonal Frequency Division Multiplexing
(OFDM) and Code Division Multiple Access (CDMA),
so it has the advantages of both the systems. MC CDMA is
a very attractive technique for high speed data transmission
over multipath fading channels. The applications of MC
CDMA includes Long Term Evaluation (LTE), WiMAX,
and Digital TV transmission. PAPR is a one of the major
problem faced by MC CDMA. The high PAPR degrades the
performance and power efficiency of the system and leads
to complexity of DAC and ADC, high bit error rate, poor
performance. High power amplifiers are require which will
increase the total cost of the system.
The PAPR reduction techniques are mainly classified
into two [4], they are signal distortion techniques and signal
scrambling techniques. Signal distortion techniques includes
companding, clipping, envelope scaling, peak cancelling,
peak windowing. In companding the transmitted
signals are compressed at the transmitter and expanded at
the receiver. Its performance is better compared to other
signal distortion techniques because here the signal distortion
is less. Signal scrambling techniques includes the
partial transmit sequence, the selective mapping, interleaving,
coding, precoding transform techniques. In precoding
transform techniques the data which is to be transmitted
is transformed into another data using discrete transforms
like discrete cosine transform (DCT), discrete wavelet transform
(DWT), discrete hartley transform (DHT). The signal
scrambling technique will not distorts the data so that Bit
Error Rate (BER) and the Power Spectral Density (PSD)
performance are unaffected.
The PAPR problem mainly affects the High Power Amplifier
(HPA) in the transmitter section. The HPA should
operate in the linear region for proper working. The high
PAPR moves the high amplitudes into saturation region resulting
in in-band radiation (IBR) and out-of- band radiation
(OBR). The IBR increases Bit Error Rate (BER), but
the OBR will leads to Adjacent Channel Interference (ACI).
Non linear distortion in HPAs results in Inter Symbol Interference
(ISI), increased co-channel interference, increased
cost, reduced battery life, loss of subcarrier orthogonality
and spectral regrowth [3].
The efficient way of reducing PAPR is combining two
suitable reduction techniques. The methods which are to
be selected depends upon the need of application. This paper
presents a combination of Discrete Hartley Transform
(DHT) with companding. DHT is simple compared to other
precoding transform techniques. Mu-law companding technique
is used here, which enlarges only small signals so
that average power increases. The advantages of Mu-law
companding technique are reduction of more PAPR, better
Power spectral density, low implementation complexity and
no constraints on modulation format and subcarrier [4]. So
by combining these two techniques the PAPR will reduce
more with less complexity in the system design. The major
advantage of this combination is that it reduces more PAPR
without affecting the BER and PSD performance.
Detailed explantion of proposed MC CDMA system is
given in section 2. Simulation of different PAPR reduction
techniques using DHT, companding, DHT and companding
are done in section 3 and compared the results with the original
MC CDMA. Finally, conclusions are given in section 4.