MC-CDMA:

 Multicarrier CDMA (MC-CDMA) is a technique that combines the advantages of OFDM and
CDMA.
 It is very effective at both combating ISI and as a mechanism to allow multiple users to share the
same channel.
Why MC-CDMA:
 In a multicarrier transmission system, the available channel bandwidth is divided into multiple sub channels
such that data symbols modulated by different subcarriers can be transmitted in parallel.
 In order to make the most of the available bandwidth, without inter-channel interference (ICI) in such a
manner that all information-bearing waveforms of the sub-channels are orthogonal on some time interval.
 For high bit rate transmission (around 30 MBit/s) over non-ideal propagation channels, OFDM parallel
transmission offers many advantages over conventional single carrier systems, such as robustness against
multipath frequency selective fading.
Why Adaptive Modulation
 The high mobility of the subscribers, in wireless communication systems, causes radio channels to vary
rapidly with time.
 Signals take different paths to reach their destination and while doing so experience different speeds of
phase rotation caused by the Doppler spread.
 These result in rapid variation of the radio channels [5]. These time varying radio channels characterized by
multi-path fading at times have very good SNR and at times are worse.
 Using a fixed modulation technique for the system, the system would have to be built for such a standard
which would take care of the worst-case scenario of the channel to offer an acceptable bit error rate (BER).
 But as the wireless channels vary rapidly with time, the fixed modulation based system is spectrally
inefficient because it could have used a higher order modulation format during the “good” channel
condition.
 To achieve robust and spectrally efficient communication over multipath fading channels, adaptive
modulation is used which adapts the transmission scheme to the current channel characteristics.
 Taking advantage of the time-varying nature of wireless channels, the adaptive modulation scheme varies
the transmission power, data rate (constellation size), coding and modulation schemes, or any combination
of these parameters according to the state of the channel [6].
 If the channel can be estimated then the transmitter can adapt to the current conditions by varying the
modulation type while maintaining a constant BER.
 This is typically done by making a channel estimate at the receiver and transmitting this estimate back to
the transmitter.
 Thus, the adaptive technique will have a higher data throughput when the channel conditions are favorable
and will reduce the throughput as the channel worsens.
 In other words, the principle of adaptive modulation consists of allocating many bits to carriers with a high
SNR, whereas on carriers with low SNR only a few or no bits at all are transmitted.

Turbo Codes
are very powerful codes that can come within a fraction of a dB of the Shannon capacity limit on AWGN
channels. Turbo codes and the more general family of codes on graphs with iterative decoding algorithms
[11, 12] have been studied extensively As first described by Berrou et al, turbo codes consist of two key
components: parallel concatenated encoding and iterative, “turbo” decoding [9, 13].

Iterative, or “turbo” decoding exploits the component-code substructure of the turbo encoder by
associating a component decoder with each of the component encoders.
Adaptive Coded Modulation
The basic idea of adaptive coded modulation is to exploit the separability of code and constellation design
inherent to coset codes.
Coded modulation is a natural coding scheme to use with variable-rate variable-power MQAM, since the
channel coding gain is essentially independent of the modulation. We can therefore adjust the power and
rate (number of levels or signal points) in the transmit constellation relative to the instantaneous SNR
without affecting the channel coding gain, as we now describe in more detail.
The coded modulation scheme is shown in Figure 9.13. The coset code design is the same as it would
be for an AWGN channel,

Abstract:
 Adaptive modulation based MC-CDMA systems can play a vital role in future generation consumer
communication electronics.
 Adaptive modulation, combined with MC-CDMA based transmission technology, is a promising way to
increase the data rate that can be reliably transmitted over the wireless radio channels.
 For 4G wireless networks, which demand very high data rate up to 100 Mbits/s with the constraints
limiting higher data rate being severe ISI due to multipath and the limited spectrum, such kind of adaptive
modulation based multi-carrier systems applied to a wide-area environment, can achieve very large
average user throughputs.
 In this paper, adaptive modulation based M-ary PSK, M-ary QAM, M-ary CPM, M-ary MHPM and
GMSK systems applied to a Turbo coded MC-CDMA system in a Rayleigh fast fading channel
environment have been investigated and the BER performance of all these digital modulation techniques
have been compared.
 Results of the comparative study indicate that the continuous phase modulation schemes like CPM,
MHPM and GMSK gives better performance as compared to PSK and QAM schemes.
 At most of the time, the MHPM system outperforms both GMSK and CPM.
 The PSK and QAM based systems perform well till the number of users is around 10.
 As a whole, the adaptive MHPM system is found to give the optimum performance among the considered
digital modulation schemes for the MC-CDMA system in 4G environment.

System Model diagram:

 The block diagram of the adaptive modulation based MCCDMA system is shown in figure .
 Binary data is first encoded using Turbo coding, followed by serial-to-parallel conversion to produce low
bit-rate streams. Each stream is then modulated using a suitable digital modulation method, such as, BPSK,
QPSK, 8 PSK, 16 QAM etc, depending on the channel estimate information provided by the receiver.
 The adaptively modulated streams are then passed through the MCCDMA transmitter block, up-converted
by an RF amplifier (not shown in this figure) and transmitted.
 The receiver performs the reverse operation to demodulate and decode the original information.
 The channel estimator estimates the quality of the channel (Carrier-to-Noise Ratio, CNR) from the pilot
symbols which are known QPSK symbols and informs the transmitter.
 Based on this channel quality estimate, the transmitter decides the modulation format to be used for the
next transmission. Moreover, it has been assumed that the receiver is aware of the modulation scheme in
use.