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SSRG International Journal of Electronics and Communication Engineering (SSRG-IJECE) – volume1 issue8 Oct 2014
Future mobile communication using OFDM and MC-CDMA techniques-Review
T. Jaya
Research Scholar, Department of ECE,
Vels University,
Chennai, India
Abstract — Emerging wireless communication systems demand the
need for high data rates and to support integrated services. To make
this possible, the primary requirement is that the physical layer
should facilitate multiuser /multi rate transmission. To enhance the
system capacity to meet the future requirements, it is very much
required to look for novel technologies or new concepts for
improving bandwidth efficiency. In this paper, an adaptive downlink
modulation using OFDM and MC-CDMA is presented for
maximizing the system capacity of future mobile communications.
The basic concept of the proposed method is that a time frame is
divided into two sub-frames, one is for OFDM and the other is for
MC-CDMA. The base station (BS) allocates a preferable modulation
scheme to each user per each time slot in accordance with their
service requirements and link conditions such as the received signal
strength indication (RSSI) level and interference signal strength [1].
Computer simulation is also conducted to evaluate the throughput
performance of the proposed system. The simulation results show
that using a combination of OFDM and MC-CDMA give better
throughput performance rather than using either OFDM or MCCDMA.
Keywords — Orthogonal frequency division multiplexing (OFDM),
Multi carrier code division multiple access (MC-CDMA), Received
signal strength indication (RSSI), Quality of Service (QoS)
Orthogonal Frequency Division multiplexing and MultiCarrier Code Division Multiple Access (MC-CDMA) [3].
A. OFDM System Model
OFDM is a multicarrier system, with the functional
block diagram shown in Figure 1 below.
Figure 1: Basic structure of OFDM system.
B. Introduction to OFDM
I. INTRODUCTION
Orthogonal frequency division multiplexing (OFDM)
is a Multi carrier modulation (MCM) technique, which offers
high spectral efficiency, immune to the multipath delay, low
inter-symbol interference (ISI), immunity to frequency
selective fading and high power efficiency [2, 3].Due to these
merits OFDM is used in high data rate communication
systems such as Digital Video Broadcasting (DVB) and
worldwide interoperability for microwave access (mobile WiMAX). However OFDM systems suffer from serious problem
of high PAPR.
OFDM is an attractive modulation scheme because of
its high immunity to multi-path fading and its capability of
offering high transmission data rate. However, the link quality
of the OFDM system could be degraded when the co-channel
interference signal strength from adjacent cells is increased.
OFDM can effectively handle frequency-selective
fading without complex equalization structures and the
enhancement of noise at the receiver. The demodulation and
modulation processes have very low complexity when the Fast
Fourier Transform (FFT) and its Inverse (IFFT) are used.
Wireless communications share the transmission media,
namely, the wireless channel, and generally involve multiple
users. Hence there is a need for a multiple access technique
[2]. There are different multiple access schemes based on
OFDM modulation. We focus on two methods: Multiuser
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E.Gopinathan
Dean, School of Engineering,
Vels University,
Chennai, India
The aim of OFDM is to divide the wide frequency
selectivity of fading channels into multiple flat fading
channels. Orthogonality property allows multiple information
signals to be transmitted in parallel over a common channel
and is detected without any interference. OFDM system
performance can be improved by channel coding.
C. Binary source
In Figure 1, at the transmitting end, the random data
generator generates binary data that is frame based. 48
samples per frame are used in the data output, and the data
rate used is 1 Mbps.
D. Data Mapping
The input data stream is available serially, and is
converted into parallel stream according to digital modulation
scheme. The data is transmitted in parallel by assigning each
data word to one carrier in the transmission. Once each
subcarrier has been allocated symbols, they are phase mapped
according to modulation scheme, which is then represented by
a complex In-phase and Quadrature-phase (I-Q) vector. In
general, the selection of modulation scheme applied to each
sub-channel depends solely on the compromise between the
data rate requirement and transmission robustness.
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SSRG International Journal of Electronics and Communication Engineering (SSRG-IJECE) – volume1 issue8 Oct 2014
E. IFFT-Frequency Domain to Time Domain Conversion
The IFFT converts frequency domain data into time
domain signal and at the same time maintains the
orthogonality of subcarriers. The real signal output can be
generated by arranging conjugate subcarriers. In this stage,
IFFT mapping, zero pad, and selector blocks are included.
Zero pad blocks adds zeros to adjust the IFFT bin size of
length L, as the number of subcarriers may be less than bin
size. Selector block reorders the subcarriers.
F. Guard Period
The effect of ISI on an OFDM signal can be
eliminated by the addition of a guard period at the start and
end of each frame. The guard period adds time overhead thus
decreasing the overall spectral efficiency of the system. Guard
duration should be longer than channel delay spread. After the
guard band has been added, the symbols are converted into
serial form. One frame length duration T = Ts + Tg, where Ts
= NT, N = number of carriers and Tg is the guard interval.
This is the OFDM base band signal, which can be up
converted to required transmission frequency. An AWGN
channel model is then applied to transmitted signal. This
model allows for the Signal to Noise Ratio (SNR) variation.
The receiver performs the reverse operation of the transmitter.
The receiver consists of removal of guard band, FFT, removal
of zero padding and de-mapping of data.
The block diagram of the MC-CDMA system is
shown in figure 2. 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, 16QAM etc, depending on the channel estimate
information provided by the receivers. Figure 3 represents the
adaptive downlink modulation scheme using OFDM and MCCDMA [4]. In this figure, a frame is divided into multiple
slots. Some slots are allocated to OFDM and others to MCCDMA. The transmission power for OFDM slots and MCCDMA slots is set to be identical to maintain the continuity of
the signal level between two modulation schemes.
C. Frame Structure
II. RELATED WORK
A. Multicarrier CDMA techniques
There are three types of multi-carrier CDMA
techniques, Multi-carrier CDMA (MC-CDMA), Multicarrier
Direct Sequence CDMA (MC DS-CDMA), and Multi-tone
CDMA (MT-CDMA). MC-CDMA combines CDMA users
spreading a narrowband signal over a wide spectrum with a
spreading sequence unique to each user. The major
classification is based on spreading operation that takes place
either in time or frequency domain [8].
Figure 3: frame structure of the adaptive downlink
modulation.
In figure 3, a frame is divided into multiple slots.
Some slots are allocated to OFDM and others to MC-CDMA.
The transmission power for OFDM slots and MC-CDMA slots
is set to be identical to maintain the continuity of the signal
level between two modulation schemes.
D. Selection Algorithm of Modulation Scheme
B. Adaptive modulation of OFDM and MC-CDMA system
Figure 4: selection algorithm for the modulation scheme.
Figure 2: Block diagram of the MC-CDMA Transceiver.
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Figure 4 presents a selection algorithm for the
modulation scheme and its parameters. When the Channel
Interference to Noise Ratio (CINR) of the channel is high and
the distance of the wireless link is short (RSSI level is high),
the BS assigns an OFDM slot with high rate sub-carrier
modulation such as 16QAM with a high coding rate. If the
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SSRG International Journal of Electronics and Communication Engineering (SSRG-IJECE) – volume1 issue8 Oct 2014
CINR is very low, the BS allocates an MC-CDMA slot with
high spreading factor and low coding rate to maintain the
communication link. The concept of this algorithm is based on
the combination of adaptively allocating the radio interface
and adaptive selection of its parameters [6]. Moreover, the
selection of the modulation scheme and its parameters will
also be established with regard to the user’s QoS.
Consequently, the adaptive downlink modulation scheme will
maximize the system capacity for wireless communications
systems and respond to a user request by allocating a
preferable modulation scheme to each time slot per user. In
this case, service areas of OFDM slots should be restricted
around the BS. Therefore, the same channel frequency can be
allocated in every cell, which will enhance the efficiency of
channel utilization. In contrast, the service area of MC-CDMA
will be overlapped because co-channel interference between
adjacent cells is mitigated using the spreading code in the
frequency domain. The selection of the spreading code per
user should consider the orthogonality between the other
codes used in the same cell [7]. As the same service areas of
MC-CDMA signals are deployed as the current cellular
systems, users will be able to establish their communication
link in high mobility environments.
structure of OFDM and MC-CDMA are changed to obtain the
throughput performance under several conditions.
IV. SIMULATION RESULTS
The throughput performance per user versus the
number of users is shown in figure 5. Initially, MC-CDMA
only is assigned to all users at any time. Similarly, OFDM is
assigned to all users at any time and set MC-CDMA equal to
0. In both cases the performances are measured. From the
simulation results, we find that the mean throughput decreases
as the number of users increases in both cases. Also, we find
that when OFDM is kept equal to 1 and MC-CDMA equal to
3, the throughput performance is almost flat when the number
of users is increased. Figures 5 and 6 also show the throughput
performance for different combinations of OFDM and MCCDMA [3]. It is found that the optimum performance is
obtained when OFDM and MC-CDMA are made equal to 1.
III. COMPUTER SIMULATION
Computer simulation was conducted to evaluate the
throughput performance of a wireless communications system
using adaptive down link modulation technology. In each
frame, OFDM or MC-CDMA slots are assigned to users
independently in accordance with channel conditions under
fast Rayleigh fading environments. Figure 4 shows the
allocation diagram of the modulation scheme per each user.
The users close to the BS (RSSI level is high) should be
allocated OFDM to provide higher bit rate, and other users far
from the BS (RSSI level is low) should be allocated MCCDMA to enhance immunity to co channel interference. If the
number of OFDM slots is insufficient, MC-CDMA should be
allocated to the users.
At the same time, the modulation type for
subcarriers, coding rate and spreading factor MC-CDMA is
selected at each slot by monitoring the RSSI and CIR level of
the control channel transmitted from BS. These procedures
actualize a QoS control that allocates high-speed data channels
for the users located near the BS. The RSSI threshold level
that achieves the BER of 10-2 is derived by assuming the noise
figure (NF) = 4 dB, absolute temperature = 290 K and
bandwidth = 40 MHz. For MC-CDMA, the spreading factor
should be changed in accordance with the number of users,
because the spreading factors should be greater than the
number of users [3].
In the simulation, co-channel interference from two
consecutive adjacent cells are considered. The target cell is set
to one and users are arranged at random with equal probability
condition in the cell. Throughput performance of the proposed
system is evaluated by changing several parameters. More
specifically, the parameters of number of users and frame
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Figure 5: mean throughput performance per user vs. number
of users.
Figure 6: mean throughput performance for OFDM slots per
user vs. number of users.
This characteristic means that MC-CDMA shows
good performance under low CIR conditions. For the
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SSRG International Journal of Electronics and Communication Engineering (SSRG-IJECE) – volume1 issue8 Oct 2014
proposed downlink adaptive modulation scheme using OFDM
and MC-CDMA, the highest mean throughput value is
achieved when the number of OFDM slots and MC-CDMA
slots are selected as one, respectively. The mean throughput
performance per user of the proposed system is superior to
that of the OFDM system when the number of users is above
three, and superior to that of the MC-CDMA system at any
case.
down-link broadband radio packet transmission,” Technical Report of
IEICE, A p99-133, RCS99-130 (1999-19).
V. CONCLUSION
This paper presented an adaptive downlink
modulation scheme using OFDM and MC-CDMA for future
mobile communications systems. The proposed scheme
maximizes the system capacity by allocating a preferable
modulation scheme to each time slot per user and offers types
of QoS services depending on user locations and channel
conditions. The detailed adaptive modulation technique and
selection algorithm were explained in this paper. Computer
simulation was conducted to evaluate the throughput
performance of the proposed system by changing the number
of users, OFDM slots and MC-CDMA slots. From the
simulation results, according to the number of users, the
proposed scheme using OFDM and MC-CDMA exceeds the
throughput performance of the OFDM system or MC-CDMA
system when the number of OFDM slots and MC-CDMA
slots are selected as one respectively. The simulation results
also indicate that the proportion of high-speed users and lower
rate users is adjustable without decreasing the total throughput
performance of the system. Thus, the proposed adaptive
downlink modulation scheme provides flexible and high
performance broadband communications services for mobile
and nomadic users.
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