Adaptive Modulation for OFDM Systems by ProQuest


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International Journal of Communication Networks and Information Security (IJCNIS)                           Vol. 1, No. 2, August 2009

              Adaptive Modulation for OFDM Systems
                                                        J.Faezah1, and K.Sabira2
                            Centre for Foundations Studies and Extension Education, Multimedia University,
                                                      75450 Melaka, Malaysia.
                                      Department of Computer and Communication Systems Engineering,
                         Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia

   Abstract: The goal for the fourth generation (4G) of mobile       by the OFDM subcarriers with highest attenuation resulting
communications system is to seamlessly integrate a wide variety of   in a poor performance. Therefore, in case of frequency-
communication services such as high speed data, video and            selective fading the error probability decreases very slowly
multimedia traffic as well as voice signals. One of the promising    with increasing average signal-to-noise ratio (SNR) [2].
approaches to 4G is adaptive OFDM (AOFDM). In AOFDM,
adaptive transmission scheme is employed according to channel
fading condition with OFDM to improve the performance. In this       This problem can be mitigated if different modulation
paper, we have considered only adaptive modulation. First we have    schemes are employed for the individual OFDM subcarriers.
investigated the OFDM system performance of uncoded adaptive         Unlike adaptive serial systems, which employ the same set
modulation using quadrature amplitude modulation (QAM) and           of parameters for all data symbols in a transmission frame,
phase shift keying (PSK). To further enhance the system, we          adaptive OFDM schemes have to be adapted to the SNR of
employ convolutional coding to OFDM system. The obtained             the individual subcarriers. This will substantially improve
results show that a significant improvements in terms of bit error   the performance and data throughput of an OFDM system.
rate (BER) and throughput can be achieved demonstrating the          For example if the subcarriers that will exhibit high bit error
superiority of the adaptive modulation schemes compared to fixed
                                                                     probabilities in the OFDM symbol to be transmitted can be
transmission schemes.
                                                                     identified and excluded from data transmission, the overall
                                                                     BER can be improved in exchange for a slight loss of system
   Keywords: Adaptive modulation, orthogonal frequency division
multiplexing (OFDM), channel coding, bit error rate(BER),            throughput. However the potential loss of throughput due to
throughput.                                                          seclusion of faded subcarriers can be mitigated by employing
                                                                     higher order modulation modes on the subcarriers exhibiting
                                                                     high SNR values.
1. Introduction
Orthogonal Frequency Division Multiplexing (OFDM) is a               Many adaptive transmission techniques have been presented
special form of multi-carrier transmission technique in which        in the literature. The combination of adaptive modulation
a single high rate data stream is divided into multiple low          with OFDM was proposed as early as 1989 by Kalet which
rate data streams. These data streams are then modulated             was further developed by Chow [3] and Czylwik [2].
using subcarriers which are orthogonal to each other. In this        Specifically the results obtained by Czylwik showed that the
way the symbol rate on each subchannel is greatly reduced,           required SNR for the BER target 10-3 can be reduced by 5dB
and hence the effect of intersymbol interference (ISI) due to        to 15dB compared to fixed OFDM depending on the
channel dispersion in time caused by multipath delay spread          scenario of radio propagation.
is reduced.
                                                                     The performance of turbo-coded adaptive modulation are
Guard interval can also be inserted between OFDM symbols             investigated in [4]. Three different modulation mode
to reduce ISI further. The orthogonality between subcarriers         allocation algorithms were discussed and compared. Further
can be maintained, even though the signal passes through a           studies on the application of turbo code in adaptive
time-dispersive channel by cyclically extending the OFDM             modulation and coding is conducted in [5]. This paper
symbols into guard interval.                                         proposed an optimal approach based on prediction of the
                                                                     average BER over all subcarriers.
The main advantages of OFDM are its multipath delay
spread tolerance and efficient spectral usage by allowing            In [6], an adaptive OFDM system with changeable pilot
overlapping in the frequency domain. Another significant             spacing has been proposed. The results showed that a
advantage is that the modulation and demodulation can be             significant improvement in the BER performance is achieved
done using inverse Fast Fourier Transformation (IFFT) and            with sacrificing a small value of the total throughput of the
Fast Fourier Transformation (FFT) operations, which are              system. A work is done on several strategies on bit and
computationally efficient.                                           power allocation for multi-antenna assisted OFDM systems
                                                                     in [7]. They found out that sometimes power and bit
In an OFDM transmission system, each subcarrier is                   adaptation is required for efficient exploitation of wireless
attenuated individually under the frequency-selective and            channels in some system conditions.
fast fading channel. The channel performance may be highly           The performance analysis of OFDM systems with adaptive
fluctuating across the subcarriers and varies from symbol to         subcarrier bandwidth is investigated by [8]. Further
symbol [1]. If the same fixed transmission scheme is used            investigations on subcarrier adaptive modulation scheme of
for all OFDM subcarriers, the error probability is dominated
International Journal of Communication Networks and Information Security (IJCNIS)                            Vol. 1, No. 2, August 2009
precoded OFDM is presented in [9] under multipath
channels.                                                                             Subcarriers are grouped into subband

This paper is organized as follows: The system model is                         The average instantaneous SNR for each subband
described in section 2. The adaptation procedure and                                             are calculated
switching thresholds used in the simulations are presented
next. Section 3 discusses the adaptive modulation systems                       The average instantaneous SNR are compared with
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