1. Performance Evaluation of OFDM and Single
Carrier Modulation in Broadband Wireless System
Using Frequency Domain Equalization
Yuan Xiaogeng, Osamu Muta and Yoshihiko Akaiwa
Graduate School of Information Science and Electrical Engineering, Kyushu University
Hakozaki 6-10-1, Higashi-ku, Fukuoka-city, 812-8581, JAPAN
E-mail:yuan@mobcom.is.kyushu-u.ac.jp, muta@is.kyushu-u.ac.jp
Abstract— In this paper, Orthogonal Frequency Division Mul-
tiplexing (OFDM) has been considered to realize a high-speed
transmission. However, OFDM system has a problem that the
peak to average power ratio becomes high and results in low
power efficiency at the power amplifier. As an alternative method,
Single Carrier transmission with Frequency Domain Equalization
(SC-FDE) system has been investigated. The purpose of this paper
is to investigate a total system performance of both OFDM and
SC-FDE systems including modulation,forward error correction
(FEC), peak-limiter (only in OFDM), adaptive predistorter,a
model of 2GHz class A/B power amplifier and the receiver with
frequency-domain equalizer in wireless communication environ-
ment. With computer simulation, we show that both OFDM and
SC-FDE systems achieve almost the same BER performance, when
the FEC is applied to both systems. In addition, the peak power
of the OFDM signal is suppressed to about 6.5dB by applying the
peak power reduction (PPR) method. QPSK-SC system shows
about 3 ∼ 8% higher power efficiency than QPSK-OFDM with
PPR, while 64QAM-SC system achieves almost the same power
efficiency as OFDM with PPR.
keywords−OFDM, PAPR, Peak power reduction, Single-
carrier transmission, Frequency-domain equalization.
I. INTRODUCTION
In recent years, high data-rate wireless transmission system
such as IEEE802.16e [1] has received increased attention.
In such a high speed transmission, inter-symbol interference
caused by frequency-selective fading severely degrades BER
performance. To overcome the effect of frequency-selective
fading, Orthogonal Frequency Division Multiplexing (OFDM)
has been considered [2][3]: In OFDM systems, inter-symbol
interference is mitigated by using a number of narrow-band
subcarriers and inserting cyclic prefix (CP) into each OFDM
symbol. However, OFDM system has a problem that the Peak
to Average Power Ratio (PAPR) becomes higher as the number
of sub-carriers increases; a high peak power causes nonlinear
distortion or a low power efficiency at the power amplifier.
Therefore, it is required to reduce the peak power of the OFDM
signal.
As a solution to PAPR problem in OFDM system, several
techniques such as partial transmit sequence (PTS) [4] and
peak-limitation[3], [5]-[7] have been known. Although the PTS
method reduces the peak power by controlling the phase of
subcarriers, this method has the disadvantage that redundant
side information is needed to inform phase rotation value
to the receiver. The amplitude limiting based methods such
as the peak reducing signal addition reduce the peak power
without sending the side information. In this paper, the power
amplification performance of the OFDM system combined
with the amplitude limitation to ease the PAPR problem is
investigated.
As an alternative method to solve the above problem, Sin-
gle Carrier transmission with Frequency-Domain Equalization
(SC-FDE) has been investigated [8]-[11]. In contrast to the
OFDM system, the SC system achieves a low PAPR. However,
the received signal is severely distorted due to frequency-
selective fading. To cope with this problem, a frequency-
domain equalization based on MMSE (minimum mean square
error) criterion [8] is applied to the received signal. Thus, with
SC-FDE system, it is expected to improve the power efficiency
at the power amplifier as compared with the OFDM system,
while keeping the required signal quality.
To achieve further power efficiency at power amplifier, it
is important to compensate for the nonlinearity of a power
amplifier. For this purpose, adaptive predistortion methods
have been investigated [12], where the predistorter distorts
the signal beforehand. The maximum power of input signal
at the power amplifier must be below its saturation level.
Therefore, combination of peak limitation and predistortion
enhances power amplification performance.
The purpose of this paper is to investigate a total system
performance of both OFDM and SC-FDE systems including
modulation, forward error correction (FEC), peak-limiter (only
in OFDM), adaptive predistorter, a model of 2GHz class
A/B power amplifier and the receiver with frequency-domain
equalizer in wireless communication environment. To cope
with the PAPR problem in the OFDM system, a peak power
reduction (PPR) method proposed in [7] is applied to the
OFDM transmitter, where the peak power of the signal is
reduced by subtracting the peak reducing signal from original
signal. To achieve further improvement in the power efficiency,
an adaptive predistortion is introduced to both systems as a
compensation method for nonlinear distortion at the amplifier.
II. THE SYSTEM DESCRIPTION
The system diagrams of OFDM system and SC-FDE system
are shown in Figs. 1(a) and (b), respectively. Figure 2 shows the

2. FEC
coding
QAM
modulator
adaptive
pre-distorter
peak
reduction
power
amplifier
channel
FFT
MMSE
equalizer
QAM
demodulator
FEC
decoding
inverse
FFT
CP
remove
CP
Freq.
interleaving
Freq.
de-interleaving
FEC
coding
QAM
modulator
adaptive
pre-distorter
peak
reduction
power
amplifier
channel
FFT
MMSE
equalizer
QAM
demodulator
FEC
decoding
inverse
FFT
CP
remove
CP
Freq.
interleaving
Freq.
de-interleaving
(a) OFDM system.
FEC
coding
QAM
modulator
adaptive
pre-distorter
power
amplifier
channel
FFT
MMSE
equalizer
QAM
demodulator
FEC
decoding
remove
CP
CP
inverse
FFT
FEC
coding
QAM
modulator
adaptive
pre-distorter
power
amplifier
channel
FFT
MMSE
equalizer
QAM
demodulator
FEC
decoding
remove
CP
CP
inverse
FFT
(b) SC-FDE system.
Fig. 1. The system diagrams.
Soft
Clipping
peak
detector
)(tsc
filtering
)(td
Peak Power Reduction Part
)(ts
Fig. 2. PAPR reduction part in OFDM system.
block diagram of the PPR method used in the OFDM system.
In the SC-FDE system (Fig. 1(b)), the band-limited SC signal
is transmitted with QAM modulation. To remove interference
between successive two transmission-blocks, CP is inserted into
each block with a similar way as OFDM [8], where each block
contains 128 symbols, when OFDM employs 128 subcarriers.
To improve BER performance, forward error correction (FEC)
is introduced into both systems. At the receiver, frequency-
domain equalization based on MMSE criterion is done. In both
systems, at the transmitter, the transmit signal is applied to the
adaptive predistorter, where the parameters of the predistorter
are adaptively calculated so as to minimize the out-of-band
radiation at the power amplifier. The details of peak power
reduction and adaptive predistortion are shown below.
A. Peak Power Reduction Methods in OFDM System
Figure 2 shows the block diagram of the peak power
reduction part in OFDM system. In Fig. 2, the OFDM signal
s(t) is applied to a peak limiter using a soft clipping function
to detect peak components, where the signal which exceeds a
given threshold level is level-limited by a soft-clipping function
[7]. The soft clipping function of peak limiter investigated in
this paper is defined as
fc(x) =
x (x ≤
√
Pth)
ax + (1 − a)
√
Pth (x >
√
Pth)
(1)
Out
PWR
DET
BPF
TABLE
Algorithm
∆τ ∆G AMP X
In
∆φ
f0
'
AGC
PWR
DET
Digital Signal Processing
@
Fig. 3. System diagram of adaptive predistorter.
where a is a constant and Pth is threshold power value of
clipping functions which is determined relatively to the average
signal power. The clipping function is based on Piecewise-
Scales Transform (PST) [13]. The difference signal is obtained
by subtracting the level-clipped signal from the original signal.
After that, the difference signal is applied to a frequency do-
main band-limited filter to remove the out-of-band components.
The filter after peak limiter causes peak re-generation. Thus, to
mitigate peak re-generation due to filtering, the peak reducing
signal is generated by using the following way.
(1) The difference signal d(t) is applied to the peak detector
in Fig. 2. The output of the peak detector is given as
˜d(nTs) =



d(nTs)
|d(nTs)|>|d((n−1)Ts)|
and |d(nTs)|>|d((n+1)Ts)|
0 (otherwise)
(2)
where Ts is sampling period.
(2) Out-of-band component of ˜d(t) is removed by the filter.
The peak power of the transmit signal is reduced by sub-
tracting the peak reducing signal from original signal.
B. Adaptive Predistorter
The system diagram of the amplifier with adaptive pre-
distorter [12] is shown in Fig. 3. An input signal is fed
successively to a delay-circuit, a gain modulator, a phase
modulator, an AGC (Automatic Gain Control) and the main
power amplifier. The instant power of input signal is measured
by power detector. The gain and phase modulator pre-distorts
the input signal based on the value of a look-up table (LUT)
corresponding to the signal level. The LUT consists of random
access memories, which are addressed by the instant input
signal power. In order to compensate for the time delay in
the predistorter, a delay-circuit is used. The AGC controls the
signal gain so as to yield a given average output power. A
band-pass filter (BPF) is used to measure the power of the
out-of-band radiation produced due to the power amplifier’s
nonlinearity. The out-of-band radiation power is the target
value that is to be minimized by the adaptive predistorter. The
content of the LUT is updated automatically by a digital signal
processor (DSP) using an iterative algorithm.

3. TABLE I
SIMULATION PARAMETERS.
Modulation QPSK, 16QAM, 64QAM
Demodulation Coherent detection
The number of subcarriers (OFDM) Nc = 128
The number of symbols per block (SC) Ns = 128
The number of FFT points 1024
Filter roll-off factor (SC) 0.25 ∼ 1
Encoding Convolution coding (K=7)
Code rate = 0.5
Decoding Soft-decision Vitervi decoding
Channel model 12-pass Rayleigh fading based
on 3GPP specification [14]
Normalized delay spread τ/T = 0.011
Guard interval duration Tg = 0.125T
III. PERFORMANCE EVALUATION
We evaluate total system performance of both OFDM and
SC-FDE systems such as Complementary Cumulative Distribu-
tion Function (CCDF), Bit Error Rate (BER), Power Spectrum
Density (PSD) at the output of a predistorted power amplifier
and their power efficiencies. The system parameters assumed
in this paper are shown in Table 1. QPSK, 16QAM and
64QAM are adopted as carrier modulation schemes. Convo-
lution encoding (K = 7) with soft-decision Vitervi decoding
is employed, where K denotes the constrain length of the code
and code-rate of R = 0.5 is used. In OFDM system, frequency-
domain (subcarrier) interleaving is applied to enhance error
correction performance. Propagation model is 12-pass Rayleigh
fading based on 3GPP specification [14], where the normalized
delay spread is τ/T = 0.011. Channel estimation for MMSE
frequency-domain equalization is done with pilot-symbol. In
this simulation, a very low Doppler frequency of 0.1Hz is
assumed (we assume the situation where channel condition is
constant within one transmission block). The characteristics of
the power amplifier assumed in this study are shown in Figs. 4
and 5. These characteristics are obtained by measurement on a
2GHz-band LDMOS class A/B power amplifier. The AM-gain
and AM-PM characteristics compensated by the predistorter
are also shown in Figs. 4 and 5.
For comparison purpose of peak power in OFDM and
SC systems, Complementary Cumulative Distribution Function
(CCDF) of instantaneous power normalized by average power
is defined as
CCDF(x) = Prob(instantaneous power > x)
The power at the CCDF= 10−4
is defined as a peak power in
this paper. CCDF performance as a function of instantaneous
power of both OFDM and SC signals normalized by average
power is shown in Fig. 6 and 7. From Fig. 6, we can see
that the peak power of the OFDM signal is suppressed to
about 6.5dB at CCDF= 10−4
by applying the PPR method
with the threshold level of Pth = 4dB. QPSK-SC signal using
roll-off factor of Ro = 0.5 achieves about 3dB lower peak
power than the OFDM signal with peak power reduction (PPR)
using Pth = 4dB, while peak power of 64QAM-SC signal is
comparable to that of 64QAM-OFDM signal using Pth = 4dB.
9
10
11
12
13
14
0 5 10 15 20 25 30 35 40
0
10
20
30
40
50
Gain[dB]
Input power [dBm]
w/o predistorter
with predistorter
Powerefficiency[%]
power efficiency
Fig. 4. AM-gain and AM-power efficiency characteristic.
0
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40
Phase[degree]
with predistorter
w/o predistorter
Input power [dBm]
Fig. 5. AM-PM Characteristic.
Figure 7 shows the effect of filter roll-off factor on CCDF of
single carrier signal. In Fig. 7, single carrier signal using a
lower roll-off factor such as Ro = 0.25 shows higher peak
power. The peak power of 64QAM-SC signal using Ro = 0.25
becomes 6.65dB at CCDF=10−4
.
Figures 8 (a)-(c) show BER performance of the OFDM
and SC-FDE system with and without FEC as a function
of Eb/N0, where QPSK, 16QAM and 64QAM are used for
carrier modulation scheme, respectively. From these figures, it
can be seen that when the FEC is applied to SC and OFDM
systems, OFDM achieves almost the same BER performance
as that of the SC-FDE system. In addition, these figures show
that frequency interleaving technique improves error correction
performance in OFDM system, since channel correlation on
successive two symbols in encoded sequence becomes lower.
It should be noted that error correction effect in SC system is
inferior to that in OFDM system even if interleaving technique
is applied. The is because that we assume a very slowly time-
varying channel. The BER degradation in OFDM system due
to peak reduction is slight, even when the threshold level is set
to 4dB.
Figure 9 shows the power efficiencies of SC and OFDM

4. 1
1 2 3 4 5 6 7 8 9 10
10
10
10
10
-1
-2
-3
-4
Instantenous power / Average power[dB]
CCDF
OFDM (QPSK,16QAM,64QAM)
SC (Roff=0.5)
w/o PPR
Pth=4dB
Pth=6dB
w/ PPR
Pth=8dB
QPSK
16QAM
64QAM
Fig. 6. CCDFs of OFDM and single carrier signals.
1
1 2 3 4 5 6 7 8 9 10
QPSK
64QAM
OFDM (w/o PPR)
CCDF
10
10
10
10
-1
-2
-3
-4
Roff=1.0
Roff=0.5
Roff=0.25
Instantenous power / Average power[dB]
Fig. 7. Effect of filter roll-off factor on CCDFs of single carrier signal.
systems. In Fig. 9(a), power efficiency in OFDM system is
improved as the threshold level becomes smaller, since peak
power of the signal is reduced by the peak limiter in Fig. 2.
In Fig. 9(b), power efficiency of SC system depends on both
roll-off factor of the transmit filter Ro and modulation level. In
case of Ro = 0.5 and QPSK modulation, SC system achieves
power efficiency of about 33% when predistorter is applied.
This is because the input signal in this case has the lowest
peak power at CCDF=10−4
as shown in Fig. 7. On the other
hand, in case of Ro = 0.5 and 64QAM modulation, almost the
same power efficiency as OFDM with PPR using Pth = 4dB
is obtained, while QPSK-SC system achieves about 3 ∼ 8%
higher power efficiency than that of QPSK-OFDM with both
predistortion and PPR.
1
0 5 10 15 20 25 30 35
BITERRORRATE
10
10
10
10
-1
-2
-3
-4
w/o FEC
w/ FEC
Eb/N0 [dB]
SC
OFDM w/o ppr &
w/ ppr (Pth=4,6dB)
OFDM w/o ppr &
w/o freq.interleaving
w/ freq.interleaving
(a) QPSK
1
0 5 10 15 20 25 30 35
10
10
10
10
-1
-2
-3
-4
BITERRORRATE
Eb/N0 [dB]
SC
w/ freq.interleaving
OFDM w/o ppr &
w/o freq.interleaving
OFDM w/o ppr &
w/ ppr (Pth=4,6dB)
w/ FEC
w/o FEC
(b) 16QAM
1
0 5 10 15 20 25 30 35
w/ freq.interleaving
OFDM w/o ppr &
w/o freq.interleaving
SC
10
10
10
10
-1
-2
-3
-4
BITERRORRATE
Eb/N0 [dB]
OFDM w/o ppr &
w/ ppr (Pth=4,6dB)
w/ FEC
w/o FEC
(c) 64QAM
Fig. 8. BER performance of OFDM and single carrier modulation.

5. 16
18
20
22
24
26
2 4 6 8 10 12
Powerefficiency[%]
w/ Prew/o Pre
Pth [dB]
(a) OFDM system
20
22
24
26
28
30
32
34
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Powerefficiency[%]
Roff-off factor Ro
QPSK16QAM 64QAM
w/o Pre
w/ Pre
(b) Single carrier system
Fig. 9. Power efficiency.
-60
-50
-40
-30
-20
-10
0
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
PowerSpectrumDensity[dB]
Normalized Frequency
SC
OFDM
Ro=0.5 &
Back off=6dB
Ro=0.25 &
Back off=6.65dB
Back off=6dB
Pth=4dB &
Back off=6dB
Back off=6.5dB
Fig. 10. Power spectrum densities (with the predistorter).
Power spectrum densities at the output signal of the power
amplifier are shown in Fig. 10, where 64QAM is used as
carrier modulation scheme and the adaptive predistorter is
applied. Out-of-band radiation caused by nonlinear distortion
is suppressed by using both the predistortion and the peak
reduction in OFDM system. From this figure, it can be seen that
64QAM-OFDM system requires almost the same back-off of
6dB as 64QAM-SC system using roll-off factor of Ro = 0.25.
IV. CONCLUSION
In this paper, we investigated a total system performance
of both OFDM and SC-FDE systems including modulation,
FEC, peak-limiter (only in OFDM), adaptive predistorter, a
model of 2GHz class A/B power amplifier and the receiver
with frequency-domain equalizer in wireless communication
environment. With computer simulation, we have shown that
both OFDM and SC-FDE systems achieve almost the same
BER performance, when the FEC is applied to both systems.
In addition, the peak power of the OFDM signal is suppressed
to about 6.5dB by applying the peak power reduction (PPR)
method. The power efficiency in QPSK-SC system is about
3 ∼ 8% higher than that of QPSK-OFDM with PPR, while
64QAM-SC system achieves almost the same power efficiency
as OFDM with PPR.
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