th
The 5 Student Conference on Research and Development –SCOReD 2007
11-12 December 2007, Malaysia
Evaluation of Modulation and Channel Coding
Candidates for Adaptive Data Communication
System
Abd. Rahim Mat Sidek and Ahmad Zuri bin Sha’ameri, Member, IEEE
Abstract-- This paper evaluates at the various modulation and
coding techniques suitable for adaptive data communication
system. The techniques evaluated are applicable to HF spectrum.
Examples of modulation and coding scheme that are evaluated
include PACTOR I, PACTOR II, PACTOR III, GTOR and
STANAG 4285. Analysis of system performance is made in the
presence of AWGN with future extension for multipath fading
conditions. Comparison will be made based on the bit-error rate
(BER), packet-error rate (PER) and the throughput. In addition,
the effective data rate for all the schemes is compared with the
Shannon capacity limit to ensure the optimal utilization of the
given channel bandwidth. The modulation and coding scheme
that meets all of the above requirements will be used for future
adaptive data communication system.
Index Terms-- Digital modulation, channel coding, multipath
fading channels, channel capacity, adaptive data communication.
I. INTRODUCTION
Similar to any wireless communication medium, data
communication over the HF spectrum are characterized by
multipath fading, path loss and interference due to white noise.
Multipath fading occurs due to solar radiation that contributes
to variability in the electron density layers in the ionosphere
[1]. Multipath fading results in time variation in the signal
amplitude and phase with time delay that minimize the
reliability of communications by increasing the bit-error rate
(BER) in data transmission.
Despite difficulties in propagation and bandwidth
limitation, HF communication is still important due to its
advantages on simplicity, ability to provide long distance
communications at low power and low cost facilities [2]. At
present, many HF digital communications are used by
military, shipping, deep sea fishing, relief operation and
amateur radio.
This research was supported by MOSTI through Postgraduate Research
Scholarship Scheme (PGD).
Abd Rahim Mat Sidek is a phD candidate at Faculty of Electrical
Engineering, Universiti Teknologi Malaysia, Skudai, 81300 Johor, Malaysia.
(phone: 607-553-5416; fax: 607-556-6272; e-mail: [email protected]).
Ahmad Zuri b. Sha’ameri is with Faculty of Electrical Engineering,
Universiti Teknologi Malaysia, Skudai, 81300 Johor, Malaysia. (phone: 607553-5674; fax: 607-556-6272; e-mail: [email protected]).
This paper presents the evaluation results of several
modulation and coding techniques used in PACTOR I,
PACTOR II, PACTOR III, GTOR and STANAG 4285 data
format in terms of bit-error rate (BER), packet-error rate
(PER) and throughput. In addition, the effective data rate are
compared with the Shannon capacity limit to ensure the
optimal utilization of the given channel bandwidth.
II. ADAPTIVE DIGITAL COMMUNICATION
Adaptive data communication system is needed to
maximize the reliability and throughput in the presence of
multipath fading and jammers. This is accomplished by
varying the modulation and coding scheme according to the
channel condition [3]. An adaptive data communication
includes additional features such as sounding and link quality
analysis used to decide which modulation and channel coding
suitable for appropriate time. This paper present analysis result
based on Additive White Gaussian Noise (AWGN) channel to
verify the throughput, effective data rate and robustness.
Future work will consider multipath fading system. The
evaluation only considers modulation and error control coding
scheme, and assumes perfect synchronization with no
compression and equalization.
A. PACTOR I
PACTOR I is an acronym for Packet Teleprinting Over
Radio is a hybrid between packet and AMTOR techniques. It
uses Frequency Shift Keying (FSK) for modulation and
Automatic Repeat Request (ARQ) for error control [2, 4].
PACTOR transmits either 12 or 24 characters, depending on
the baud rate, either 100 or 200. It uses checksum (CRC-16)
and the errors are detected at the receiver by comparing the
checksum with the accompanying data. The receiver is
responsible to request new data, retransmission of data or
change the system baud rate depends on channel propagation.
B. PACTOR II
PACTOR II is claimed as a robust and powerful m-ary
Differential Phase Shift Keying (DPSK) mode which operates
well under varying conditions [4]. It uses hybrid error control
which is convolution code with constraint 9 and CRC-16 for
ARQ. For adaptive purposes, the system has four level
combinations of different modulation and error control which
depends on the channel state. Unlike PACTOR I, the system
always remain the speed at 200 baud for every level of
combination and the rapid changes is based on modulation
type and code rate of convolution code.
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C. PACTOR III
PACTOR III is the fastest digital mode in PACTOR family
and it uses almost the 3 kHz bandwidth. The system uses multi
sub-carrier modulation (MCM) also called Orthogonal
Frequency Division Multiplexing (OFDM) where each subcarrier using DPSK for modulation and hybrid error controls
similar to PACTOR II [4]. The speed always remains at 100
baud and the numbers of sub-carrier changes according to
channel conditions. In PACTOR’s family, the link quality
analysis is done by evaluating the data input rate, packet
statistics including error rate and number of retries.
D. GTOR
GTOR (Golay -TOR) is an FSK mode that offers 3 times
faster transfer rate as compared to PACTOR [4, 10]. To ensure
error free transmission, it utilizes Golay code with 3 bits error
correction capability together with CRC-16 for ARQ. Gtor
tries to perform all transmissions at 300 baud but drops to 200
baud if difficulties are encountered and finally down to 100
baud.
E. STANAG 4285
The STANAG 4285 is one of the HF digital modes used by
NATO and compatible to MIL STD 188-110A [5]. It uses
Phase Shift Keying (PSK) as modulation incorporates with
convolution code constraint 7 and the symbol rate of 1200
baud. Some of the transmission is repeated a few times to
increase reliability but apparently reduce the throughput to
only 75 bps. Even though it consists of five different levels
combination but it does not has an auto-baud field. Thus, the
transmitter and receiver need to be set up to the desired data
rate at both stations before it operates.
F. PACTOR II (M-ARY FSK)
The systems in previous subsection use PSK as their
modulation due to spectrum efficiency and its theoretically
better performance compared FSK. But, for M-ary
modulation, the performance of M-ary FSK is better compared
to M-ary PSK especially when M ≥ 4 [6]. Since there is no
published system based on M-ary FSK, PACTOR II format is
adopted for M-ary FSK for comparison purposes.
III. THEORY
The digital modulation technique used is M-ary-PSK and
M-ary FSK together with error control coding to increase
reliability and throughput. There are two types of error
control; detect errors or to detect and correct the errors.
A. M-ary PSK
The basic structure of digital modulation and detection for
M-ary PSK is when M=2. It uses two phases which are
separated by 180°, which normally at 0° and 180°. This
modulation is the most robust among conventional digital
modulation but only modulate at 1 bit/symbol. If x(t) is a
received signal, then output receiver y(t) is
Tb
y (t ) =
∫ x(t )[ x (t ) − x
1
0
0 (t )]dt
0 < t < Tb
(1)
where x0(t) and x1(t) are reference signal used at receiver. The
receiver structure is based on matched filter and requires
perfect synchronization. By sampling y(t) at interval nTb, the
binary bit equal to “1” if accumulate value of y(t) is positive
and “0” for negative value. The BER for 2-PSK is
 SNR 

 Rs 


PB = Q 
(2)
where SNR is signal-to-noise ratio and Rs is symbol rate. The
function Q( ) is referred as the Q function and is defined as
Q ( x) =
∞
 − z2
exp
∫  2
2π x
1

dz

(3)
Signal-to-noise ratio (SNR) in decibels with A is signal
amplitude and N0 is Gaussian noise power with zero-mean is
 A2 

 2N 0 
SNR = 10 log
(4)
For M≥4, the symbols error performance PE(M) with coherent
detection can be express as
 SNR 

 2 Rs 
P ( M = 4) ≈ 2Q
E
 SNR
π 
sin
M 
 2 Rs
(5)
P ( M > 4) ≈ 2Q

E
(6)
The symbol error rate is related to the BER by
P
E
P ≈
B log M
2
(7)
M-ary PSK is implemented by STANAG 4285 [6] and at
receiver detection it needs reference signals as shown in
Equation 1. To minimize problem due synchronization to
error, non-coherent detection for M-ary PSK is utilized which
also known as differential PSK. But, the performance is also
reduce together with less complexity and for M=2, the BER
[7] is
 SNR 
PB = 0.5 exp
(8)

 2 Rs 
The BER performance for multi sub-carrier modulation is the
same as conventional single carrier digital modulation. Even
though data is transmitted over many sub-carriers, the
detection process is performed one sub-carrier at a time.
B. M-ary FSK
M-FSK modulation is characterized by the information
being contained in the frequency of sub-carrier. The
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bandwidth of the system increases with the number of M and
for non-coherent detection, symbol error rate is defined [7] as
P (M ) <
E
M −1
2
 SNR 

 4R 
s 

exp −
(9)
The relationship between BER (PB ) and symbol error rate (PE)
is
P
B
P
E
=
k −1
2
k
2 −1
achieved error free condition. On the average, throughput for
SWARQ [8] is
(
k
T =
1− P
d
h T R
D s
(10)
PER =
(11)
C. Forward Error Correction (FEC)
FEC utilizes parity or redundant bits to detect and correct
the errors during transmission and can be categorized as a
block code and convolution code. Block code combats the
errors in blocks of data and different block codes exhibit
different performance depend on how many bit of errors it can
detect and corrects. FEC reduces the BER and the coded BER
is derived based on the binomial probability density function
(pdf). The coded BER is
n 1  n
k
n−k
P ≈ ∑
P (1 − P )


C k =t +1 n  k  B
B
(13)
where k is information bits, TDRS is channel symbol time and
Pd is probability of detectable error. The PER can obtained by
determine probability of undetectable error, Pe and utilize
following equation.
where k is number of bits within a symbol
k = log2(M)
)
P
e
−
1 P
d
(14)
The Pd and Pe respectively depend on BER and error detection
capability. To calculate the PER, the Pd and Pe is calculated
first using the binomial distribution [1, 8]
n= N −1 n  k
n−k
P = ∑  PC 1 − PC
k
k =0  k 
(
)
(15)
where N is the packet length. The detectable and undetectable
errors are
Pd =
d = 2t +1
∑P
(16)
k
k =1
(12)
where PB is uncoded BER, n is number of code word and t is
the number of correction capability.
Convolution code basically builds memory to information
bits which is not only determined by present information bits
but also determined by specified polynomial. Constraint
length, L determines the polynomial and complexity of the
codes. Existing systems use the Viterbi decoder at the receiver
with constraint length L ≤ 9 at Viterbi decoder and softdecision.
D. Automatic Repeat Request (ARQ)
ARQ is an alternative error control scheme to FEC. It
detects error and performs retransmission until error free.
There are 3 types of ARQ strategies which is Stop-and-Wait
ARQ (SWQRQ), Go-Back-N ARQ (GBNARQ) and Selective
Repeat ARQ (SRARQ). The systems evaluated use SWARQ
which is the simplest among them. After a message is
transmitted, the SWARQ transmitter simply waits for the
receiver to acknowledge correct reception before transmitting
the next message. The acknowledgement of correct reception
is called an ACK and is returned to the transmitter if no errors
occur. If errors occur during transmission, a negative
acknowledgement (NAK) is returned and the message is
retransmitted. ARQ scheme is not reduce BER performance
but it will ensure free error transmission.
The throughput of the systems depends on BER and
transmission time including retransmission in order to
Pe =
N −1
∑P
(17)
k
k = d +1
E. Shannon Capacity Theorem
Shannon [8] showed with a given SNR and bandwidth, the
optimal system capacity, C can stated as
C = W log (1 + SNR )
2
(18)
For M-ary system with transmission rate Rs symbols/sec, the
effective data rate expressed by [1, 9] is
[(
) (
)
]
D = 1 + 1 − P log 1 − P + P log P R
t
B
2
B
B
2 B s
(19)
With a limitation of HF bandwidth, those systems were
measured in term of channel capacity to determine the
effective usage of channel bandwidth.
IV. PERFORMANCE E VALUATION
From the equation in previous section, the BER, PER,
throughput and channel capacity were obtained. An example is
presented using the GTOR packet structure found in [10] for
100 bits/sec at SNR 30 dB. The uncoded BER based on
Equation (9) is
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 1000 
1
 = 0.041
P = exp −
B 2
 4(100) 
V. RESULT
(20)
Provided with Golay code forward error correction, the coded
BER using Equation (12) is
n = 24 1  n 
k
n−k
P = ∑
(21)
 0.041 (1 − 0.041)
C k =3+1 n  k 
= 2.7x10-3
To calculate the PER, the Pd and Pe calculated first using the
defined using binomial distribution in Equation (15)-(17) are
n = 95  n 
k
n−k
P = ∑  0.0027 (1 − 0.0027 )
k
k
k =0  
(22)
Pd = P1 + P2 + …..+ P16
= 0.2286
(23)
Pe = P17 +P18 + …. + P95
= 5.4862x10-26
(24)
Thus, the PER is
PER =
5.4862 x10
−26
1 − 0.2286
= 7.1121x10 − 26
(25)
By using Equation (13) and (19), the throughput and effective
data rate respectively is
Th =
80
2.4
(1 − 0.2286 ) = 25.71bps
Dt = 1+[(1-0.0027)log2(1-0.0027) + 0.0027log20.0027]100
(26)
(27)
= 97.3071 bps
The normalized channel capacity for the system is
Dt/B = 97.3071/3000 = 0.0324 bps/hertz
(28)
For a given SNR, normalized Shannon capacity limit for HF
channel is
C
= log 2 (1 + 1000) = 9.97 bps/hertz
(29)
W
The proposed systems were evaluated mathematically as
shown in Equation (20)–(29) but, certain part where the
equation is very complex such as coded BER for convolution
code at constraint 9, then the results were obtained by
simulation. The analysis result and throughput performance
were stated in Table 1 and Figure 1 respectively.
The performance of the various systems, discussed in the
earlier sections, are calculated when an Addictive White
Gaussian Noise (AWGN) channel is used. Even though
AWGN channel does not reflect the true nature of a real HF
channel with multipath fading issues but it provides a base line
analysis for each system. The analysis presented in Figure 1 is
focus in on the lower end SNR in order to determine which
systems is power efficient. Thus, the reasons for putting the
graph limit up to only to SNR 40 dB.
Figure 1 shows the throughput (in bits per second) with
appropriate SNR that corresponds to a reference BER of 10-3.
For all the systems analyzed, a lower SNR means that the
system is more robust in the presence of AWGN. PACTOR II
and PACTOR III are the most robust at SNR of 28 dB but the
throughput for PACTOR III is double compared to PACTOR
II. The highest throughput achieved among the systems is
STANAG 4285 level 4 which is almost 700 bps at 36 dB.
Unlike PACTOR I and II, the throughput for STANAG
4285 increases vertically at the same SNR. This is because
uses repeated coding for various coding scheme where the
repetition increase for more robust coding scheme. Since the
channel is AWGN, the effect of repeated code in STANAG
4285 is not obvious. Therefore, its BER remains the same at
36 dB for coding scheme with different throughput. The
repeated code is essential against multipath fading especially
in the presence of flat fading [11].
The performance of GTOR is better than PACTOR with 3
dB different in coding gain as shown in Figure 1. Even though
both systems use the same modulation and ARQ scheme, the
presence of Golay code in GTOR increase the robustness
compared to an ARQ system. Those systems with BER equal
to 10-3 achieved almost error free transmission with the small
PER as shown in Table 1. For STANAG 4285, the PER is
higher because it only utilize forward error correction without
ARQ scheme.
From modulation perspective, the systems with 2-PSK
present better performance compared to 2-FSK but the
performance changes for M ≥ 4. It is observed that 8-FSK
PACTOR II level 3 can operate well at 33 dB which
equivalent with 4-DPSK PACTOR II level 2. However, M-ary
FSK uses more bandwidth compared with M-ary PSK. Hence,
there is the trade-off between bandwidth, BER and spectrum
efficiency. PACTOR II level 1 is a robust and spectrum
efficient system but with increasing of M, it need higher SNR
to maintain the required BER. While, PACTOR II with M- ary
FSK modulation is less robust at initially but increases with M
as well as the bandwidth. Figure 1 shows that it’s still can
operate well below 34 dB.
Figure 2 presents the effective utilization of channel
capacity and limitation based on Shannon capacity theorem.
The plot is representing effective data rate for each system
within 3 kHz HF bandwidth as stated in Table 1. The
interpretation of the results can be described in two different
directions following the arrows on the figure.
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TABLE I
THE PERFORMANCE OF EVALUATED SYSTEM FOR BELOW 40 DB RECEIVED SIGNAL
700
STANAG 4285-4
Throughput (bits per second)
600
500
PACTORIII-3
400
300
STANAG 4285-3
200
STANAG 4285-2
PACTORIII-2
100
0
26
PACTORII(4FSK)
PACTORII(FSK)
PACTORIII-1
PACTORII-1 GTOR(100)
28
30
PACTORII-2
PACTORII(8FSK)
PACTORI(100)
GTOR(200)
32
34
36
Signal-to-Noise Ratio (dB)
GTOR(300)
PACTORI(200)
STANAG 4285-1
38
40
Figure 1. SNR requirements for AWGN Channels to achieve BER of 10-3
Moving to the left means increase robustness and moving
up close to Shannon limit shows how well the systems utilize
the channel capacity. PACTOR II and PACTOR III level 1 is
most robust systems but fewer throughputs while STANAG
4285 level 4 is utilized optimal channel capacity for data
communication. For STANAG 4285 level 1 until level 3, they
have same effective data rate even thought different
throughput because the effect of repeated code.
From Figure 2, the higher throughput and optimal channel
capacity is achieved by increasing the data rate with M-ary
modulation but at the cost of higher SNR to ensure reliability.
Therefore, an adaptive system should always determine the
current SNR to decide which level of transmission is suitable
to ensure optimum use of channel capacity and also provides
higher throughput and reliability. From the analysis, it is found
that channel capacity for each level within a system is
increasing with reduction in BER and it remains constant
when BER achieves 10-3.
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2
Effective Data rate/Bandwidth (bps/hertz)
10
1
10
0
10
STANAG 4285-4
STANAG 4285-1
STANAG 4285-2
STANAG 4285-3
PACTORII(8FSK)
-1
PACTORII(4FSK)
10
PACTORII-1
PACTORIII-1
PACTORII(FSK)
PACTORIII-2
PACTORII-2
GTOR(300)
GTOR(200)
-2
26
PACTORI(200)
Throughput
PACTORI(100)
GTOR(100)
10
PACTORIII-3
Robust
28
30
32
34
36
Signal-to-Noise Ratio (dB)
38
40
Figure 2. Channel utilization with comparison Shannon Capacity Theorem at BER is 10-3
[6]
A.B. Reynolds, W.D. Blair, “Tactical High Frequency Communications
in the Land Arena – The Current State of the Art”, Commonwealth of
Australia, 2001.
[7]
W.C.Y. Lee, “Mobile Communications Engineering – Theory and
Applications”, 2nd Edition, Singapore: McGraw-Hill, 1998.
[8]
R. E. Ziemer, R.L.Peterson, "Introduction to Digital Communication”,
2nd Edition, New Jersey: Prentice Hall, 2001.
[9]
K. S. Shanmugam, “Digital and Analog Communication Systems”,
Singapore : John Wiley & Sons, Inc, 1985
VI. CONCLUSION
The proposed systems were evaluated using AWGN as
channel model to give base line analysis in term of BER, PER
and throughput. The analysis conducted based on modulation
and channel control coding. The performance is presented in
Table 1 and can further enhance by using channel equalizer
and multiple antenna [11]. PACTOR II and III presented a
good performance in term of robustness and optimal
utilization of channel bandwidth. But if multipath fading is
considered, it is expected PACTOR III to be an ideal choice
for HF digital communication because of its characteristic
which suitable to against frequency selective fading problem.
VII. REFERENCES
[1]
B. Sklar, “Digital Communications – Fundamentals and Application”,
2nd Edition, New Jersey: Prentice Hall, 2001.
[2]
S. Ford, “The HF Digital-Tower of Babel”, QST January 2001, pp. 5053.
[3]
S. T. Chung and A. J. Goldsmith, “Degrees of Freedom in
Adaptive Modulation: A Unified View,” IEEE Trans. Commun., vol.
49, pp. 1561-1571, Sept. 2001.
[4]
E.C. Jones, H.P. Helfert, “PACTOR I, II and III Protocol Description
and Speed Comparison to Other Digital Modes”, Hanau Germany, 2006.
[Online] Available : http://ecjones.org/pactor.html
[5]
B.A. Clark, “STANAG 4285 Conformance Test Procedures”, Arizona,
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[10] D.G. Reed, “The ARRL Handwork for Radio Amateurs”, 79th edition,
Newington: ARRL 2002.
[11] E.S. Warner, I.K. Proudler, “Adaptive multi-channel equalization
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Proc. April 2003 IEEE Radar Sonar Navig. Vol. 150, No. 2.
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