Int. J. Communications, Network and System Sciences, 2011, 4, 630-637
doi:10.4236/ijcns.2011.410077 Published Online October 2011 (http://www.SciRP.org/journal/ijcns)
Copyright © 2011 SciRes. IJCNS
Chip Interleaved Multirate and Multimedia Transmission
in Self-Encoded Communication System with Iterative
Detection over Fading Channels
Liang Chi, Won-Mee Jang, Lim Nguyen
Department o f C omputer an d El e ct ro ni c s E n gineering, University of Nebraska-Lincoln,
Omaha, USA
E-mail: lchi@unomaha.edu, {wjang1, lnguyen1}@unl.edu
Received July 26, 2011; revised August 26, 2011; accepted September 14, 2011
Abstract
In this paper, we investigate the self-encoded multirate and the multimedia (SEMM) transmission. In SEMM
system, multiple applications transmit their information simultaneously with different bit rate via
self-encoded spreading spectrum (SESS), where the spreading codes are derived from the previous bits rather
than the pseudorandom code generator. At the transmitter, a block chip interleaving is employed to combat
the deep fading over the wireless channels. At the receiver, a decorrelation scheme separates the combined
signals to reduce the crosstalk between different applications, and provides a better estimation for the de-
spreading sequence. In addition, interference cancelation (IC) is also adopted to improve both the correlation
detection and iterative detection (ID) performance. The simulation results show that the proposed scheme
significantly improves the performance over fading channels.
Keywords: Multirate, Multimedia, Decorrelation, Iterative Detection
1. Introduction
In previous work about the self-encoded multiple access
(SEMA) system, every application transmits data with
the same bit rate [1] and the chip interleaving SEMA
with iterative detection achieves a significant perform-
ance improvement [2].
In SEMM transmission, each application employs
SESS with different processing gain, where SESS is a
time-varying spreading scheme which must update the
despreading code with the detection bit. One challenge in
SEMM system is to separate the combined signal at the
receiver for each application. Another problem exists
that the detection bit affected by multi-application inter-
ference is not accurate especially at lower SNR. In our
paper, we employ a modified decorrelation scheme in
which an extended signature vectors (ESV) is proposed
for spreading signature [3] to overcome the two difficul-
ties.
Iterative detection has been shown to improve the bit
error rate (BER) performance for its time diversity [2].
The fundamental of the iterative detection is that the in-
formation bit exists in the next N spreading sequence
consequently, where N is the chip length of the spreading
spectrum. However, in SEMM system, the information
bits are merged by the multi-application interference
through the wireless channel. Therefore, an inference
cancellation (IC) scheme is introduced to remove the
multi application interference and realize the iterative
detection.
The rest of this paper is organized as follows: in sec-
tion 2, SEMM system model is introduced; in section 3,
the numerical results are shown and in section 4, we
make a conclusion for the scheme. The important nota-
tions in this paper are with following:
i
l
l: the length of spreading sequences in application i;
max : the maximum length of all the spreading se-
quence;
u: the total applications number in the SEMM sys-
tem;
i: the number of the spreading sequence of applica-
tion i in one transmission circle;
m
ik : the extend spreading sequence in applica-
tion i;
pth
k
j
r: the received vector from the row of the chip
interleaver;
th
j
L. CHI ET AL.631
i
q: the row of output of Q;
th
i
ik
g
: the despreading sequence in application i;
th
k
G: the extended signature vector of the SEMM sys-
tem with size of max
M
l;
d: the correlation detection output;
ˆ
d: the decorrelation detection output.
2. System Model
2.1. SEMM System
Figure 1 shows the block diagram of SEMM transmis-
sion system. In the transmitter, each application employs
the SESS as the code scheme where the spread spectrum
is derived from the data source itself rather than the
pseudorandom noise generator. The spreading chip from
each application are combined together and sent to the
chip interleaving blocks. The size of the block interleav-
ing is andmax of all applications. At the
receiver, deinterleaving is employed first, and then each
application is correlated by its own despreading sequence
with the crosstalk from other applications. To reduce this
crosstalk, a decorrelation scheme is needed to separate
the combined signal. Furthermore, interference cancela-
tion removes the multiple application interference to re-
alize the iterative detection. In SEMM system, to realize
the decorrelation scheme, we propose an extended sig-
nature vectors (ESV) to accommodate with the multirate
transmission. The result of the decorrelation scheme
plays an important role in SEMM system: on one side,
the result updates the dispreading sequence for all appli-
cations; and on the other side, the interference cancela-
tion still need the decorrelation result.
NNNl
Figure 2 shows an example block diagram of SESS
system with N/T chip rate. The current transmission bit is
spread by the output of a delay register which stores the
previous N bits, where N is the longest sequence from all
applications. As a result, the spreading sequence is not
only randomly generated and independent from the cur-
rent symbol, but also dynamically changing from one
symbol to the next [1]. At the receiver, the recovered
data provide an estimate of the transmitter code for de-
spreading. The recovery data comes from the correlation
detector definitely.
2.2. Transmitter
Figure 3 shows an example SEMM transmission with
3u
applications. From the diagram and our notation,
in application 1, the number of spreading sequences is
14m
of length 1 = 4 chips/bit; Application 2 is l
22m
, 28l
chips/bit; and Application 3 is 31m
,
316l
chips/bit. Therefore, the total number of se-
quences in this time period is .
12
Mmm 3
7m
1
b
2
b
N
b
1
S
~
d
^
d
1
d
2
d
N
d
S–1
Figure 1. Block diagram of SEMM system.
Copyright © 2011 SciRes. IJCNS
L. CHI ET AL.
632
Figure 2. SESS communication syste m.
Figure 3. SESS multi-rate system transmission at the transmitter.
The spreading length of the system ismax 3. The
spreading sequence of the application can
be expressed as
16ll
ik
s
th
kth
i
 

1,2, ,
ikikikik i
bb bl

 

s (1)
where is the previous transmitted bit of
application.
()
ik
bqth
q
th
k
This is used to encode the current bit as

0
ik
b

0
ik ikik
bes (2)
During one transmission period, then encoded se-
quence are extended to by appending zeros at
proper positions. The expanded sequence can be ex-
pressed as
ik
emax
l

max
1i
i
ikiklk l
kl 

pe00 (3)
The encoded and expanded symbols from all applica-
tions are combined as
11
= ΣΣ
i
m
M
ik

zi
pk
(4)
where z is a 1× row vector as a input of the inter-
leaver.
max
l
The combined signal undergoes block chip interleav-
Copyright © 2011 SciRes. IJCNS
L. CHI ET AL.633
ing with fading is shown in Figure 4. We also assume
that each column has the same fading, i
where fading
vector
is defined as
max
12
,,,l
 
(5)
2.4. Receiver
From Figure 1, the signal from the channel will be as-
signed to the de-interleaver first. Therefore, the
element of the signal vector
th
k
th
j
j
r as the input of the
interleaver can be expressed as
 
T
jkk i
rk nkA
λz
(6)
where k, j are from 1 to N; and mn is the amplifier of
the output to input. The noise of
A
th
nth
m
i
nk are
following with AWGN distribution. The complete se-
quence of the block channel output can be given as
th
j
 
1,2, ,
jj jj
rr r
r
N
(7)
where j is from 1 to N also. After the chip-interleaved
sequences are received, we can reconstruct the original
spreading sequence. The output of the de-interleaver is
an N × N matrix which could be written as
12
,,,
TTT
N
Qrr r
N
(8)
We define the row of Q as
th
ii
q
 
12
,,,
i
ri riri 

q (9)
The correlation detection output is an
d1
M
row
vector and the element can be written as
k
dq
T
ia
g
b
(10)
where 1, 2,,kM
;
1,2, ,;1,2, ,;1,2, ,;
aab
iNaubm
g
th
b stands for
the dispreading sequence for application a.
In the SEMM, the signature vectors G for decorrela-
tion scheme is different from that of the multi-access
system with same bit rate. The columns and rows must
be extended for every application to accommodate with
SEMM transmission.
11
11
11 11
112 1
ll
ll


11 1
22
22
22 2
11 1
21 11
122 1
11 2
11 1
12 1
11
uu
uu
uu u
ll m
ll
ll
ll m
ul l
lu l
ll um









g
g
Gg
g
g
g
g
00
00
00
00
00
00
00

g
g
00
00
(11)
Figure 4. Block chip interleaving.
Copyright © 2011 SciRes. IJCNS
L. CHI ET AL.
634
In Equation (10), 1k stands for the row vec-
tor with elements of zero. The extended signature vectors
can be used to generate the correlation matrix S [4-6]
01k
T

S
GG (12)
and the output of the decorrelation scheme is
1
dSd
i
m
(13)
We have mentioned that the ESV G plays an important
role in interference cancellation part. In fact, we can de-
vide the ESV G by row according to the application in
SEMM system. It is clear that the first 1 rows of
are composed by despreading sequence of Application-1;
the next 2 rows are composed by dispreading se-
quence of Application 2; and the last u
m rows are
composed by the despreading sequence of Application-u.
So the sub-matrix derived from is
mG
m
i
G G

1:()
i
vvm
GG (14)
where
1
1
i
i
k
v
(15)
G (m:n) means the sub-matrix begins from the mth
row of S and terminates at the nth row of G. So G can be
written as
1

G
2
u





G
G
G
(16)
For interference cancellation, the remaining matrix
and with fading are defined as
k
G
k
d
1


G
2
1
1
kk
k
u







G
G
G
G
G (17)
11
ˆ
ˆ
d


22
11
11
ˆ
ˆ
ˆ
kkk
kk
uu
d
d
d
d










d (18)
The interference cancelation result is
ikk


CGdq (19)
From the discussion above, we find that the ESV plays
the key role in the SEMM system which makes the de-
correlation and interference cancelation available. After
interference cancellation, the iterative detection can be
employed as a single application [2,7].
3. Simulation Result
In the following, we present the performance of SEMM
with three applications in additive white Gaussian noise
(AWGN) and Rayleigh fading channels. The simulation
result shows that the SEMM performance approaches
single rate and is significantly better over the multirate
random sequence. In this simulation, the SEMM employs
three applications with chip length of 128 chips/bit, 64
chips/bit and 32 chips/bit.
3.1. AWGN Channel
Figure 5 compares the SEMM system and single rate in
AWGN channel. Notice that the SEMM performance
approaches to the single rate. Taking application 1 with
128 chips/bit in SEMM as an example, at the BER of
3
10
, SEMM is just 0.5 dB slightly worse than single
rate. The performance different at higher SNR is primar-
ily due to the multi-application interference.
In Figure 6, both SEMM and multirate random
schemes are employed the decorrelation. However, itera-
tive detection is applicable for SEMM only which is able
to achieve about 3 dB gain over random sequence. That
is because the iterative detection supplies a time diversity
and double the receive energy to improve the BER per-
formance.
3.2. Flat Fading Channel
In Figure 7 SEMM and single rate employ the iterative
detection over Rayleigh fading channel. At the BER of
3
10
, the SEMM is only at most 3 dB loss than the single
rate. Still, the SEMM performance approaches to the
single rate in Rayleigh fading channel.
Figure 8 shows the comparison of SEMM and multi-
rate random sequence in the Rayleigh fading channel.
Because iterative detection is only available for SEMM
system, it achieves about 15 dB gain over random se-
quence at the BER of 3
10
.
From all diagrams of the simulation result, we can make
a conclusion that the performance of SEMM system works
successfully in both AWGN and Rayleigh channel. With
iterative detection scheme, the performance of the multi-
rate system can approach to the single rate system.
C
opyright © 2011 SciRes. IJCNS
L. CHI ET AL.
Copyright © 2011 SciRes. IJCNS
635
4. Conclusions
In this paper, we investigate the self-encoded multirate
and the multimedia (SEMM) transmission. In SEMM
system, the spreading sequence is time varied and up-
dates by its own bit information. At transmitter side, chip
interleaving is employed to combat the fading. At the
receiver, we realize the decorrelation for multirate
012345678
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
BER Performance in AWGN Channel
S NR (d B )
BER
32 chips/bit multirate
64 chips/bit multirate
128 c hi ps / bi t m ul t i rat e
128 chips/bi t s i ngl e rate
64 chips/bi t s i ngl e rate
32 chips/bi t s i ngl e rate
Figure 5. The performance of the SEMM in AWGN channel.
0 1 23 4 56 78
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
S E SS and Random Sequenc e B ER P erformanc e i n A WGN Channel
S NR ( dB)
BER
32 c hi ps/bit Random M ultirate
64 c hi ps/bit Random M ultirate
128 c hi ps/bit Random M ul t i rat e
32 c hi ps/bit SE S S Mult i rate
64 c hi ps/bit SE S S Mult i rate
128 c hi ps/bit SES S M ul ti rat e
Figure 6. SEMM and random spreading sequ e nc e, multirate AWGN channel.
L. CHI ET AL.
636
0 12 3456 78910
10
-4
10
-3
10
-2
10
-1
10
0
Rayleigh Fading Pe rforman ce
S NR ( dB)
BER
32 c hi ps/bit Mult i rat e
64 c hi ps/bit Mult i rat e
128 c hips/bit Mult i rat e
32 c hi ps/bit si ngl e rat e
64 c hi ps/bit si ngl e rat e
128 c hips/bi t s i ngl e rat e
Figure 7. SEMM and single rate perfor manc e, r ay lei gh fading channel.
05 10 15 20 25
10
-4
10
-3
10
-2
10
-1
10
0
Multi rate System , Rayleigh fadi ng
SNR (dB )
BER
32 c hi ps/bi t Random
64 c hi ps/bi t Random
128 c h i ps/bit Random
32 chips/bit SESS ID
64 chips/bit SESS ID
128 c h i ps/bit SE SS ID
Figure 8. SEMM and random sprea ding se quence, multirate, rayleigh fading channel.
transmission. By the way, the decorrelation scheme does
not depend on the code scheme and we still realize the
multirate random sequence decorrelation in our simula-
tion. Once from other applications, the crosstalk is re-
moved the interference cancellation is employed to
eliminate the multiapplication interference. Therefore,
the iterative detection can be taken on as single rate. The
simulation result shows that SEMM system with iterative
detection works successfully in AWGN and Rayleigh
channel. The performance approaches to the single rate
and is significantly improved over multirate random se-
quence.
5. Acknowledgements
This work was supported by the contract award FA9550-
08-1-0393 from the Air Force Office of Scientific Re-
search.
Copyright © 2011 SciRes. IJCNS
L. CHI ET AL.637
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