A Low Interference Time-Slicing Code Assignment for the 2D-Spread MC-DS-CDMA Systems

Abstract

In addition to some design flexibilities, the low interference property of code division multiple access (CDMA) makes multi-carrier direct sequence CDMA (MC-DS-CDMA) suitable for possible unlicensed transmissions in the licensed bands. Introducing the concept of time division multiple access (TDMA) into MC- DS-CDMA may further create a new dimension dealing with the interference problem. In this paper, we proposed a time slicing code assignment for the two-dimensional (2D) spread MC-DS- CDMA. The time slicing technique distributes interference equally over transmission time intervals (TTIs) by assigning users different frequency-domain spreading code groups and transmitting in their corresponding TTIs. Combing with the previous interference avoidance code assignment and power control mechanism for the 2D-spread MC-DS-CDMA, the proposed time slicing scheme is proved to be capable of eliminating interference more efficiently. In terms of the call admission rate, the improvement can be as large as 10% in one of the considered cases. A good operating point of time slicing can also maintain a high throughput, while achieving lower call dropping rate and lower power consumption. The results of this paper can provide some important and interesting hints for the system designers.

Full text

Page 1

A Low Interference Time-Slicing Code Assignment
for the 2D-Spread MC-DS-CDMA Systems
Chih-Wen Chang1and Chien-Cheng Kuo
Institute of Computer and Communication Engineering, National Cheng Kung University, Taiwan
Abstract—In addition to some design flexibilities, the low inter-
ference property of code division multiple access (CDMA) makes
multi-carrier direct sequence CDMA (MC-DS-CDMA) suitable for
possible unlicensed transmissions in the licensed bands. Introduc-
ing the concept of time division multiple access (TDMA) into MC-
DS-CDMA may further create a new dimension dealing with the
interference problem. In this paper, we proposed a time slicing
code assignment for the two-dimensional (2D) spread MC-DS-
CDMA. The time slicing technique distributes interference equally
over transmission time intervals (TTIs) by assigning users different
frequency-domain spreading code groups and transmitting in
their corresponding TTIs. Combing with the previous interference
avoidance code assignment and power control mechanism for the
2D-spread MC-DS-CDMA, the proposed time slicing scheme is
proved to be capable of eliminating interference more efficiently. In
terms of the call admission rate, the improvement can be as large
as 10% in one of the considered cases. A good operating point of
time slicing can also maintain a high throughput, while achieving
lower call dropping rate and lower power consumption. The results
of this paper can provide some important and interesting hints for
the system designers.
Index Terms—MC-DS-CDMA, 2D spreading, code assignment,
power control, time slicing, interference mitigation.
I. INTRODUCTION
The unceasing demand for ubiquitous access to network is
a strong driving force behind the revolutions in the wireless
communication technologies. However, the limited spectrum
resources can never quench the thirst. Accordingly, unlicensed
transmission in the licensed band stands out to show its potential
to fit this need, for example, the cognitive radio (CR) [1]. With
no doubt, low interference is the necessary characteristic to
facilitate the unlicensed transmissions. Based on the CDMA
techniques, MC-DS-CDMA possesses some desirable attributes
for the low interference transmissions and additional design
flexibilities [2], [3]. In the MC-DS-CDMA systems, however,
the key point to accommodate users lies in the level of multiple
access interference (MAI) but the amount of available code
resources [4]–[6]. Thus, how to eliminate MAI is a fatal
issue for possibly applying MC-DS-CDMA in the unlicensed
transmissions.
In the literature, several aspects has been explored to elim-
inate interference for MC-DS-CDMA, including the adaptive
subcarrier allocation [7], [8], code and waveform design [9]–
[12], and the novel joint code assignment and power control
techniques [4]–[6]. One systematic way to incorporate the
concept of TDMA into the MC-DS-CDMA may further create a
new dimension dealing with the MAI problem. In this paper, we
propose a time-slicing code assignment for the two-dimensional
(2D)-spread, time and frequency domain, MC-DS-CDMA with
power control. The term ”time slicing” originated from [13]
1The contact author’s e-mail is cwchang@ee.ncku.edu.tw
2This work was supported by the National Science Council, Taiwan, under
the contracts 96-2221-E-006-020 and 97-2221-E-006-085-MY3.
is an example of putting TDMA concept into orthogonal
frequency division multiplex (OFDM) to save power for digital
video broadcasting-handheld (DVB-H) with bursty traffic.
On top of our previous joint interference avoidance code
assignment and power control scheme [4]–[6], the time slicing
here plays an important role in balancing the interferers between
TTIs. With time slicing, TTIs are numbered according to the
number of the frequency spreading code groups (Nf). That is
users associated with different frequency spreading codes may
belong to different groups. Different groups will be arranged
to transmit in their corresponding TTIs. To achieve this, we
define a vacancy quotient (VQ) to quantize the available code
resources for a group of frequency spreading codes. When
receiving a new coming call request, VQ is pre-calculated and
a code belonging to the code group with highest VQ value will
be assigned. Thanks to the aids of VQ and TTI arrangement,
the MAI stemmed from the non-orthogonality of frequency-
domain spreading codes can be largely eliminated and equally
distributed to each TTI.
From the simulation results, we have several interesting ob-
servations. First, with a proper value of Nf, the call admission
rate can significantly increased and the same system throughput
can be maintained, although time slicing makes the available
code resources less in every TTI. Second, a proper Nf can
also contribute to lower call dropping rate and lower power
consumption. Third, as Nfgets larger, more incoming calls will
be blocked owing to the lack of code resources but the level of
interference. Thus, when we ultimately seek for an interference
free situation via time slicing, it may not always lead to good
results. These observations provide some important hints for the
system designers. Also, one can say that time slicing reachs a
new frontier of dealing with the MAI problem in the MC-DS-
CDMA systems.
The rest of this paper is organized as follows. We address
the motivation and problem formulation in Section II. Some
necessary background and the definition of the VQ value can
also be found in Section II. In Section III, we propose a novel
time slicing code assignment strategy. Simulation results and
conclusion are provided in Section IV and V, respectively.
II. MOTIVATION AND PROBLEM FORMULATION
A. The MAI in the 2D-Spread MC-DS-CDMA
Following the same system model and the assumptions of
[4]–[6], we consider a downlink multi-rate MC-DS-CDMA with
time- and frequency-domain spreading, the so-called 2D-spread
MC-DS-CDMA. In such a system, the orthogonal variable
spreading factor (OVSF) code tree has a two-dimensional struc-
ture, as shown in Fig. 1. In this figure, the time- and frequency-
domain spreading factors are 8 and 4, respectively. As derived in
equations (7)∼(9) of [4], the MAI with the 2D OVSF code tree
results from the non-orthogonality of the frequency spreading
978-1-4244-2517-4/09/$20.00 ©2009 IEEE

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code tree for frequency-domain
spreading code [1, -1, -1, 1 ]
code tree for frequency-domain
spreading code [1, -1, 1, -1 ]
code tree for frequency-domain
spreading code [1, 1, -1, -1 ]
code tree for frequency-domain
spreading code [1, 1, 1, 1 ]
Fig. 1.
spreading factor is four.
A two dimensional OVSF code tree when the frequency-domain
codes. Moreover, the relationships and the amount of the MAI
can be well described by the grid representation of a 2D
OVSF code tree. In the grid representation, the code resources
are illustrated by a set of rectangles. Code channels with the
same time-domain spreading factor Gt can be represented by
a set of equal-sized rectangles. The area of each rectangle is
proportional to the transmission rate of the corresponding code
channel (or inversely proportional to Gt).
Consideringthesamescenarios
2 of[4]3, the candidate
?
C(4)
8,8
, where C(i)
code in the l-th layer associated with the i-th frequency-
domain code tree, where the time-domain spreading factor
Gt = 1 ∼ 2(l−1)from the top layer (l = 1) to the bottom
layer (l = 4) of the OVSF code tree. The corresponding grid
representation of this scenario is shown in Fig. 2.
To make this example simple, here, we only consider the
candidacy of C(1)
the definition of the related codes in equation (1) of [4]4,
one can know that C(1)
amount of MAI. To be specific, C(1)
by
C(2)
2,2
and
C(1)
2,1
Now, two interesting questions arise: (1) what if the amount
of MAI goes beyond the affordable level of C(1)
(2) Can we further distinguish C(3)
MAI level ? To answer the first question, conventionally, the
coming call request will be blocked. For the second question,
however, the answer is positive when time slicing technique is
applied. Thus, with time slicing, this call request can be served
using C(3)
as
for
8,3,C(4)
shown
a
8,4,C(4)
in Fig.
are
8,7,
codes
8,2,C(4)
request
8,5,C(4)
C(1)
8,8,C(3)
?
8,1,C(3)
8,2,C(3)
8,3,C(3)
8,4,C(4)
8,1,C(4)
8,6,C(4)
2l−1,n(n = 1,...,2(l−1)) denotes the n-th
8,8and C(3)
8,1for the request. According to
8,8and C(3)
8,1are affected by the same
8,8and C(3)
2,1,C(2)
, respectively.
8,1are interfered
?
2,2,C(3)
???
8,8and C(3)
8,8in terms of the
8,1?
8,1from C(1)
8,1.
B. The Concept of Time Slicing
In [13], time slicing turns off radio frequency (RF) front-end
when the desired service is not in transmission. In this paper,
3Owning to the page-limit, the Fig. 2 of [4] is not reiterated in this paper.
4Related Codes ≡ {The used codes positioned in the same column of the
grip representation}. Codes belonging to the same related codes group interfere
each other.
: used code
?????????????
?????????????
?????????????
?????????????
Layer 2
Layer 3
Layer 4
Aggregate
Layer 2~4
) 3 (
1 , 8
C
(
1 , 8
C
) 3 (
2 , 8
C
(
2 , 8
C
) 3 (
3 , 8
C
(
3 , 8
C
) 3 (
4 , 8
C
) 4
C
) 4
) 4(
4 , 8
) 4(
5 , 8
C
) 4(
7 , 8
C
) 4(
6 , 8
C
) 4 (
8 , 8
C
????????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
?????????????
) 4
) 1 (
8 , 8
C
) 4 (
8 , 8
C
) 4 (
7 , 8
C
) 4 (
6 , 8
C
) 4(
5 , 8
C
) 4 (
4 , 8
C
) 4 (
3 , 8
C
) 4 (
2 , 8
C
) 4 (
1 , 8
C
) 3 (
1 , 8
C
) 3 (
2 , 8
C
) 3 (
3 , 8
C
) 3 (
4 , 8
C
) 1 (
1 , 2
) 2(
1 , 2
C
C
) 1 (
8 , 8
C
) 2(
2 , 2
) 3 (
2 , 2
C
C
Fig. 2.
MC-DS-CDMA system with time and frequency domain spreading.
The grid representation of Fig. 2 of [4] for the code resources in the
based on the interference level during each TTI, time slicing
allocates TTIs to users utilizing different frequency spreading
codes. Figs. 3(a) and 3(b) respectively shows the traditional
continuous transmission and time slicing principle during two
consecutive TTIs for the 2D-spread MC-DS-CDMA. As shown
in the figure, all the K users with 2D spreading codes transmit
simultaneously using continuous transmission scheme, while
these codes are divided into two groups to transmit during two
different TTIs when time slicing is applied. One should note
that the length of the TTI is unchanged when time slicing is
applied. Recall the example shown in Fig. 2, what will it be
when we apply time slicing to it ? Fig. 4 shows the answer. As
shown in the figure, the 1stand 2ndcode trees are arranged to
transmit during the m-th TTI, while the 3rdand 4thcode trees
are allocated the (m+1)-th TTI. Now, C(3)
free. Fortunately, the aforementioned coming call request can
now be served.
8,1can be interference
C. Vacancy Quotient of a Frequency Spreading Code Group
Assume that the number of frequency spreading codes and
groups of which are Gf and Nf, respectively. It gives M =
Gf/Nf frequency spreading codes in one group. A user is
requesting a code with time-domain spreading factor equal
to Gt. Then, the possible candidate codes are
1 ≤ i ≤ Gt and 1 ≤ j ≤ Gf. The vacancy quotient (VQ)
of the k-th frequency spreading code group can now be defined
as following.
?
C(j)
Gt,i
?
for
V Qk=
kM
?
j=(k−1)M+1
Gt
?
i=1
e(j)
Gt,i, 1 ≤ k ≤ Nf ,
(1)

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Code K
Code 2
…
Code 1
Frequency spreading
code domain
Time
m th TTI
m+1 th TTI
(a) Continuous transmission
Code n
Code 2
…
Code 1
Frequency spreading
code domain
Time
m th TTI
with allocating
1st code group
m+1 th TTI
with allocating
2ndcode group
Code K
Code n+2
…
Code n+1
(b) Time slicing principle
Fig. 3.
principle during two consecutive TTIs for the 2D-spread MC-DS-CDMA.
(a) The traditional continuous transmission and (b) time slicing
) 3 (
1 , 8
C
(
1 , 8
C
) 3 (
2 , 8
C
(
2 , 8
C
) 3 (
3 , 8
C
(
3 , 8
C
) 3 (
4 , 8
C
) 4
C
) 4
) 4(
4 , 8
) 4(
5 , 8
C
) 4(
7 , 8
C
) 4(
6 , 8
C
) 4 (
8 , 8
C
) 4
) 1 (
1 , 2
) 2(
1 , 2
C
C
) 1 (
8 , 8
C
) 2(
2 , 2
C
) 3 (
2 , 2
C
Fig. 4. Applying time slicing to the example of Fig. 2.
where e(j)
otherwise e(j)
candidate code is a code which can preserve orthogonality either
in time or frequency domain between any other codes in use. A
group with a higher value of VQ will be assigned to the coming
user.
Gt,i= 1 if C(j)
Gt,i= 0. One should note that the available
Gt,iis an available candidate code;
III. THE TIME SLICING CODE ASSIGNMENT
As shown in Fig. 5, the proposed time slicing code assign-
ment mainly consists of four steps. In the first step, according
to the VQ value, the new coming user will be assigned a
frequency-domain spreading code group, in other words, being
allocated to a particular TTI as mentioned before. In the second
step, a code group will be randomly assigned if there are more
than one group having the same VQ value in the first step. The
third step is to apply the so-called interference avoidance (IA)
code assignment to pick a code within its corresponding code
group [4], [5]. At last, the user will transmit signals using the
power control mechanism in [6].
IV. SIMULATION RESULTS
In this section, we illustrate the effectiveness of the proposed
time slicing code assignment for the power-controlled MC-DS-
CDMA with 2D spreading, in terms of call admission rate,
1st step?Pick a code group with highest
VQ value
2nd step?Pick a code group randomly
4th step?Power control mechanism
New call arrival
Satisfying all power constraints
This is a new call
Call blocking
Call dropping
NO
YES
Exceed the call dropping threshold
NO
YES
NO
YES
If more than two groups tie
3rd step?Pick a code which produces less
MAI by the IA code assignment
strategy
NO
YES
Fig. 5.
MC-DS-CDMA with 2D spreading.
The flowchart of the proposed time slicing code assignment for the
call blocking rate, call dropping rate, received SINR, power
consumption and total throughput.
A. Simulation Environment
In the simulation, we follow the same assumptions as [4]–
[6]. In the downlink MC-DS-CDMA system, the subcarriers
carrying the same data bits experience independent flat Rayleigh
fading channel. The background noise is modelled by the
white Gaussian noise with power spectrum density of N0 =
−103.2 (dBm). A new call is generated by the Poisson arrival
process with arrival rate (λ) of 1/2 per time unit and the
paired departure rate (μ) can be one of {1/32, 1/48, 1/64,
1/80, 1/96, 1/112, 1/144, 1/176}. Thus, there are on average
λ/μ = 16 ∼ 88 active calls in the system. We simulate
5,000 incoming calls for each combination of λ and μ. With
128 parallel substreams, the frequency-domain spreading factor
(Gf) is 8 and the time-domain spreading factors (Gt) are 4,
8, 16, or 32. Each call requests a code of 8R (Gt = 4), 4R
(Gt= 8), 2R (Gt= 16), or R (Gt= 32) with a probability
according to the code traffic pattern [ 1 1 2 8 ], where R is
the unit data rate. A code traffic pattern of [ a b c d ] means
that the times of requesting data rates 8R, 4R, 2R, and R are
proportional to a : b : c : d, respectively. The data rate of each
user is fixed during his call holding time. To clearly indicate
the traffic load brought by the active calls with different data
rates, we define an effective traffic load in the following. With
the time-domain spreading factor selecting from Gt=[ 4 8 16
32 ] and the code traffic pattern of [ a b c d ], the effective
traffic load (ρ) is defined as
μ×8R × a + 4R × b + 2R × c + R × d
a + b + c + d
For λ = 1/2 and μ = 1/80 and the code traffic pattern [ 1 1 2
8 ], the effective traffic load ρ is 250% of the utilization of the
time-domain resources. The maximum transmission power of a
substream is Po = 1 mW and that of BS is Ptotal = 10W.
The target signal to interference and noise (SINR) ratio is 5dB.
A new call will be blocked if it can not reach the target SINR
or without sufficient code resources. Furthermore, an active call
ρ =λ
×
1
32R
. (2)

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1 1.52 2.53 3.54 4.55 5.5
40
50
60
70
80
90
100
Effective traffic load (ρ)
Average admission rate (%)


Nf=1
Nf=2
Nf=4
Nf=8
Fig. 6.
Nf= 1,2,4,8 code groups.
Average call admission rate for various effective traffic loads with
will be dropped if it call can not reach the SINR target for a
consecutive period of time (Tdrop), where Tdrop= 0.01 seconds
is assumed to be a frame time interval [14]. Note that the power
control mechanism will be executed 15 times per frame. The
transmission time interval (TTI) is 2 milliseconds. The number
of code groups (Nf) can be one of [ 1 2 4 8 ]. Note that
Nf = 1 is actually equivalent to the conventional continuous
transmission scheme.
B. Performance Comparison
Fig. 6 shows the average call admission rate for various
effective traffic loads with Nf= 1,2,4,8 code groups. One can
find that with Nf= 2 and 4, the proposed time slicing code as-
signment outperforms the conventional continuous transmission
with Nf = 1. For example, at ρ = 4.5 with Nf = 4, the call
admission rate can be raised from 67% to 77%. However, with
Nf= 8, the time slicing technique can not be effective, owing
to the lack of code resources. That is because, with Nf = 8,
only one eighth of the code resources can be supplied in each
TTI. This phenomenon can also be explained by Fig. 7.
Fig. 7 shows the average percentage of call blocking (a)
because of the surplus MAI and (b) because of the lack of code
resources for various effective traffic loads with Nf= 1,2,4,8
code groups. As shown in the figure, with Nf= 1 and 2, 100%
of the call blocking results from the surplus of MAI, which
leads to the unsatisfactory received SINR. On the contrary, with
Nf = 8, 100% of the call blocking is caused by the short
supply of the code resources. However, it is interesting to find
that, with Nf = 4, the call blocking event can happen due to
the both reasons. Somehow, we can say that in our considered
cases, Nf= 4 can be a better and flexible time slicing operating
point.
Fig. 8 shows average call dropping rate for various effective
traffic loads with Nf= 1,2,4,8 code groups. From this figure,
one can see the effectiveness of the time slicing technique
to distribute interference into every TTI. Comparing with the
performance curve of Nf = 1, Nf = 2 and Nf = 4 can
preserve lower call dropping rate while allowing higher call
admission rate simultaneously (Fig. 6).
Fig. 9 shows the average received SINR for various effective
traffic loads with Nf = 1,2,4,8 code groups. Recalling the
simulation setup, the power control scheme in this paper is to
target the received SINR at five dB. Thus, with Nf = 8, the
12345
0
10
20
30
40
50
60
70
80
90
100
Effective traffic load (ρ)
(a)
Percentage of blocking (%)


Nf=1
Nf=2
Nf=4
Nf=8
12345
0
10
20
30
40
50
60
70
80
90
100
Effective traffic load (ρ)
(b)
Percentage of blocking (%)


Nf=1
Nf=2
Nf=4
Nf=8
Fig. 7.
and (b) because of the lack of code resources for various effective traffic loads
with Nf= 1,2,4,8 code groups.
Average percentage of call blocking (a) because of the surplus MAI
1 1.52 2.53 3.54 4.55 5.5
0
1
2
3
4
5
6
Effective traffic load (ρ)
Average droping rate (%)


Nf=1
Nf=2
Nf=4
Nf=8
Fig. 8.
Nf= 1,2,4,8 code groups.
Average call dropping rate for various effective traffic loads with
received SINR can be perfectly maintained at five dB because
only one frequency-domain spreading code can be used during
each TTI in this case. Note that the MAI in the 2D-spread
MC-DS-CDMA system results from the non-orthogonality of
the frequency-domain spreading codes. However, the dynamics
of the MAI level leads to the loosed controlled received SINR
as in [6].
Fig. 10 shows the average power consumption for various
effective traffic loads with Nf = 1,2,4,8 code groups. From
this figure, one can see that with the help of the time slicing,
the average power consumption can be largely reduced. For
example, at ρ = 3.5, the power consumption can reduce from
0.0163(W) to 0.0135(W) and 0.0004(W) with four (Nf = 4)
and eight code groups (Nf= 8), respectively.
Fig. 11 shows the average total throughput for various
effective traffic loads with Nf = 1,2,4,8 code groups. To
facilitate the comparison, the throughput statistics are collected
every eight TTIs because the largest number of the frequency
domain spreading code groups is Nf= 8. Also, the throughput
is evaluated based on the unit data rate R. Obviously, the
throughput performance of Nf = 2 and 4 can be maintained
at the same level as that of Nf = 1, although the available
code resources in every TTIs can only be 1/Nf. However, the
low call admission rate of Nf = 8 (Fig. 6) results in the low
throughput performance.

Page 5

1 1.522.53 3.54 4.55 5.5
5
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
6
Effective traffic load (ρ)
Average received SINR (dB)


Nf=1
Nf=2
Nf=4
Nf=8
Fig. 9.
1,2,4,8 code groups.
Average received SINR for various effective traffic loads with Nf=
1 1.52 2.53 3.54 4.55 5.5
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
Effective traffic load (ρ)
Average power consumption (W)


Nf=1
Nf=2
Nf=4
Nf=8
Fig. 10.
Nf= 1,2,4,8 code groups.
Average power consumption for various effective traffic loads with
1 1.52 2.53 3.54 4.55 5.5
200
250
300
350
400
450
500
550
600
650
Effective traffic load (ρ)
Average total throughput (R/8TTI)


Nf=1
Nf=2
Nf=4
Nf=8
Fig. 11. Average total throughput for various effective traffic loads with Nf=
1,2,4,8 code groups.
V. CONCLUSION
In this paper, we introduced the concept of TDMA into
MC-DS-CDMA and proposed a low interference time slicing
code assignment for the 2D-spread MC-DS-CDMA with power
control. The time slicing here was to equally distributed interfer-
ence over every TTI. To facilitate the time slicing technique, we
defined a VQ value to indicate the available code resources for
a group of frequency spreading codes. After this VQ-evaluation
process, a user can be assigned a frequency-domain spreading
code group and arranged to transmit in a proper TTI. Cooper-
ating with IA code assignment and power control mechanism
for the 2D-spread MC-DS-CDMA, the proposed scheme has
shown its ability to perform much better than the traditional
continuous transmission scheme. In terms of call admission rate,
the improvement can be 10%. Unfortunately, time slicing can
not be operated ultimately to reach an interference free situation.
A good operating point of time slicing can not only maintain a
high throughput but also achieve lower call dropping rate and
lower power consumption. In our considered cases, we found
that four groups of the frequency-domain code (Nf= 4) can be
an good operating point. An interesting research direction for
the future work can be the adaptive adjustment of the number
of code groups based on the MAI level.
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