11-08-2012, 12:02 PM
Space Division Multiple Access for Cellular CDMA
[attachment=31079]
Abstract-
Space Division Multiple Access (SDMA) will
form an important part of the new Wideband Code Division
Multiple Access (WCDMA) standard that will realise
the Universal Mobile Telephone System (UMTS). This paper
addresses a few issues of importance when SDMA techniques
are used in a cellular CDMA system. Firstly a theoretical
analysis of a SDMA/CDMA system is performed.
The analysis is focused on a single cell, multipath Rayleigh
fading scenario. As system performance measure Bit Error
Rate (BER) is used as criteria to investigate the influence
of user location and number of antennas. An important parameter
in a SDMA system is the antenna array element
spacing. In our analysis a Uniform Linear Array (ULA) is
considered and a measure is defined to determine the optimal
antenna element spacing in a CDMA cellular environment.
INTRODUCTION
The rapid growth in wireless communication services and
networks have placed enormous strain on available natural
resources, ie. bandwidth. The mere fact that the Federal
Communications Commission (FCC) in the United States
was able to auction 90 MHz of spectrum allocated to PCS
for $17.2 billion is clear evidence of this 111. Added to this
are the additional strain placed on the available spectrum
by services such as Wireless Local Loop (WLL), Cordless
Phones and broadband access services [2], [3], [4]. This
path of rapid evolution has brought with it some of the
biggest engineering challenges to date. A couple of years
ago, the design of a communication system capable of operating
in multiple propagation environments, delivering a
broad range of services each with its own quality requirements
on a global scale to a person with a single handheld
terminal [4] would have sounded far fetched. Today this
is reality. The definition of a Universal Mobile Telephone
System (UMTS) is currently receiving a great deal of attention
from standardization bodies such as the ITU, and
current evolution paths predict the availability of UMTS
services early in the next century. This process have been
the driving force behind the design of new coding, modulation,
planning and access methodologies.
OPTIMUMA NTENNAS PACINGC RITERIA
In this section we derive a parameter for the optimum
antenna spacing from (11) in a cellular CDMA environment.
It is clear that if E{a2} = 0 the system will have
optimal performance, i.e. no multiple access interference.
This can only happen when there is no multiple access interference
or when (Re [11;y~k])2 = 0, which is essentially
the spatial correlation between and $(&ThIis. is only
possible when the interferers, relative to the reference user,
are in the antenna nulls. Since we are considering a multiple
access system, we would like to optimise E(a2} in the
presence of multiple acces interference. The only parameter
which can be optimised for this purpose when a ULA is
assumed, is to determine an optimal antenna spacing. In
order to achieve this, we define the following.
MOBILEL OCATIOND ISTRIBUTION
The position of mobiles in a cellular system has a significant
influence on the system performance. To describe this
effect, the cellular multiple access system is assumed to be
constituted by a number of non-overlapping cells, each of
radius R (in our analysis we only consider one cell). Each
cell has a base station located at its center, with mobile
users distributed throughout the cell. The position of a
mobile user in the cellular structure is fully defined by its
distance from the reference base station T, and its angle,
$o, measured from some reference, both of which can be
considered random variables. This is shown in Figure 3.
As the performance of the cellular system depends on the
spatial distribution of mobiles, true pdf's for both the angular
and distance distributions of mobiles are required in
order to calculate the performance of the system. These
are, however, not available in the literature and various
approximations such as uniform angular and distance distributions
[7]
CONCLUSIONS
In this paper the performance of a multipath Rayleigh
faded single cell §DMA/CDMA system was investigated.
A criteria for the optimum antenna spacing in a cellular
CDMA system employing a ULA was derived. The
tion is based on reducing the multiple access interference
by using the spatial correlation as criteria. This criteria
can also be used to determine the optimum antenna geometry
in a cellular CDMA system. A novel mobile subscriber
probability density function was derived to more realistically
calculate system performance.
From the results it is clear that antenna arrays are an
effective way to reduce interference and to increase system
capacity. What is of essential importance is to have an algorithm
to determine the angle of arrival of the reference
user and to be able to determine where the strongest interferers
axe. In general, it is clear that, in addition to
an antenna array, a RAKE receiver in a multipath environment
is essential for acceptable system performance. It
should be noted that the antenna arrays have the ability to
increase the system capacity substantially, but additional
gain can be achieved from a combination with other antenna
diversity schemes.
[attachment=31079]
Abstract-
Space Division Multiple Access (SDMA) will
form an important part of the new Wideband Code Division
Multiple Access (WCDMA) standard that will realise
the Universal Mobile Telephone System (UMTS). This paper
addresses a few issues of importance when SDMA techniques
are used in a cellular CDMA system. Firstly a theoretical
analysis of a SDMA/CDMA system is performed.
The analysis is focused on a single cell, multipath Rayleigh
fading scenario. As system performance measure Bit Error
Rate (BER) is used as criteria to investigate the influence
of user location and number of antennas. An important parameter
in a SDMA system is the antenna array element
spacing. In our analysis a Uniform Linear Array (ULA) is
considered and a measure is defined to determine the optimal
antenna element spacing in a CDMA cellular environment.
INTRODUCTION
The rapid growth in wireless communication services and
networks have placed enormous strain on available natural
resources, ie. bandwidth. The mere fact that the Federal
Communications Commission (FCC) in the United States
was able to auction 90 MHz of spectrum allocated to PCS
for $17.2 billion is clear evidence of this 111. Added to this
are the additional strain placed on the available spectrum
by services such as Wireless Local Loop (WLL), Cordless
Phones and broadband access services [2], [3], [4]. This
path of rapid evolution has brought with it some of the
biggest engineering challenges to date. A couple of years
ago, the design of a communication system capable of operating
in multiple propagation environments, delivering a
broad range of services each with its own quality requirements
on a global scale to a person with a single handheld
terminal [4] would have sounded far fetched. Today this
is reality. The definition of a Universal Mobile Telephone
System (UMTS) is currently receiving a great deal of attention
from standardization bodies such as the ITU, and
current evolution paths predict the availability of UMTS
services early in the next century. This process have been
the driving force behind the design of new coding, modulation,
planning and access methodologies.
OPTIMUMA NTENNAS PACINGC RITERIA
In this section we derive a parameter for the optimum
antenna spacing from (11) in a cellular CDMA environment.
It is clear that if E{a2} = 0 the system will have
optimal performance, i.e. no multiple access interference.
This can only happen when there is no multiple access interference
or when (Re [11;y~k])2 = 0, which is essentially
the spatial correlation between and $(&ThIis. is only
possible when the interferers, relative to the reference user,
are in the antenna nulls. Since we are considering a multiple
access system, we would like to optimise E(a2} in the
presence of multiple acces interference. The only parameter
which can be optimised for this purpose when a ULA is
assumed, is to determine an optimal antenna spacing. In
order to achieve this, we define the following.
MOBILEL OCATIOND ISTRIBUTION
The position of mobiles in a cellular system has a significant
influence on the system performance. To describe this
effect, the cellular multiple access system is assumed to be
constituted by a number of non-overlapping cells, each of
radius R (in our analysis we only consider one cell). Each
cell has a base station located at its center, with mobile
users distributed throughout the cell. The position of a
mobile user in the cellular structure is fully defined by its
distance from the reference base station T, and its angle,
$o, measured from some reference, both of which can be
considered random variables. This is shown in Figure 3.
As the performance of the cellular system depends on the
spatial distribution of mobiles, true pdf's for both the angular
and distance distributions of mobiles are required in
order to calculate the performance of the system. These
are, however, not available in the literature and various
approximations such as uniform angular and distance distributions
[7]
CONCLUSIONS
In this paper the performance of a multipath Rayleigh
faded single cell §DMA/CDMA system was investigated.
A criteria for the optimum antenna spacing in a cellular
CDMA system employing a ULA was derived. The
tion is based on reducing the multiple access interference
by using the spatial correlation as criteria. This criteria
can also be used to determine the optimum antenna geometry
in a cellular CDMA system. A novel mobile subscriber
probability density function was derived to more realistically
calculate system performance.
From the results it is clear that antenna arrays are an
effective way to reduce interference and to increase system
capacity. What is of essential importance is to have an algorithm
to determine the angle of arrival of the reference
user and to be able to determine where the strongest interferers
axe. In general, it is clear that, in addition to
an antenna array, a RAKE receiver in a multipath environment
is essential for acceptable system performance. It
should be noted that the antenna arrays have the ability to
increase the system capacity substantially, but additional
gain can be achieved from a combination with other antenna
diversity schemes.