Multiple Access Schemes for Cellular Systems
including FDMA, TDMA, CDMA and OFDMA
a summary or tutorial about the basics of the different multiple access
schemes used within cellular technology to allow multiple users to access the
cellular system. These include FDMA, TDMA, CDMA and OFDMA.
In any cellular system or cellular technology, it is
necessary to have a scheme that enables several multiple users to gain access to
it and use it simultaneously. As cellular technology has progressed different
multiple access schemes have been used. They form the very core of the way in
which the radio technology of the cellular system works.
There are four main multiple access schemes that are used in
cellular systems ranging from the very first analogue cellular technologies to
those cellular technologies that are being developed for use in the future. The
multiple access schemes are known as FDMA, TDMA, CDMA and OFDMA.
Requirements for a multiple access scheme
In any cellular system it is necessary for it to be able have
a scheme whereby it can handle multiple users at any given time. There are many
ways of doing this, and as cellular technology has advanced, different
techniques have been used.
There are a number of requirements that any multiple access
scheme must be able to meet:
- Ability to handle several users without mutual interference.
- Ability to be able to maximise the spectrum efficiency
- Must be robust, enabling ease of handover between cells.
FDMA - Frequency Division Multiple Access
FDMA is the most straightforward of the multiple access
schemes that have been used. As a subscriber comes onto the system, or swaps
from one cell to the next, the network allocates a channel or frequency to each
one. In this way the different subscribers are allocated a different slot and
access to the network. As different frequencies are used, the system is
naturally termed Frequency Division Multiple Access. This scheme was used by all
analogue systems.
TDMA - Time Division Multiple Access
The second system came about with the transition to digital
schemes for cellular technology. Here digital data could be split up in time and
sent as bursts when required. As speech was digitised it could be sent in short
data bursts, any small delay caused by sending the data in bursts would be short
and not noticed. In this way it became possible to organise the system so that a
given number of slots were available on a give transmission. Each subscriber
would then be allocated a different time slot in which they could transmit or
receive data. As different time slots are used for each subscriber to gain
access to the system, it is known as time division multiple access. Obviously
this only allows a certain number of users access to the system. Beyond this
another channel may be used, so systems that use TDMA may also have elements of
FDMA operation as well.
CDMA - Code Division Multiple Access
CDMA uses one of the aspects associated with the use of
direct sequence spread spectrum. It can be seen from the article in the cellular
telecoms area of this site that when extracting the required data from a DSSS
signal it was necessary to have the correct spreading or chip code, and all
other data from sources using different orthogonal chip codes would be rejected.
It is therefore possible to allocate different users different codes, and use
this as the means by which different users are given access to the system.
The scheme has been likened to being in a room filled with
people all speaking different languages. Even though the noise level is very
high, it is still possible to understand someone speaking in your own language.
With CDMA different spreading or chip codes are used. When generating a direct
sequence spread spectrum, the data to be transmitted is multiplied with
spreading or chip code. This widens the spectrum of the signal, but it can only
be decided in the receiver if it is again multiplied with the same spreading
code. All signals that use different spreading codes are not seen, and are
discarded in the process. Thus in the presence of a variety of signals it is
possible to receive only the required one.
In this way the base station allocates different codes to
different users and when it receives the signal it will use one code to receive
the signal from one mobile, and another spreading code to receive the signal
from a second mobile. In this way the same frequency channel can be used to
serve a number of different mobiles.
OFDMA - Orthogonal Frequency Division Multiple Access
OFDMA is the form of multiple access scheme that is being
considered for the fourth generation cellular technologies along with the
evolutions for the third generation cellular systems (LTE for UMTS / W-CDMA and
UMB for CDMA2000).
As the name implies, OFDMA is based around OFDM. This is a
technology that utilises a large number of close spaced carriers.
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Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a
form of transmission that uses a large number of close spaced carriers
that are modulated with low rate data. Normally these signals would be
expected to interfere with each other, but by making the signals
orthogonal to each another there is no mutual interference. This is
achieved by having the carrier spacing equal to the reciprocal of the
symbol period. This means that when the signals are demodulated they
will have a whole number of cycles in the symbol period and their
contribution will sum to zero - in other words there is no interference
contribution. The data to be transmitted is split across all the
carriers and this means that by using error correction techniques, if
some of the carriers are lost due to multi-path effects, then the data
can be reconstructed. Additionally having data carried at a low rate
across all the carriers means that the effects of reflections and
inter-symbol interference can be overcome. It also means that single
frequency networks, where all transmitters can transmit on the same
channel can be implemented. |
To utilise OFDM as a multiple access scheme for cellular
technology, two different methods are used, one for the uplink and one for the
downlink. In the downlink, the mobile receives the whole signal transmitted by
the base station and extracts the data destined for the particular mobile. In
the uplink, one or more carriers are allocated to each handset dependent upon
the data to be transmitted, etc. In this way the cellular network is able to
control how the data is to be sent and received.
Situation today
Although the current 3G cellular systems use CDMA as their
basis, elements of TDMA and FDMA are also used. Both the major schemes, UMTS and
CDMA2000 have a limit on the number of users who are able to use a single
channel. In some instances two or more channels may be allocated to a particular
cell. This means that the system still uses an element of FDMA.
Additionally UMTS incorporates some timeslots, and this means
that the scheme uses elements of TDMA.
While CDMA is currently the dominant technology, both the
other forms of access scheme are still in evidence, not just in legacy
technologies, but utilised as part of the main access scheme in the latest 3G
systems. In addition to this, the new cellular technologies being developed for
the 3.99G or 4G cellular systems use OFDMA, and this appears to be the
technology of the future.
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