IS-95, cdmaOne Mobile Phone System
a basic introduction or tutorial about the basics of IS-95 A and B ( IS 95
) marketed under the brand name cdmaOne
IS-95 was the first CDMA mobile phone system to gain
widespread use and it is found widely in North America. Its brand name is
cdmaOne and the initial specification for the system was IS95A, but its
performance was later upgraded under IS-95B. It is this later specification that
is synonymous with cdmaOne. Apart from voice the mobile phone system is also
able to carry data at rates up to 14.4 kbps for IS-95A and 115 kbps for IS-95B.
The IS-95 system was introduced by Qualcomm. They had been
investigating the use of direct sequence spread spectrum techniques for military
use when it was realised that it could be used as a multiple access technology
for mobile communications. Previous systems had used frequency division multiple
access (FDMA) to time division multiple access (TDMA). The principle of FDMA is
that different users use different frequencies. This techniques was used for the
analogue systems such as AMPS, TACS and NMT. The TDMA principle is used in GSM.
Here in different users are allocated different time slots on a given channel.
CDMA
The CDMA or code division multiple access system used for
IS-95 is very differentto other multiple access schemes used in previous
cellualr systems. However it offers a number of advantages and as a result has
been widely used in many cellular technologies.
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Note on CDMA:
CDMA, Code Division Multiple Access, is a multiple
access scheme used by many 3G cellular technologies, and other forms of
wireless technology. It uses a process called Direct Sequence Spread
Spectrum where spreading codes are used to spread a signal out over a
given bandwidth and then reconstituting the data in the receiver by
using the same spreading code. By supplying different spreading codes to
different users, several users are able to utilises the same frequency
without mutual interference. |
The advantage of using CDMA over FDMA and TDMA is that it
enables a greater number of users to be supported. The improvement in efficiency
is hard to define as it depends on many factors including the size of the cells
and the level of interference between cells and several other factors.
Unlike the more traditional cellular systems where
neighbouring cells use different sets of channels, a CDMA system re-uses the
same channels. Signals from other cells will be appear as interference, but the
system is able to extract the required signal by using the correct code in the
demodulation and signal extraction process. Often more than one channel is used
in each cell, and this provides additional capacity because there is a limit to
the amount of traffic that can be supported on each channel.
Downlink signal
The downlink transmission (i.e. base station to the mobile)
within IS-95 consists of a number of elements. There is the pilot channel and
other further channels each with their own functions. The pilot channel
corresponds to the control channel in GSM and enables the mobile to estimate the
path loss and as a result of this to set its power level accordingly. In
addition to this there are other channels for paging, speech, data etc. The
speech is encoded using a voice encoder. Error correction is then applied to
this data to enable it to be carried even under poor conditions. This brings the
data rate up to 19.2 kbps. The next stage in the generation of the signal is to
multiple the data by a Walsh code - the form of orthogonal code used to spread
the signal when generating the CDMA signal itself. As this is a 64 bit code,
this multiplies the data rate by 64 to bring the overall data rate to 1.228
Mbps. This signal is then transmitted.
Uplink signal
The uplink signal for IS-95 is generated in a different way.
Although the same voice encoder is used, the resulting data has a greater degree
of error correction or protection applied. Accordingly the resulting data rate
is brought up to 28.8 kbps. A more complicated method of spreading using a Walsh
code is used. However this only results in 307 kbps data stream. Further
spreading is required. This is provided by using a different form of orthogonal
spreading code known as a PN code. This is multiplied with the signal to
increase its data rate by four to bring it up to the final data rate of 1.228
Mbps, the same as the downlink signal.
Soft handover
The reason that the uplink and downlink transmissions for
IS-95 are generated in a different way results from the fact that it is
difficult to synchronise the mobile handsets. Each one is a different distance
away from the base station and the time delays will be different. As a result
synchronisation is not possible. For the Walsh codes to maintain their
orthogonality and to operate correctly they must be properly synchronised. PN
codes do not require synchronisation and can be used more successfully under
these circumstances.
One of the advantages of CDMA is the fact that handover can
be made easier and more reliable. Normally when handing over from one from a
base station in one cell to the base station in the next, it is necessary for
the system to arrange for a new channel to be used. The mobile then changes
channel and hopes to be able to receive the signal on the new one
satisfactorily. Obviously there is a degree of risk, and occasionally a hand
over does not proceed smoothly. With CDMA it is possible to use what is termed a
soft hand over. As transmissions from the base stations in adjacent cells may be
made on the same frequency, it is possible for a mobile to receive signals from
two base stations at once. Normally the mobile would reject the signal from the
second base station, but it is possible to arrange for it to receive signals
from the two stations and this proves to be very useful during handover. During
the period of the handover the two base stations transmit the same signal
enabling the mobile to receive the signal via two routes at the same time. This
means that during this handover phase the mobile should not loose the signal.
Then as the mobile moves further into the second cell and the signal is firm, it
can rely on one station only and the handover is complete.
This approach considerably reduces the risk of loosing the
connection during handover, and it also minimises the risk of a short break in
the speech during this period. However it is not free and there is an associated
cost. The mobile needs two decoders to monitor and decode the two signals and
this increases the complexity of the mobile. On the network side it means that
two channels are used instead of one and this reduces the overall capacity. Some
estimate this could be as high as 40%. This is dependent upon the speed of
handover and the degree of overlap in the cells. The figure given is obviously a
worst case scenario, but despite this the advantages are deemed to outweigh the
reduction in capacity and increased mobile complexity.
IS-95 has been successfully installed in many areas of the
world, chiefly in North America. IS 95 also has the advantage that it has an
evolutionary migration path to 3G with CDMA2000 to give the higher data rates
that are needed for video streaming and data transfer whilst retaining
compatibility with the existing networks.
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