IS-95, cdmaOne Mobile Phone System

- a basic introduction to 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 different. Although a complete summary of CDMA will not be included here, the basic principle of CDMA is that different codes are used to distinguish between the different users. CDMA uses a form of modulation known as direct sequence spread spectrum. Here a signal is generated that spreads out over a wide bandwidth. A code known as a spreading code is used to perform this action. By using a group of codes known as orthogonal codes, it is possible to pick out a signal with a given code in the presence of many other signals with different orthogonal codes. In fact many different baseband "signals" with different spreading codes can be modulated onto the same carrier to enable many different users to be supported. By using different orthogonal codes interference between the signals is minimal. Conversely when signals are received from several mobile stations, the base station is able to isolate each one as they have different orthogonal spreading codes. In fact the system has been likened to hearing many people in a room speaking different languages. Despite a very high noise level it is possible to pick out the person speaking your own language - English for example.

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.