IS-95 Air Interface - forward and reverse links

- basics of the IS-95, IS-95A and IS-95B, cdmaOne air interface looking at the CDMA spreading, forward and reverse links and basic channel organisation.

The IS-95 air interface was very different to anything that had been previously used as a result of the fact that CDMA was used. The IS 95 forward link and the IS-95 reverse link are different in their construction in view of the fact that there are different requirements for each.

The IS-95 air interface is based around the use of CDMA within a 1.25 MHz channel. Using this, the system can use the same channel for a large number of signals, each one allocated a different code to access the system.

IS-95 forward and reverse links

In order to be able define the directions of the transmission within a cellular system, the links to and from the base station are defined. The names used within IS-95 and cdma2000 are different to those used for GSM and UMTS.

For the IS-95 air interface, the convention is:

  • Forward link:   the link from the base station to the handset.
  • Reverse link:   the link from the handset to the base station.

IS-95 air interface reverse and forward links

IS-95 air interface reverse and forward links

IS-95 CDMA and Spreading codes

A CDMA system is based around Direct Sequence Spread Spectrum techniques. Here the function of the system revolves around the use of the orthogonal chip or spreading codes. There are two forms of codes used within IS-95 which are used in slightly different ways:

  • Walsh codes:   The Walsh Codes used within IS-95 are a fixed 64 bit length. This provides for a maximum of 64 individual physical channels to be carried by the system. However the number of usable channels is more usually limited by the level of interference that results from the number of active channels and their data within the cell, and also interference from adjacent cells.
  • PN codes:   The pseudo-random or PN code is used in assembling the IS-95 air interface signal. They consist of a series of bits that have random properties. Three PN codes are used within IS-95, two short codes and one long one.

    These codes are used because it is found that if one code is time shifted then the code becomes uncorrelated with the same unshifted code. This means that the same code can be used twice, but time shifted to provide two uncorrelated sequences. In fact when used in the network, the codes are synchronised by GPS timing signals.

    In order to utilise the PN codes in IS-95, an approach using a mask is adopted. Codes transmitted at a different time offset are uncorrelated, and therefore by adopting a known given set of offsets it is possible to create a set of "codes" that can be used. The short codes are 32768 bits long and offset by 64 bits. This gives a total of 512 offsets that can be used within IS-95. As the short code is transmitted at the CDMA data rate of 1.2288 Mbps this equates to a full cycle time of 26.667 mS.

The IS-95 system combines these Walsh codes with two pseudorandom noise codes for each communication channel - in this way many different channels can be made.

The different channels can either be used for control applications, or for carrying the data payload. The different channels can then be separated by the receiver, as the receiver is only able to correlate those for which it has been provided with the required Walsh codes. All other signals using other Walsh codes just appear as noise.

IS-95 air interface radio signal construction

CDMA transmissions are very different to those used for the previous FDMA and TDMA systems. Here the system occupied a relatively narrow bandwidth. For a CDMA system a much wider bandwidth is used. In the case of IS-95 / cdmaOne a channel bandwidth of 1.25 MHz is used. The actual bandwidth being determined by the chip rate which is 1.228 Mcps. Additionally, in another departure from previous practice, adjacent cells are able to use the same frequency, the different codes used by the adjacent cells enabling them to work alongside each other.

The characteristics of the transmission between the forward and reverse links are slightly different. Much of this arises from the fact that the two links are on different frequencies, and the path lengths are different in terms of the number of wavelengths travelled. Also the requirements for the paths are slightly different.

The forward link is transmitted by a single station where synchronisation between the different channels is implicit as they are all generated within the same equipment. The IS-95 reverse link is generated by a large number of different handsets that are not easily synchronised and are at varying distances from the base station.

A further requirement is that the modulation format must take account of the power requirements for the handset.

IS-95 forward link

The IS-95 forward link uses a form of modulation known as Quadrature Phase Shift Keying (QPSK). This allows two bits per symbol to be transmitted and is therefore an efficient method of transmitting the data.

The IS-95 forward link 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.

IS-95 reverse link

In the reverse channel a form of modulation known as Offset Quadrature Phase Shift Keying (OQPSK). This is a form of quadrature phase shift keying, QPSK where the quadrature (Q) component of the signal is offset from the in-phase (I) component by half a symbol period. This form of modulation is used because the amplitude of the waveform does not cross zero, and this reduces the peak to average ratio. This has advantages for power amplifiers because it reduces the peak to average ratio and enables more efficient signal amplifiers to be made in the mobile where battery consumption is a key parameter.

The IS-95 reverse link is generated in a different way to that of the forward link. 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. To increase the bandwidth to the full 1.25 MHz, a further degree of spreading is provided by using an additional 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.

By Ian Poole

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