- Extended range for mobiles

The closure of the AMPS mobile telephone network in Australia posed a challenge to find a suitable replacement system: one that could provide coverage for sparsely populated country areas taking in great distances as well as dense city populations. The introduction of the chosen replacement, CDMA, has not been without its teething problems, however...

For those living in metropolitan areas, the AMPS closure may have had little impact, except of course for the annoyance of changing handsets. If you chose to go with the new CDMA system, the changeover was practically seamless. The new system shares the same spectral allocation in the 800 MHz band as the AMPS network and the new handsets were conveniently designed to operate on both systems during the changeover period.

However, for those living in the bush the experience has been less than satisfying it would seem.

One of the greatest virtues of the AMPS analogue system over the GSM system is that of cell range. What’s more, this range could be considerably improved with a good external antenna, making it practical in many areas for inshore vessels to use at sea. However, data transmission is not possible and the number of system users is very limited.

Before the introduction of CDMA, Telstra Australia was obviously aware of the unique situation of country mobile users and carried out tests to ensure that it would be a suitable replacement for AMPS, namely that it would provide comparable range. These tests were conducted in New South Wales at Mt. Dowe, Peak Alone and off the coast of Ulladulla with a standard car kit, without a booster. The results, announced in mid 1999, claimed coverages of 100-130 kilometres from the base station, which were not available from other land based digital technologies.

In reality it would appear that some country CDMA customers have been less than satisfied with regular coverage and rather than fitting an external higher gain antenna, as recommended, have preferred to use old AMPS type booster amplifiers. Telstra has found evidence of this in the Riverina area of NSW around Deniliquin, Hay, Hillston and Jerilderie.

The use of these amplifiers has had such a detrimental effect on system operation that the Australian Communications Authority has moved to ban such devices so that anyone possessing or using a non-compliant amplifier with their CDMA or GSM phone may be fined up to $220,000. CDMA is seriously compromised by the use of these amplifiers, as it interferes with the basic technology that permits the spread spectrum system to operate.

Since the development of mobile phone technology, one of the basic aims has been to develop a system that can accommodate as many users as possible, thus optimising return efficiencies for the high basic costs of setting up network hardware.

Different radio techniques have been used to accommodate multiple users sharing bandwidth with increasing efficiencies. The initial analogue systems like AMPS used FDMA (Frequency Division Multiple Access) to divide up the bandwidth, allocating a single 25 kHz channel to each call so that calls could be separated out by frequency.

With the advent of TDMA (Time Division Multiple Access) it became possible to allocate 8 time slots to 8 users over a 200 kHz bandwidth divided into 25 kHz channels with 8 users per channel.

The use of time as a separator is achieved through the development of base station and handset power control techniques ensuring regulation of time slots, so that each call can reach the base station in correct time sequence no matter where the mobile is within the cell. The base station regularly sends a signal to the mobile which is echoed back to base and, depending on the time taken, the mobile is instructed to advance or retard transmission. This also reduces interference levels.

The development of the ability to control the transmission power of the mobiles led to the re-evaluation of a spread spectrum technique invented in World War II, known as CDMA (Code Division Multiple Access). This technique permitted many more users to utilise the same system, but up until then had not been workable in the mobile environment due to a phenomenon known as near-far effect. Initially its use was confined to small aperture satellite services, and also, due to its security virtues, to military applications, where it was used to avoid jamming and detection of sensitive transmissions.

The restricted use was due to the inability to eliminate narrowband interference caused by louder signals with greater amplitude drowning out lower power signals. This prevented correct demodulation of the wanted signal. The propagation path loss difference between those located close to the base station and those in fringe areas can be many tens of dB without regulation! (This is not the case in satellite systems where signal levels do not vary greatly.)

Through control of the transmission power of the mobiles it became possible to ensure all accessing signals could be received at similar amplitude, thus eliminating near-far effect and enabling the same frequency spectrum to be reused in all the cells.

The use of amplifier boosters that are not compatible with CDMA power regulation, therefore, prevents the elimination of near-far effect. This makes it impossible for the system to operate correctly, undermining the foundations of the basic concept of successful CDMA operation.

When the amplified signal fails to respond to requests by the base station to reduce power, the base station itself tries to rectify the problem by reducing its own power, thus reducing the size of the cell, so that the outer areas of the cell become out of range. The closer the amplified signal is to the base station, the louder the signal is and greater the reduction in cell area.

Of course when talking about range, the actual size of cells and the network itself are all important. One of the most attractive features of the AMPS system is range. In AMPS, the analogue signal, which is in continuous wave form, fades out gradually, as the distance from the base station increases. Propagation, however, can continue for much further, especially at sea, where, with a high efficiency antenna system, more than double the range may be achievable.

In GSM, however, the signal is regulated to have only enough power to reach the cell boundary in the correct time. If there is no further cell to hand it on to, the pulsed digital signal then drops out abruptly. Cell size is uniform being normally restricted to 35 km. This creates distinct network boundaries.

In CDMA all signals share the entire bandwidth and are separated by code so there is no need to restrict the size of cells in this way. Cell size is determined, rather, on the basis of the number of users, as system capacity is determined by what is perceived to be an acceptable signal to noise ratio. The greater the number of users in a cell, the higher the level of noise.

This makes it possible to have small cells in areas with high population density and what are known as boomer cells in country areas with scattered widespread populations. (There are a number of boomer cells in country and coastal locations.)

The organisation of cell size becomes a matter of strategy to develop the best economical coverage for an area. However, although cell sizes may be much larger, the signal is still digital and power controls will ensure that it drops off abruptly when it reaches the network boundary.

The banning of the use of non compliant amplifiers may be perceived as bad news for country mobile users, but in reality it is not, as this is not the only way to improve performance and range. The best and most effective way is to use a high performance higher gain external antenna. This will improve your communications all round, especially in fringe areas. What’s more, it will give you far more efficient use of your battery whereas using an amplifier will result in a significant drain on your battery, so you will need to recharge it more often.

In May 2001 Telstra carried out comparison tests on the use of external antennas and amplifiers for the Australian Communications Authority. External antennas were found to give the same improvement in coverage performance without undermining the CDMA system.

Moreover, what many people forget is that an amplifier will increase the power of transmission only - a good antenna improves reception as well. So, even if a compliant amplifier becomes available in the future you are certainly better off with your external antenna, whether you are on land or in coastal waters - presuming you want to receive calls as well.

To-date not many countries have introduced CDMA making it not a good solution if you require overseas roaming. However for countries that have similar geography and widespread population, it deserves a closer look. Otherwise you may be faced with greater infrastructure costs with far greater cell numbers.

For further information on CDMA please see our article CDMA: Private transmissions - cellular in code in the What’s New section.

Marine Cellular Mobile Systems