Nowadays cellular mobile phones are almost commonplace as more and more people respond to the convenience of receiving and making telephone calls from wherever they happen to be. Questions of network range become important, especially as users want to extend their usage to more remote locations, harbours or coastal waters, as do questions of reception quality.

When using cellular mobile phones, the quality of reception can vary quite considerably. While cellular signals are no different from other line-of-sight UHF transmissions, the different systems used to propagate the signal have a significant impact on what you hear. Consistent quality is directly related to modulation, multiple channel access and call transfer methods from one cell and frequency to the next, as well as how the system responds to fading and interference from adjacent cells and other radio sources.

At these frequencies, 800-960 MHz, waves do not defract well and are vulnerable to obstruction by obstacles, man-made or geographical, and the compensating process of diffraction,or filling in of shadow areas is significantly less effective. Signals are also susceptible to fading caused by secondary signals (often reflected off large buildings) which arrive after the direct signal, known as multipath fading and to interference from other radio sources.

Because the two cellular systems, analog AMPS and digital GSM, are very different in the processes they use, the resultant speech quality provided varies quite considerably. With the digital system, speech quality remains constant for some distance from the base station then declines as it approaches the 35km (22 mile) cell boundary, cutting out sharply at the actual boundary without warning. The analog signal, however, fades out gradually, as the distance from the base station increases, but continues for much further, especially at sea where, with a high efficiency antenna system, more than double the range may be achievable.

In the AMPS analog system the signal is frequency modulated (FM), that is the amplitude is made constant and the frequency is varied. With the GSM digital system the signal is also FM modulated but using GMSK (gaussian minimum-shift keying). In FM and GMSK amplitude limiters are used to achieve a better noise immunity. This process provides a constant transmission amplitude output during the burst and permits more efficient amplifiers (Class C) to be used, resulting in increased mobile battery efficiency.

However, the digital signal can operate with a very low error rate in poor signal to noise ratios which results in better use of the spectrum and greater resilience to co-channel interference. Whereas with the analog signal, the audio quality decreases with the signal to noise ratio.

In GSM, TDMA (time-division multiple access) is used to allocate time slots to 8 users over a 200 kHz bandwidth divided into 25 kHz channels (or groups) with 8 users per channel. To enable a signal to reach the base station in correct time sequence no matter where the mobile is within the cell, the base station controls when it transmits, based on how long it takes a signal to travel the 70km (44 miles) to the boundary and back. Echo-back circuits at the mobile measure the distance and calculate the synchronisation delay.

The base station regularly sends a signal to the mobile which is echoed back to base and, based on the time taken, the mobile is instructed to advance or retard transmission. Any signals that arrive after the maximum time limit are ignored as being foreign to the cell. For this reason, if there is not another cell to hand the signal on to, the signal drops out at the boundary.

The method used in AMPS is quite different. Each call is allocated a single channel (25 kHz) through a process known as FDMA (frequency-division multiple access). Because each signal has its own channel, it is not necessary to co-ordinate time slots as in the GSM system. This permits cell range to be extended in fringe areas but can also contribute to noise interference between adjacent cells.

The limitation of the GSM cell range to 35km dates back to the origins of the system in Europe where cell ranges over 35km were not considered necessary. In Asia and Australia, where population is often scattered over much larger regions, the advantages of increased cell range are obvious.

While AMPS has the advantage of range for users in fringe areas, coastal waters and harbours, GSM is better equipped to provide quality reception, being able to operate with carrier/interference ratios of 10-12 dB compared to around 17-18 dB in analog systems.

Because the signal cuts out abruptly at the cell boundary, there is less interference from signals from neighbouring cells. Through the use of a voice activity detector transmission only takes place when speech is detected. This approximately halves the time when interference by another mobile can occur.

As a caller moves from the jurisdiction of one cell (and frequency) to another, methods of handoff are critical to continuity. With GSM mobile-assisted handover, the base station continuously monitors other base stations in its vicinity measuring signal strength and error rate to decide when to handover. The monitoring of bit error rate in particular is instrumental in achieving much better informed handover decisions than are possible with AMPS. In fact, data transmissions are not possible in AMPS as there is a short break in transmission during handoff, but large enough for data to be lost.

To minimise fading caused by multipath interference, which is mostly restricted to a single frequency, with the GSM system it is possible to change the frequency at random intervals (approximately 217 times per second) within each cell. This is known as frequency hopping. However, this feature has not been adopted universally. Because fading at a particular location is unlikely to extend more than ½ wavelength, two receiving antennas are used on the premise that one will be able to receive the signal where the other cannot.

However, due to the pulsed nature of the digital signal, when disruption to the signal does occur due to obstruction or multipath fading, for example, the call will cut out sharply without warning, even in the vicinity of the base station, rather than fade away as it does with the AMPS continuous wave.

Taken as a whole, GSM can provide a better quality transmission unless disrupted by obstruction or multipath fading, but if you are operating in fringe areas and need range AMPS will be much more useful. However, whether you have a digital or an analog mobile, communications will benefit from a high performance higher gain antenna system designed for your system, especially when you are operating in fringe areas, where signal power is significantly reduced, regardless of system. With a more efficient antenna, there is the added benefit of reduced drain on the mobile battery, as signals can be transmitted at the required field intensity using less power.

The design of cellular networks does not always take into account the needs of the mariner or fringe area dwellers. Frequently harbours and coastal waters are not fully included within the system and this means that mobiles phones work spasmodically, if at all, in these areas. If you are communicating from remote building sites or from temporary fringe area base stations, similar problems can occur. With Moonraker type CEL800 antenna system it is possible to improve and extend the range of AMPS analog communications .

System efficiency is critical with GSM digital networks where the signal can cut out abruptly on encountering obstruction or multipath fading. Moonraker type CEL900 antenna system is specifically designed to improve communications for GSM digital cellular mobile phone users and give increased performance in fringe areas.

Marine Cellular Mobile Systems