Introduction to Mobile Telephone System

Analog Systems (1st Generation)

Advanced Mobile Phone Service (AMPS)
Advanced Mobile Phone Service (AMPS) was the original analog cellular system in the United States. It is still in widespread use and by1997; AMPS systems were operating in over 72 countries [3]. The AMPS system continues to evolve to allow advanced features such as increased standby time, narrowband radio channels, and anti-fraud authentication procedures. In 1974, 40 MHz of spectrum was allocated for cellular service [4] that provided only 666 channels. In 1986, an additional 10 MHz of spectrum was added to facilitate expansion [5] of the system to 832 channels. The frequency bands for the AMPS system are 824 MHz to 849 MHz (uplink) and 869 MHz to 894 MHz (downlink). Of the 832 channels, AMPS systems are divided into A and B bands to allow for 2 different service providers. There are two types of radio channels in an AMPS system; dedicated control channels and voice channels. On each system (A or B), mobile telephones scan and tune to one of 21 dedicated control channels to listen for pages and compete for access to the system. The control channel continuously sends system identification information and access control information. Although the control channel data rate is 10 kbps, messages are repeated 5 times, which reduces the effective channel rate to below 2 kbps. This allows a control channel to send 10 to 20 pages per second. The AMPS cellular system is frequency duplex with its channels separated by 45 MHz. The control channel and voice channel signaling is transferred at 10 kbps. AMPS cellular phones have three classes of maximum output power. A class 1 mobile telephone has a maximum power output of 6 dBW (4 Watts), class 2 has a maximum output power of 2 dBW (1.6 Watts), and the class 3 units are capable of delivering only -2 dBW (0.6 Watts). The output power can be adjusted in 4 dB steps and has a minimum output power of -22 dBW (approximately 6 milliwatts).

Total Access Communication System (TACS)
The Total Access Communication System (TACS) is very similar to the US EIA-553 AMPS system. Its primary differences include changes to the radio channel frequencies, radio channel bandwidths, and data signaling rates. The TACS was introduced to the U.K. in 1985. After its introduction in the UK in 1985, over 25 countries offered TACS service. The introduction of the TACS system was very successful and the system was expanded to add more channels through what is called Extended TACS (ETACS). The TACS system was deployed in 25kHz radio channels, compared to the 30kHz channels used in AMPS. This narrower radio bandwidth reduced the data speed of the signaling channel. The frequency ranges of most TACS systems are 890 MHz to 915 MHz for the uplink and 935 MHz to 960 MHz for the downlink. The TACS system was initially allocated 25 MHz although 10 MHz of the 25 MHz was reserved for future pan-European systems in the UK. An additional 16 MHz of radio channel bandwidth was added to allow for Extended TACS (ETACS). The ETACS system is a frequency duplex system with its channels separated by 45 MHz. The control channel and voice channel signaling is transferred at 8 kbps. There are 4 power classes for ETACS mobile telephones. Class 1 mobile telephones have a maximum output of 10 Watts, class 2 has 4 Watts, class 3 has 1.6 Watts, and class 4 has 0.6 Watts. Similar to AMPS, mobile telephones can be adjusted in 4 dB steps and have a minimum transmit power level of approximately 6 milliwatts. The TACS system has also been modified for use in Japan. This Japanese version is called JTACS. The only significant changes were the frequency bands and number of channels. The TACS system has also been modified to create the Narrowband TACS (NTACS) system. NTACS reduced the radio channel bandwidth from 25 kHz to 12.5 kHz and changed the in-band 8 kbps signaling on the voice channel to 100 bps sub-band digital signaling.

Nordic Mobile Telephone (NMT)
There are two Nordic Mobile Telephone (NMT) systems; NMT 450 that is a low capacity system and NMT 900 that is a high capacity system. The Nordic mobile telephone (NMT) system was developed by the telecommunications administrations of Sweden, Norway, Finland, and Denmark to create a compatible mobile telephone system in the Nordic countries [6]. The first commercial NMT 450 cellular system was available at the end of 1981. Due to the rapid success of the initial NMT 450 system and limited capacity of the original system design, the NMT 900 system version was introduced in 1986. There are now over 40 countries that have NMT service available. Some of these countries use different frequency bands or reduced number of channels. The NMT 450 system uses a lower frequency (450 MHz) and higher maximum transmitter power level which allows a larger cell site coverage areas while the NMT 900 system uses a higher frequency (approximately the same 900 MHz band used for TACS and GSM) and a lower maximum transmitter power which increases system capacity. NMT 450 and NMT 900 systems can co-exist which permits them to use the same switching center [7]. This allows some NMT service providers to start offering service with an NMT 450 system and progress up to a NMT 900 system when the need arises. Some operations of the NMT systems are very different from most other cellular systems. When NMT mobile telephones access the cellular system, they can either find an unused voice channel and negotiate access directly or begin conversation without the assistance of a dedicated control channel. Because scanning for free voice channels can be very time consuming, the NMT 900 system does allow for the use of a dedicated control channel called the calling channel. The NMT 900 system also allows discontinuous reception, which increases the standby time of the portable phones. The NMT 450 system is frequency duplex with 180 channels (except Finland which only has 160 channels) [8]. The radio channel bandwidth is 25 kHz and the frequency duplex spacing is 10 MHz. The NMT 900 system has 999 channels or 1999 interleaved channels. Signaling on the NMT systems is performed at 1200 bps on the control (calling) channel (NMT 900) and voice channel. Because of the slow signaling rate and robust error detection/correction capability, no repeated messages are necessary. NMT 450 base stations can transmit up to 50W. This high power combined with the lower 450 MHz frequency allows cell site size of up to approximately 40 km radius. NMT 900 base stations are limited to a maximum of 25W that allows a maximum cell size radius of up to approximately 20 km [9]. There are three power levels (high, medium, and low) for NMT mobile phones and two power levels (high and low) for portables. NMT 450 mobile telephone power levels are: High 15W, Medium 1.5W, and Low 0.15W. NMT 450 portable telephones; High 1.0W, Low 0.1W. NMT 900 mobile telephones: High 6.0W, Medium 1.0W, Low 0.1W and NMT 900 portable telephones: High 1.0W, Low 0.1W. The NMT system is unique as it included various types of anti-fraud protection. NMT mobile telephones hold a three-digit password that is stored in the telephone and cellular switching center and is unknown to the customer. This password is sent to the cellular system during system access along with the mobile telephone number. The NMT system has also added a Subscriber Identity Security (SIS) system that provides additional anti-fraud protection. Not all NMT telephones have SIS capability.

Narrowband AMPS (NAMPS)
Narrowband Advanced Mobile Phone Service (NAMPS) is an analog cellular system that was commercially introduced by Motorola in late 1991 and was deployed worldwide. Like the existing AMPS technology, NAMPS uses analog FM radio for voice transmissions. The distinguishing feature of NAMPS is its use of a “narrow” 10 kHz bandwidth for radio channels, a third of the size of AMPS channels. Because more of these narrower radio channels can be installed in each cell site, NAMPS systems can serve more subscribers than AMPS systems without adding new cell sites. NAMPS also shifts some control commands to the sub-audible frequency range to facilitate simultaneous voice and data transmissions. In 1991, the first NAMPS standard, named IS-88, evolved from the US AMPS specification (EIA-553). The IS-88 standard identified parameters needed to begin designing NAMPS radios, such as radio channel bandwidth, type of modulation, and message format. During development, the NAMPS specification benefited from the narrowband JTACS radio system specifications. During the following years, advanced features such as ESN authentication, caller ID, and short messaging were added to the NAMPS specification.

Japanese Mobile Cellular System (MCS)
apan launched the world’s first commercial cellular system in 1979. Because this system had achieved great success, several different types of cellular systems have evolved in Japan. These include the MCS-L1, MCSL2, JTACS and NTACS systems. The MCS-L1 was the first cellular system in Japan, which was developed and operated by NTT. The system operates in the 800 MHz band. The channel bandwidth is 25 kHz and the signaling is at 300 bps. The control channels are simulcast from all base stations in the local area. This limits the maximum capacity of the MCS-L1 system. Because the MCS-L1 system could only serve a limited number of customers, the MCS-L2 system was developed. It uses the same frequency bands as the MCS-L1 system. The radio channel bandwidth was reduced from 25 kHz to 12.5 kHz with 6.25 kHz interleaving. This gives the MCS-L2 system 2,400 channels. The control channels transfer information at 2,400 bps and the voice channels can use either in-band (blank and burst) signaling at 2,400 bps or sub-band digital audio signaling at 150 bps. MCS-L2 mobile telephones have diversity reception (similar to diversity receive used in base stations). While this increases the cost and size of the mobile telephones, it also increases the performance and range of the cellular system.

CNET is an analog cellular system that is used in Germany, Portugal, and South Africa [10]. The first CNET system started operation in Germany in 1985. The primary objective of the CNET system was to bridge the gap of cellular systems in Germany until the digital European system could be introduced [11]. The CNET system operates at 450 MHz with 4.44 MHz transmit and receive bands. The frequency bands are 461.3 to 465.74 MHz and 451.3 to 455.74 MHz. The primary channel bandwidth is 20 kHz with 10 kHz channel inter- leaving. The CNET system continuously exchanges digital information between the mobile telephone and the base station. Every 12.5 msec, 4 bits of information are sent during compressed speech periods [12]. CNET mobile telephones also use an Identification Card (IC), which slides into the telephone to identify the customer. This allows customers to use any compatible CNET telephone.

The MATS-E system is used in France and Kuwait [13]. The MATS-E system combines many of the features used in different cellular systems. MATS-E uses the standard European mobile telephone frequency bands; 890-915 MHz and 935-960 MHz. The channel bandwidth is 25 kHz that provides 1,000 channels. The MATS-E is a frequency duplex system separated by 45 MHz. Each cell site has at least one dedicated control channel with a signaling rate of 2400 bps. Voice channels use FM modulation with sub-band digital audio signaling with a data rate of 150 bps