First-Generation Mobile Networks

3.3.1 AMPS

First-generation mobile networks are analog systems. Some of the more widely deployed first-generation networks include AMPS and NMT. In this section we focus the discussion on AMPS.

The Advanced Mobile Phone Service (AMPS) is in wide use even today, almost 25 years after it was introduced. AMPS was conceived by Bell Labs in the 1970s, and improvements in the form of digital AMPS (D-AMPS) were made in the late 1980s. The AMPS air interface is specified in EIA/TIA-553. AMPS is based on FDMA.

The FCC allocated a total of 50 MHz (25 MHz on the A side and B side) in the 800-MHz spectrum for AMPS. Each voice channel is allocated a 30-KHz portion of the bandwidth within the AMPS frequency allocations. Because each carrier has 25 MHz of spectrum, this provides a total of 832 (25 MHz/30 KHz) cellular channels (forward and reverse). However, since the same frequency cannot be used in adjacent cells, the 416 duplex channels are a theoretical maximum (actual number of valid voice channels equals 312). AMPS uses the seven-cell frequency reuse method. Control channels are used to set up and clear calls as well as other control messages. Each band (25 MHz) contains 21 control channels. When a mobile station is not in session, it must monitor designated control channels. It tunes and locks into the strongest channel to receive system information. The forward control channel (FOCC) is a data stream from the base station to the mobile, and the reverse control channel (RECC) is from the mobile to the base station. Voice conversation is carried over the forward voice channel (FVC) and the reverse voice channel (RVC).

The identifiers used in AMPS are as follows:

• The mobile station's electronic serial number (ESN)

• The mobile operator's system identification (SID)

• The mobile station's mobile identification number (MIN)

The ESN for a mobile is a 32-bit number that uniquely identifies a mobile and is set up by the mobile manufacturer. System IDs (SIDs) are 15-bit binary numbers that are assigned to cellular systems. One of the uses of the SID is to determine a home network from a roaming network. The MIN is a 34-bit number that is derived from the mobile terminal's 10-digit telephone number.

The network utilizes the IS-41 protocol for mobility and authentication procedures. The MSC provides the capability for call processing, and the HLR and VLRs keep track of the mobile as it moves. The mobile terminal is responsible for updating its location as it moves in the cellular network.

Data services in AMPS are straightforward and analogous to dial-up networking. Because AMPS is an analog technology, it is possible to make use of standard modems directly with AMPS. Data rates are at a maximum of 14.4 Kbps irrespective of the modem protocol (v.90 or others).

3.3.2 D-AMPS

D-AMPS, or digital AMPS, is a hybrid air interface that uses both first-generation and second-generation technology. The D-AMPS specification is detailed in IS-54-B. The primary reason for introducing D-AMPS in the early 1990s in North America was to overcome some of the shortcomings of AMPS technology. The co-channel interference problem of AMPS limited its capacity significantly, and the 30-KHz channel assigned to each user is excess capacity on a per user basis. The hybrid nature of D-AMPS comes from the fact that second-generation TDMA technology is placed on AMPS traffic channels.

The AMPS channels are still used, but the content and formats of the 30-KHz channels are modified. The channels defined for D-AMPS are as follows:

• FOCC— Forward analog control channel; direction: base station (BS) to mobile station (MS) control channel

• FVC— Forward voice channel; direction: BS to MS voice channel

• FDTC— Forward digital traffic channel; direction: BS to MS digital user and control channel

• RECC— Reverse analog control channel; direction: MS to BS control channel

• RVC— Reverse analog voice channel; direction: MS to BS voice channel

• RDTC— Reverse digital traffic channel; direction: MS to BS digital user and control channel

The FDTC and RDTC can be split up into fast associated control channel (FACCH) and slow associated control channel (SACCH), which are used for signaling. One of the improvements that was made in the handoff process was the involvement of the mobile in the handoff procedure. Mobile assisted handoff was introduced in D-AMPS. The MS keeps measuring the quality of the forward channel and sends these measurements to the BS to allow the network to make a more informed decision.

First-generation AMPS and D-AMPS mobile networks continue to exist even today, especially in the United States. They complement coverage of second-generation digital networks such as GSM and IS-95. Most mobile terminals are dual mode (i.e., they incorporate a second-generation (2G) digital radio as well as the analog radio). With roaming agreements in place, 2G network operators can claim nationwide coverage. However, it is expected that the lifetime of these analog networks is coming to an end and will be decommisioned slowly in the next few years. One of the reasons for decommissioning these networks is to reclaim the spectrum for other uses.