Mobile radio communications dates back to Marconi. For a time it was the principal application of radio. This was ship-to-shore and ship-to-ship communications. The pioneer in this was the Marconi Company of the United Kingdom. It spread to land vehicles and aircraft in the 1920s. Since 1980, mobile radio communication has taken on a more personal flavor.
Cellular radio systems have extended the telephone network to the car, to the pedestrian, and even into the home and office. A new and widely used term in our vocabulary is personal communications. It is becoming the universal tether. No matter where we go, on land, at sea, and in the air, we can have near instantaneous
two-way communications by voice, data, and facsimile. At some time it will encompass video.
Personal radio terminals are becoming smaller. There is the potential of becoming wristwatch size. However, the human interface requires input–output devices that have optimum usefulness. A wristwatch-size keyboard or keypad is rather difficult to operate. A hard-copy printer requires some minimum practical dimensions,
and so forth. A new name has entered our vocabulary, wireless. The British have been using the term since Marconi. It is relatively new in North America, and with a different flavor in meaning. I think we can define wireless as a telecommunication method that does not require wires to communicate. From our perspective, wireless and radio are synonymous.
Reference 28 states that there were 1 billion (1 × 109) by the end of 2002;
and Reference 27 expects there to be 3 billion (3 × 109) traditional telephone and wireless users in the world by the year 2010. It is our opinion that this latter estimate may be on the low side.
1.2 Scope and Objective
This chapter presents an overview of “personal communications” or what many call wireless. Much of the discussion deals with cellular radio and wireless LANs (WLANs), and it extends this thinking inside of buildings. The coverage most necessarily includes propagation for the several environments, propagation
impairments, and methods to mitigate these impairments, access techniques, bandwidth limitations, and ways around this problem. It will cover several mobile radio standards and compare a number of existing and planned systems. The chapter objective is to provide an appreciation of mobile/personal communications.
Space limitations force us to confine our discussion to what might be loosely called “land mobile systems.”
2 BASIC CONCEPTS OF CELLULAR RADIO
Cellular radio systems connect a mobile terminal to another user, usually through the PSTN. The “other user” most commonly is a telephone subscriber of the PSTN. However, the other user may be another mobile terminal. Most of the connectivity is extending POTS to mobile users. Data and facsimile services are
in various stages of implementation (see Chapter 13, Section 6). Some of the terms used in this section have a strictly North American flavor. The heart of the system for a specific serving area is the mobile telephone switching office (MTSO). The MTSO is connected by a trunk group to a nearby telephone exchange providing an interface to, and connectivity with, the PSTN. The area to be served by a cellular geographic serving area (CGSA) is divided into small geographic cells which ideally are hexagonal. Cells are initially laid out with centers spaced about 4 to 8 miles (6.4 to 12.8 km) apart. The basic system components are the cell sites, the MTSO, and mobile units. These mobile units may be hand-held or vehicle-mounted terminals.
Each cell has a radio facility housed in a building or shelter. The facility’s radio equipment can connect and control any mobile unit within the cell’s responsible geographic area. Radio transmitters located at the cell site have a maximum effective radiated power (ERP∗) of 100 watts. Combiners are used to connect multiple transmitters to a common antenna on a radio tower, usually between 50 and 300 ft (15 and 92 m) high. Companion receivers use a separate antenna system mounted on the same tower. The receive antennas are often arranged in a space-diversity configuration.
The MTSO provides switching and control functions for a group of cell sites.
A method of connectivity is required between the MTSO and the cell site facilities.
The MTSO is an electronic switch and carries out a fairly complex group of processing functions to control communications to and from mobile units as they move between cells as well as to make connections with the PSTN. Besides making connectivity with the public network, the MTSO controls cell site activities
and mobile actions through command and control data channels. The connectivity between cell sites and the MTSO is often via DS1 on wire pairs or on microwave facilities, the latter being the most common.
A typical cellular mobile unit consists of a control unit, a radio transceiver, and an antenna. The control unit has a telephone handset, a push button keypad to enter commands into the cellular/telephone network, and audio and visual indications for customer alerting and call progress. The transceiver permits full
duplex transmission and reception between a mobile and cell sites. Its ERP is nominally 6 watts. The unit is usually vehicle-mounted. Hand-held terminals combine all functions into one small package that can be easily held in one hand. The ERP of a hand-held is a nominal 0.6 watts. It seems that this package
is being made smaller and smaller.
In North America, cellular communication is assigned a 25-MHz band between 824 and 849 MHz for mobile unit-to-base transmission and a similar band between 869 and 894 MHz for transmission from base to mobile. The original North American cellular radio systems was called AMPS (advanced mobile telephone
system). The original system description was contained in an entire issue
Bell System Technical Journal (BSTJ) of January 1979. The present AMPS is based on 30-kHz channel spacing using frequency modulation. The peak deviation is 12 kHz. The cellular bands are each split into two to permit competition.
Thus only 12.5 MHz is allocated to one cellular operator for each direction of transmission. With 30-kHz spacing, this yields 416 channels. However, nominally 21 channels are used for control purposes, with the remaining 395 channel available for cellular end-users.
Common practice with AMPS is to assign 10 to 50 channel frequencies to each cell for mobile traffic. Of course, the number of frequencies used depends on the expected traffic load and the blocking probability. Radiated power from a cell site is kept at a relatively low level with just enough antenna height to
cover the cell area. This permits frequency reuse of these same channels in nonadjacent cells in the same CGSA with little or no co-channel interference. A well-coordinated frequency reuse plan enables tens of thousands of simultaneous calls over a CGSA. Here four channel frequency groups are assigned in a way that avoids the same frequency set used in adjacent cells. If there were uniform terrain contours, this plan could be applied directly.
However, real terrain conditions dictate further geographic separation of cells that use the same frequency set. Reuse plans with 7 or 12 sets of channel frequencies provide more physical separation and are often used depending on the shape of the antenna pattern employed.
With user growth in a particular CGSA, cells may become overloaded. This means that grade of service objectives are not being met due to higher than planned traffic levels during the busy hour (BH). In these cases, congested cells can be subdivided into small cells, each with its own base station,
These smaller cells use lower transmitter power and antennas with less height, thus permitting greater frequency reuse.
An important issue in wireless communication systems is multiple random access: communication links can be activated at any moment while several links can be active simultaneously. As multi-access and random-access are properties mainly determined by the chosen data-communication technique it is important to keep these requirements in mind from the very beginning. Three possible concepts to realize a multi-access communication system are in use:
Frequency Division Multiple Access, commonly used in conventional telephone systems: every user gets a certain frequency band assigned and can use this part of the spectrum to perform its communication. If only a small number of users is active, not the whole resource (frequency-spectrum) is used. Assignment of the channels can be done centrally or by carrier-sensing in a mobile. The latter possibility enables random-access.
Time Division Multiple Access, applied nowadays in mobile phone systems: every user is assigned a (set of) time-slots. Transmission of data is only possible during this time-slot, after that the transmitter has to wait until it gets another time-slot. Synchronization of all users is an important issue in this concept. Consequently, there must be a central unit (base-station) that controls the synchronization and the assignment of time-slots. This means that this technique is difficult to apply in random-access systems.
Code Division Multiple Access (Spread Spectrum). A unique code is assigned to each user. This code is used to ``code'' the data message. As codes are selected for low cross-correlation properties, all users can transmit simultaneously in the same frequency channel while a receiver is still capable of recovering the desired signal. Synchronization between links is not strictly required and so random-access is possible. A practical application at the moment is the cellular-cdma phone system IS-95 [Qua92].
From the above list it is clear that both fdmaand cdma are candidate transmission techniques to enable multiple random access. There are however a number of reasons for choosing cdma over fdma. The first alternative provides [Sch94, SOSL85a, Dix84]:
- Interference limited operation. In all situations the whole frequency-spectrum is used. As a result the more active users are present, the higher the interference level will be.
- Privacy due to unknown codes. The applied codes are - in principle - unknown to a hostile user. This means that it is hardly possible to detect the message of another user.
- Applying spread spectrum implies the reduction of multi-path effects. By using a wide frequency-band, the influence of narrow-band fades is reduced.
- Random access possibilities. Users can start their transmission at any arbitrary time (no infrastructure required).
- Good anti-jamming performance. Small-band interference is reduced as explained in the next section.