Biblio5

Figure 15.23 IEEE 802.14 upstream channel model. (From Ref. 7.)

the ranging adjustment indicates that the ranging offset at that station is to be decreased, resulting in later times of transmission at the station. The station implements the ranging adjustment with a resolution of at most 1 symbol duration for the symbol rate in use for the given burst. In addition, the accuracy of the station burst transmission timing is TBD ± TBD symbol, relative to the minislot boundaries that are derived at the station based on ideal processing of time-stamp message signals received from the headend.

15.7.7.5 Modulation and Bit Rates. QPSK and 16-QAM are the modulation choices for upstream transmission. Table 15.2 tabulates the upstream data rates and modulation rates. Figure 15.23 is the upstream channel model. The reader should note the numerous upstream channel impairments illustrated in the model.

REVIEW EXERCISES

1. Define a CATV headend. What are its functions?

2. List at least three impairments we can expect from a broadband CATV amplifier (downstream).

3. A signal splitter divides a signal in half, splitting into two equal power levels if the input to a 3-dB splitter were −7 dBm (in the power domain) and the output on each leg would be −10 dBm. Is this a true statement? What is missing here?

4. What was/ is the purpose of LOS microwave at a CATV headend?

5. What is the purpose of a set-top converter?

6. What does the term beat mean in CATV parlance?

7. Define composite triple beat.

REFERENCES

1. W. O. Grant, Cable Television, 3rd ed., GWG Associates, Schoharie, NY, 1994.

2. K. Simons, Technical Handbook for CATV Systems, 3rd ed., Jerrold Electronics Corp., Hatboro, PA, 1968.

3. E. R. Bartlett, Cable Television Technology and Operations, McGraw-Hill, New York, 1990.

4. D. N. Carson, in “CATV Amplifiers: Figure of Merit and the Coefficient System,” in 1966

REFERENCES 475

IEEE International Convention Record, Part I, Wire and Data Communications, pp. 87–97, IEEE, New York, 1966.

5. Electrical Performance for Television Transmission Systems, EIA/ TIA-250C, Telecommunication Industry Association, Washington, DC, 1990.

6. Lightwave Buyers’ Guide Issue, Pennwell Publishing Co., Tulsa, OK, 1997.

7. Multimedia Modem Protocol for Hybrid Fiber-Coax Metropolitan Area Networks, IEEE Std. 802.14, Draft R2, IEEE, New York, 1997.

8. Private communication, Robert Fuller, Chairman, IEEE 802.14 Committee, April 4, 1997. 9. Digital Multi-Programme Systems for Television, Sound and Data Services for Cable Dis-

tribution, ITU-T Rec. J.83, ITU, Geneva, Sept. 1995.

10. The IEEE Standard Dictionary of Electrical and Electronic Terms, 6th ed., IEEE, New York, 1996.

478 CELLULAR AND PCS RADIO SYSTEMS

lying cellular system for the United States and in some Latin-American countries. It uses FM radio, allocating 30 kHz per voice channel.

AMPS set the scene for explosive cellular radio growth and usage. What set AMPS apart from previous mobile radio systems is that it was designed to interface with the PSTN. It was based on an organized scheme of adjoining cells and had a unique capability of handoff when a vehicle moves through one cell to another, or when another cell receives a higher signal level from the vehicle, it will then take over the call. Vehicles can roam from one service area to another with appropriate handoffs.

In the late 1980s there was pressure to convert cellular radio from the bandwidthwasteful AMPS to some sort of digital regime. As the reader reviews this chapter, it will be seen that digital is also bandwidth-wasteful, even more so than analog FM. Various ways are described on how to remedy the situation: first by reducing the bandwidth of a digital voice channel, and second by the access/ modulation scheme proposed. Of this latter proposal, two schemes are on the table in North America: a TDMA scheme and a CDMA scheme. They are radically different and competing.

Meanwhile, the Europeans critiqued our approaches and came up with a better mousetrap. It is called Ground System Mobile (GSM) (from the French), and there is some pressure that it be adopted in the United States. GSM is a TDMA scheme, fairly different from the U.S. TIA standard (IS-54C).

As mentioned earlier, PCS is an outgrowth of cellular radio; it uses a cellular concept. The cells, however, are much smaller, under 1-km diameter. RF power is much lower. As with cellular radio, TDMA and CDMA are vying for the national access/ modulation method. Unlike the North American popular press, which discriminates between PCS and cellular radio, ITU-R takes a more mature and reasonable view of the affair by placing the two in the same arena. Earlier, CCIR/ ITU-R called their conceptual PCS future public land mobile telecommunication system (FPLMTS). The name has now changed to UMTS (universal mobile telecommunication system). The FPLMTS/ UMTS concept breaks down into three terrestrial operational areas: (1) indoor environments (range to 100 m), (2) outdoor environments (100 m to 35 km) for more rural settings, and (3) an intermediate region called outdoor-to-indoor environments, where building penetration is a major theme. They also describe satellite environments.

16.1.2 Scope and Objective

This chapter presents an overview of mobile and personal communications. Much of the discussion deals with cellular radio and extends this thinking inside buildings. The coverage most necessarily includes propagation for the several environments, propagation impairments, methods to mitigate those 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 the discussion to what might loosely be called “land mobile systems.”

16.2 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 “plain old telephone service” (POTS) to mobile users. Data and facsimile services

Figure 16.1 Conceptual layout of a cellular radio system.

are in various stages of implementation. Some of the terms used in this section have a strictly North American flavor.

Figure 16.1 illustrates a conceptual layout of a cellular radio system. The heart of the system for a specific serving area is the 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.1 Cells are initially laid out with centers spaced about 4 –8 m (6.4 –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 W.2 Combiners are used to connect multiple transmitters to a common antenna on a radio tower, usually between 50-ft and 300-ft (15-m 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

1CGSA is a term coined by the U.S. FCC. We do not believe it is used in other countries.

2Care must be taken with terminology. In this instance ERP and EIRP are not the same. The reference antenna in this case is the dipole, which has a 2.15-dBi gain.

480 CELLULAR AND PCS RADIO SYSTEMS

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 W. Hand-held terminals combine all functions into one small package that can easily be held in one hand. The ERP of a hand-held is a nominal 0.6 W.

In North America, cellular communication is assigned a 25-MHz band between 824 MHz and 849 MHz for mobile unit-to-base transmission and a similar band between 869 MHz and 894 MHz for transmission from base to mobile.

The first and most widely implemented North American cellular radio system was called AMPS (advanced mobile phone system). The original system description was contained in an entire issue of the 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 channels available for cellular end-users.

Common practice with AMPS is to assign 10 –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 cochannel interference. A well-coordinated frequency reuse plan enables tens of thousands of simultaneous calls over a CGSA.

Figure 16.2 illustrates one frequency reuse method. Here four channel frequency groups are assigned in a way that avoids the same frequency set 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; see Section 4.2.1). In these cases, congested cells can be subdivided into smaller cells, each with its own base station, as shown in Figure 16.3. With smaller cells, lower transmitter power and antennas with less height are used, thus permitting greater frequency reuse. These subdivided cells can be split still further for even greater frequency reuse. However, there is a practical limit to cell splitting, often with cells with a 1-mi (1.6-km) radius.

Radio system design for cellular operation differs from that used for LOS microwave operation. For one thing, mobility enters the picture. Path characteristics are constantly changing. Mobile units experience multipath scattering, reflection, and/ or diffraction by

482 CELLULAR AND PCS RADIO SYSTEMS

MTSO to each cell site. These data links transmit information for processing calls and for controlling mobile units. In addition to its “traffic” radio equipment, each cell site has installed signaling equipment, monitoring equipment, and a setup radio to establish calls.

When a mobile unit becomes operational, it automatically selects the setup channel with the highest signal level. It then monitors that setup channel for incoming calls destined for it. When an incoming call is sensed, the mobile terminal in question again samples signal levels of all appropriate setup channels so it can respond through the cell site offering the highest signal level, and then tunes to that channel for response. The responsible MTSO assigns a vacant voice channel to the cell in question, which relays this information via the setup channel to the mobile terminal. The mobile terminal subscriber is then alerted that there is an incoming call. Outgoing calls from mobile terminals are handled in a similar manner.

While a call is in progress, the serving cell site examines the mobile’s signal level every few seconds. If the signal level drops below a prescribed level, the system seeks another cell to handle the call. When a more appropriate cell site is found, the MTSO sends a command, relayed by the old cell site, to change frequency for communication with the new cell site. At the same time, the landline subscriber is connected to the new cell site via the MTSO. The periodic monitoring of operating mobile units is known as locating, and the act of changing channels is called handover. Of course, the functions of locating and handover are to provide subscribers satisfactory service as a mobile unit traverses from cell to cell. When cells are made smaller, handovers are more frequent.

The management and control functions of a cellular system are quite complex. Handover and locating are managed by signaling and supervision techniques, which take place on the setup channel. The setup channel uses a 10-kbps data stream that transmits paging, voice channel designation, and overhead messages to mobile units. In turn, the mobile unit returns page responses, origination messages, and order confirmations.

Both digital messages and continuous supervision tones are transmitted on the voice radio channel. The digital messages are sent as a discontinuous “blank-and-burst” inband data stream at 10 kbps and include order and handover messages. The mobile unit returns confirmation and messages that contain dialed digits. Continuous positive supervision is provided by an out-of-band 6-kHz tone, which is modulated onto the carrier along with the speech transmission.

Roaming is a term used for a mobile unit that travels such distances that the route covers more than one cellular organization or company. The cellular industry is moving toward technical and tariffing standardization so that a cellular unit can operate anywhere in the United States, Canada, and Mexico.

16.3 RADIO PROPAGATION IN THE MOBILE ENVIRONMENT

16.3.1 Propagation Problem

Line-of-sight microwave and satellite communications covered in Chapter 9 dealt with fixed systems. Such systems are optimized. They are built up and away from obstacles. Sites are selected for best propagation.

This is not so with mobile systems. Motion and a third dimension are additional variables. The end-user terminal often is in motion; or the user is temporarily fixed, but that point can be anywhere within a serving area of interest. Whereas before we dealt with point-to-point, here we deal with point-to-multipoint.

One goal in line-of-sight microwave design was to stretch the distance as much as possible between repeaters by using high towers. In this chapter there are some overriding circumstances where we try to limit coverage extension by reducing tower heights, what we briefly introduced in Section 16.2. Even more important, coverage is area coverage, where shadowing is frequently encountered, Examples are valleys, along streets with high buildings on either side, verdure such as trees, and inside buildings, to name a few typical situations. There are two notable results. Transmission loss increases notably and such an environment is rich with multipath scenarios. Paths can be highly dispersive, as much as 10 ms of delay spread (Ref. 3). If a user is in motion, Doppler shift can be expected.

The radio-frequency bands of interest are UHF (ultra high frequency, the frequency band from 300 –3000 MHz), especially around 800 MHz and 900 MHz, and 1700 MHz to 2000 MHz. In certain parts of the world, there is usage in the 400-MHz band.

16.3.2 Propagation Models

We concentrate on cellular operation. There is a fixed station (FS) and mobile stations (MSs) moving through the cell. A cell is the area of responsibility of the fixed station, a cell site. It usually is pictured as a hexagon in shape, although its propagation profile is more like a circle with the fixed station in its center. Cell radii vary from 1 km (0.6 mi) in heavily built-up urban areas to 30 km (19 mi) or somewhat more in rural areas.

16.3.2.1 Path Loss or Transmission Loss. We recall the free space loss (FSL) formula in Section 9.2.3. It simply stated that FSL was a function of the square of the distance and the square of the frequency plus a constant. It is a very useful formula if the strict rules of obstacle clearance are obeyed. Unfortunately, in the cellular situation, it is impossible to obey these rules. Then to what extent must this free space loss formula be modified by the proximity of the earth, the effects of trees, buildings, and hills in, or close to, the transmission path?

There have been a number of models that have been developed that are used as a basis for the calculation of transmission loss, several assumptions are made:

•That we will always use the same frequency band, often 800 MHz or 900 MHz. Thus it is common to drop the frequency term (the 20 logF term) in the FSL formula and include a constant that covers the frequency term. If we wish to use the model for another band, say, 1800 MHz, a scaling factor is added.

•That we will add a term to compensate for the usual great variance between the cell site antenna height when compared with the mobile (or hand-held) antenna height. We often call this the height-gain function, and it tends to give us an advantage. It

is often expressed as −20 log(hThR) where HT is the height of the transmit antenna (cell site) and HR is the height of the receive antenna (on the mobile platform). These are comparative heights. Commonly, the mobile platform antenna height is taken as 6 ft or 3 m.

•That there is a catch-all term for the remainder of the losses, which in some references is expressed as b (in dB);

•That at least three models express the free space loss as just 40 logdm (d is distance in meters).