Biblio5

494 CELLULAR AND PCS RADIO SYSTEMS

If we are spreading a voice channel over a very wide frequency band, it would seem that we are defeating the purpose of frequency conservation. With spread spectrum, with its powerful antijam properties, multiple users can transmit on the same frequency with only some minimal interference one to another. This assumes that each user is employing a different key variable (i.e., in essence, using a different time code). At the receiver, the CDMA signals are separated using a correlator that accepts only signal energy from the selected key variable binary sequence (code) used at the transmitter, and then despreads its spectrum. CDMA signals with unmatching codes are not despread and only contribute to the random noise.

CDMA reportedly provides an increase in capacity 15-times that of its analog FM counterpart. It can handle any digital format at the specified input bit rate such as facsimile, data, and paging. In addition, the amount of transmitter power required to overcome interference is comparatively low when utilizing CDMA. This translates into savings on infrastructure (cell site) equipment and longer battery life for hand-held terminals. CDMA also provides so-called soft handoffs from cell site to cell site that make the transition virtually inaudible to the user (Ref. 13).

Dixon (Ref. 15) lists some advantages of the spread spectrum:

1. Selective addressing capability;

2. Code division multiplexing is possible for multiple access;

3. Low-density power spectrum for signal hiding;

4. Message security; an

5. Interference rejection.

Of most importance for the cellular user (Ref. 14), “when codes are properly chosen for low cross correlation, minimum interference occurs between users, and receivers set to use different codes are reached only by transmitters sending the correct code. Thus more than one signal can be unambiguously transmitted at the same frequency and at the same time; selective addressing and code-division multiplexing are implemented by the coded modulation format.”

Processing gain is probably the most commonly used parameter to describe the performance of a spread spectrum system. It quantifies the signal-to-noise ratio improvement when a spread signal is passed through the appropriate processor. For instance, a certain spread spectrum processor has an input S/ N of 12 dB and an output S/ N of 20 dB, then its processing gain is 8 dB. Processing gain is expressed by the following:

More commonly, processing gain is given in a dB value; then

Example. A certain cellular system voice channel information rate is 9.6 kbps and the RF spread bandwidth is 9.6 MHz. What is the processing gain?

Figure 16.13 A typical RAKE receiver used with direct sequence spread spectrum reception.

delay line tap has the same sign (phase shift) as the chips in the sequence, we have a match. This is illustrated in Figure 16.12b. As shown here, the short sequence of seven chips is enhanced with the desired signal seven times. This is the output of the modulo-2 adder, which has an output voltage seven times greater than the input voltage of one chip.

In Figure 16.12c we show the correlation process collapsing the spread signal spectrum to that of the original bit spectrum when the receiver reference signal, based on the same key as the transmitter, is synchronized with the arriving signal at the receiver. Of overriding importance is that only the desired signal passes through the matched filter delay line (adder). Other users on the same frequency have a different key and do not correlate. These “other” signals are rejected. Likewise, interference from other sources is spread; there is no correlation and those signals also are rejected.

Direct sequence spread spectrum offers two other major advantages for the system designer. It is more forgiving in a multipath environment than conventional narrowband systems, and no intersymbol interference (ISI) will be generated if the coherent bandwidth is greater than the information symbol bandwidth.

If we use a RAKE receiver, which optimally combines the multipath components as part of the decision process, we do not lose the dispersed multipath energy. Rather, the RAKE receiver turns it into useful energy to help in the decision process in conjunction with an appropriate combiner. Some texts call this implicit diversity or time diversity.

When sufficient spread bandwidth is provided (i.e., where the spread bandwidth is greater or much greater than the correlation bandwidth), we can get two or more independent frequency diversity paths by using a RAKE receiver with an appropriate combiner such as a maximal ratio combiner. Figure 16.13 is a block diagram of a RAKE receiver.

16.7 FREQUENCY REUSE

Because of the limited bandwidth allocated in the 800-MHz band for cellular radio communications, frequency reuse is crucial for its successful operation. A certain level of interference has to be tolerated. The major source of interference is cochannel interference from a “nearby” cell using the same frequency group as the cell of interest. For the 30-kHz bandwidth AMPS system, Ref. 6 suggests that C/ I be at least 18 dB. The pri-

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Figure 16.14 Definitions of R and D.

mary isolation derives from the distance between the two cells with the same frequency group. In Figure 16.2 there is only one cell diameter for interference protection.

Refer to Figure 16.14 for the definition of R and D. D is the distance between cell centers of repeating frequency groups and R is the “radius” of a cell. We let:

The D/ R ratio is a basic frequency reuse planning parameter. If we keep the D/ R ratio large enough, cochannel interference can be kept to an acceptable level. Lee (Ref. 8)

calls a the cochannel reduction factor and relates path loss from the interference source to R−4.

A typical cell in question has six cochannel interferers, one on each side of the hexagon. So there are six equidistant cochannel interference sources. The goal is C/ I ≥ 18 dB or a numeric of 63.1. So

Then

a c 4.4.

This means that D must be 4.4 times the value of R. If R is 6 mi (9.6 km) then D c 4.4 × 6 c 26.4 mi (42.25 km).

Lee (Ref. 8) reports that cochannel interference can be reduced by other means such as directional antennas, tilted beam antennas, lowered antenna height, and an appropriately selected site.

One way we can protect a cell that is using the same frequency family as a nearby cell is by keeping that cell base station below line-of-sight of the nearby cell. In other words, we are making our own shadow conditions. Consider a 26.4-mi path. What is the height of earth curvature midpath? From Section 9.2.3.3, h c 0.667(d/ 2)2/ 1.33 c 87.3 ft (26.9 m). Providing the cellular base station antennas are kept under 87 ft, the 40-dB/ decade rule of Lee holds. It holds so long as we are below line-of-sight conditions.

The total available (one-way) bandwidth is split up into N sets of channel groups. The channels are then allocated to cells, one channel set per cell on a regular pattern, which repeats to fill the number of cells required. As N increases, the distance between channel sets (D) increases, reducing the level of interference. As the number of channel sets (N ) increases, the number of channels per cell decreases, reducing the system capacity. Selecting the optimum number of channel sets is a compromise between capacity and quality. Note that only certain values of N lead to regular repeat patterns without gaps. These are N c 3, 4, 7, 9, and 12, and then multiples thereof.

Figure 16.15 A cell layout based on N c 7.

Figure 16.15 shows a repeating 7 pattern for frequency reuse. This means that N c 7 or there are 7 different frequency sets (or families) for cell assignment. Cell splitting will take place, especially in urban areas, in some point in time because the present cell structure cannot support the busy hour traffic load. Cell splitting, in effect, provides more frequency slots for a given area and relieves the congestion problem. Macario (Ref. 11) reports that cells can be split as far down as 1 km in radius.

Cochannel interference tends to increase with cell splitting. Cell sectorization can reduce the interference level. Figure 16.16 shows a threeand a six-sector plan. Sectorization breaks a cell into three or six parts each with a directional antenna. With a standard cell (using an omnidirectional antenna), cochannel interference enters from six directions. A six-sector plan can essentially reduce the interference to just one direction. A separate channel frequency set is allocated to each sector.

The three-sector plan is often used with a seven-cell repeating pattern (Figure 16.15) resulting in an overall requirement for 21 channel sets. The six-sector plan with its improved cochannel performance and rejection of secondary interferers allows a fourcell repeat plan (Figure 16.2) to be employed. This results in an overall 24-channel set requirement. Sectorization entails a larger number of channel sets and fewer channels per sector. Outwardly it appears that there is less capacity with this approach; however, the ability to use much smaller cells results in a higher capacity operation.

16.8 PERSONAL COMMUNICATIONS SERVICES (PCS)

16.8.1 Defining Personal Communications

Personal communications services (PCS) are wireless. This simply means that they are radio based. The user requires no tether. The conventional telephone is connected by a wire pair through to the local serving switch. The wire pair is a tether. We can only walk as far with that telephone handset as the “tether” allows.

Both of the systems we have dealt with in the previous sections of this chapter can be classified as PCS. Cellular radio, particularly with the hand-held terminal, gives the user tetherless telephone communication. Paging systems provided the mobile/ ambulatory user a means of being alerted that someone wishes to talk to that person on the telephone or of receiving a short message. The cordless telephone is certainly another example that has extremely wide use around the world with more than 200 million sets. We provide a brief review of cordless telephone sets in the following.

New applications are either on the horizon or going through field tests (1998). One that seems to offer great promise in the office environment is the wireless PABX. It

500 CELLULAR AND PCS RADIO SYSTEMS

Figure 16.16 Breaking up a cell into three sectors (left) and six sectors (right).

will almost eliminate the telecommunication manager’s responsibilities with office rearrangements. Another is the wireless LAN (WLAN).

Developments are expected such that PCS cannot only provide voice communications but facsimile, data, messaging, and possibly video. GSM provides all but video. Cellular digital packet data (CDPD) will permit data services over the cellular system in North America.

Donald Cox (Ref. 17) breaks PCS down into what he calls “high tier” and “low tier.” Cellular radio systems are regarded as high-tier PCS, particularly when implemented in the new 1.9-GHz PCS frequency band. Cordless telephones are classified as low tier. Table 16.1 summarizes some of the more prevalent PCS technologies.

16.8.2 Narrowband Microcell Propagation at PCS Distances

The microcells discussed here have a radial range of ≤1 km. One phenomenon is the Fresnel break point, which is illustrated in Figure 16.17. This figure illustrates that signal level varies with distance R as A/ Rn, where R is the distance to the receiver. For distances greater than 1 km, n is typically 3.5 to 4. The parameter A describes the effects of environmental features in a highly averaged manner (Ref. 18).

Typical PCS radio paths can be of an LOS nature, particularly near the fixed transmitter where n c 2. Such paths may be down the street from the transmitter. The other types of paths are shadowed paths. One type of shadowed path is found in highly urbanized settings, where the signal may be reflected off high-rise buildings (see Figure 16.4). Another is found in more suburban areas, where buildings are often just two stories high.

When a signal at 800 MHz is plotted versus R on a logarithmic scale, as in Figure 16.17, there are distinctly different slopes before and after the Fresnel break point. We call the break distance (from the transmit antenna) RB. This is the point for which the Fresnel ellipse about the direct ray just touches the ground. Such a model is illustrated in Figure 16.18. The distance RB is approximated by:

For R < RB, n is less than 2, and for R > RB, n approaches 4.

It was found that on non-LOS paths in an urban environment with low base station antennas and with users at street level, propagation takes place down streets and around corners rather than over buildings. For these non-LOS paths the signal must turn corners by multiple reflections and diffraction at vertical edges of buildings. Field tests reveal that signal level decreases by about 20 dB when turning a corner.

In the case of propagation inside buildings where the transmitter and receiver are on the same floor, the key factor is the clearance height between the average tops of furniture and the ceiling.

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Figure 16.17 Signal variation on a line-of-sight path in a rural environment. (From Ref. 18, Figure 3. Reprinted with permission.)

Bertoni et al. (Ref. 18) call this clearance W. Here building construction consists of drop ceilings of acoustical material supported by metal frames. That space between the drop ceiling and the floor above contains light fixtures, ventilation ducts, pipes, support beams, and so on. Because the acoustical material has a low dielectric constant, the rays incident on the ceiling penetrate the material and are strongly scattered by the irregular structure, rather than undergoing specular reflection. Floor-mounted building furnishings such as desks, cubicle partitions, filing cabinets, and workbenches scatter the rays and prevent them from reaching the floor, except in hallways. Thus it is concluded that propagation takes place in the clear space, W.

Figure 16.19 shows a model of a typical floor layout in an office building. When both the transmitter and receiver are located in the clear space, path loss can be related to the Fresnel ellipse. If the Fresnel ellipse associated with the path lies entirely in the clear space, the path loss has LOS properties (1/ L2). Now as the separation between the trans-

Figure 16.18 Direct and ground-reflected rays, showing the Fresnel ellipse about the direct ray. (From Ref. 18, Figure 18. Reprinted with permission.)

Figure 16.19 Fresnel zone for propagation between transmitter and receiver in clear space between building furnishings and ceiling fixtures. (From Ref. 18, Figure 35. Reprinted with permission.)

mitter and receiver increases, the Fresnel ellipse grows in size so that scatterers lie within it. This is shown in Figure 16.20. Now the path loss become greater than free space.

Bertoni et al. report one measurement program where the scatterers have been simulated using absorbing screens. It was recognized that path loss will be highly dependent on nearby scattering objects. Figure 16.20 was developed from this program. The path loss in excess of free space calculated at 900 MHz and 1800 MHz where W c 1.5 m is plotted in Figure 16.20 as a function of path length L. The figure shows that the excess path loss (over LOS) is small at each frequency out to distances of about 20 m to 40 m, respectively, where it increases dramatically.

Propagation between floors of a modern office building can be very complex. If the floors are constructed of reinforced concrete or prefabricated concrete, transmission loss can be 10 dB or more. Floors constructed of concrete poured over steel panels show much greater loss. In this case (Ref. 18), signals may propagate over other paths involving diffraction rather than transmission through the floors. For instance, signals can exit the building through windows and reenter on higher floors by diffraction mechanisms along the face of the building.

Figure 16.20 Measured and calculated excess path loss at 900 MHz and 1800 MHz for a large office building having head-high cubical partitions, but no floor-to-ceiling partitions. (From Ref. 18, Figure 36. Reprinted with permission.)