Biblio5

220 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.13 BER performance for several modulation types.

rience that mountains move (i.e., map error), buildings grow, grain elevators appear where none were before, east of Madrid a whole high-rise community goes up, and so forth.

Another point from experience: If someone says “line-of-sight” conditions exist on a certain path, don’t believe it! Line of sight must be precisely defined. We reiterate that for each obstacle in the LOS microwave path, earth curvature with proper K-factor must be added to obstacle height, 0.6 of the first Fresnel zone must be added on top of that, and then 50 ft for trees and 10 ft more for growth must be added if in a vegetated area (to avoid foliage-loss penalties).7

7Often it is advisable to add 10 ft (or 3 m) of safety factor on top of the 0.6 first Fresnel zone clearance to avoid any diffraction-loss penalties.

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Much of the survey is to verify findings and conclusions of the path profile. Of course, each site must be visited to determine the location of the radio equipment shelter, the location of the tower, whether site improvement is required, the nearest prime power lines, and site access, among other items to be investigated.

Site/ path survey personnel must personally inspect the sites in question, walking/ driving the path or flying the path in a helicopter, or a combination thereof. The use of GPS receivers are helpful to verify geographical positions along the path, including altitudes.8

9.2.4 Fades, Fading, and Fade Margins

In Section 9.2.3.4.2 we showed how path loss can be calculated. This was a fixed loss that can be simulated in the laboratory with an attenuator. On very short radio paths below about 10 GHz, the signal level impinging on the distant-end receiving antenna, assuming full LOS conditions, can be calculated to less than 1 dB. If the transmitter continues to give the same output, the RSL will remain uniformly the same over long periods of time, for years. As the path is extended, the measured RSL will vary around a median. The signal level may remain at that median for minutes or hours, and then suddenly drop and then return to the median again. In other periods and/ or on other links, this level variation can be continuous for periods of time. Drops in level can be as much as 30 dB or more. This phenomenon is called fading. The system and link design must take fading into account when sizing or dimensioning the system/ link.

As the RSL drops in level, so does the Eb/ N0. As the Eb/ N0 decreases, there is a deterioration in error performance; the BER degrades. Fades vary in depth, duration, and frequency (i.e., number of fade events per unit of time). We cannot eliminate the fades, but we can mitigate their effects. The primary tool we have is to overbuild each link by increasing the margin.

Link margin is the number of dB we have as surplus in the link design. We could design an LOS microwave link so we just achieve the RSL at the distant receiver to satisfy the Eb/ N0 (and BER) requirements using free-space loss as the only factor in link attenuation (besides transmission line loss). Unfortunately we will only meet our specified requirements about 50% of the time. So we must add margin to compensate for the fading.

We have to determine what percentage of the time the link meets BER performance requirements. We call this time availability.9 If a link meets its performance requirements 99% of the time, then it does not meet performance requirements 1% of the time. We call this latter factor unavailability.

To improve time availability, we must increase the link margin, often called the fade margin. How many additional dB are necessary? There are several approaches to the calculation of a required fade margin. One of the simplest and most straightforward approaches is to assume that the fading follows a Rayleigh distribution, often considered worst-case fading. If we base our premise on a Rayleigh distribution, then the following fade margins can be used:

8GPS stands for geographical positioning system, a satellite navigation system that is extremely accurate. 9Other texts call this “reliability.” The use of this term should be deprecated because it is ambiguous and confusing. In our opinion, reliability should relate to equipment failure rate, not propagation performance.

222	CONCEPTS IN TRANSMISSION TRANSPORT
	TIME AVAILABILITY (%)		REQUIRED FADE MARGIN (dB)
	90		8
	99		18
	99.9		28
	99.99		38
	99.999		48

More often than not, LOS microwave systems consist of multiple hops. Here our primary interest is the time availability at the far-end receiver in the system after the signal has progressed across all of the hops. From this time availability value we will want to assign an availability value for each hop or link.

Suppose a system has nine hops and the system time availability specified is 99.95%, and we want to calculate the time availability per hop or link. The first step is to calculate the system time unavailability. This is simply 1.0000 − 0.9995 c 0.0005. We now divide this value by 9 (i.e., there are nine hops or links):

0.0005/ 9 c 0.0000555.

Now we convert this value to time availability:

Per-hop time availability c 1.0000000 − 0.0000555 c 0.9999445 or 99.99445%.

We recommend that for digital links, an additional 2 dB of fade margin be added to the Rayleigh values to compensate for path anomalies which could not be handled by automatic equalizers.

The most common cause of fading is called multipath. Refer to Figure 9.14. As the term implies, signal energy follows multiple paths from the transmit antenna to the receive antenna. Two additional paths, besides the main ray beam, are shown in Figure 9.14. Most of the time the delayed signal energy (from the reflected/ refracted paths) will be out of phase with the principal ray beam. These out-of-phase conditions are what cause fading. On digital links we also have to concern ourselves about dispersion. This is delayed signal energy caused by the multipath conditions. Of course, the delayed

Figure 9.14 Multipath is the most common cause of fading.

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energy arrives after the main ray beam pulse energy, spilling into the subsequent bit position, greatly increasing the probability that the bit position two will be in error. This is called intersymbol interference (ISI).

To achieve the fade margin we must overbuild the link, which will increase the RSL above that of the RSL if we just designed the link to meet the objectives of the unfaded conditions. We must add to the link margin that number of decibels indicated by the Raleigh fading table on the facing page, plus the recommended additional 2 dB.

Probably the most economic way to overbuild a link is to increase the antenna aperture. Every time we double the aperture (i.e., in this case, doubling the diameter of the parabolic dish), we increase the antenna gain 6 dB (see Eqs. (9.17a) and (9.17b)). We recommend the apertures for LOS microwave antennas not exceed 12 ft (3.7 m). Not only does the cost of the antenna get notably greater as aperture increases over 8 ft (2.5 m), but the equivalent sail area of the dish starts to have an impact on system design. Wind pressure on large dishes increases tower twist and sway, resulting in movement out of the capture area of the ray beam at the receive antenna. This forces us to stiffen the tower, which could dramatically increase system cost. Also, as antenna aperture increases, gain increases and beamwidth decreases.

Other measures we can take to overbuild a link are:

•Insert a low-noise amplifier (LNA) in front of the receiver-mixer. Improvement: 6–12 dB.

•Use an HPA (high-power amplifier). Usually a traveling-wave tube (TWT) amplifier; 10 W output. Improvement: 10 dB.

•Implement FEC (forward error correction). Improvement: 1–5 dB. Involves adding a printed circuit board at each end. It will affect link bandwidth (See Ref. 3 for description of FEC.)

•Implement some form of diversity. Space diversity is preferable in many countries. Can be a fairly expensive measure. Improvement: 5–20 dB or more. Diversity is described in Section 9.2.5.

It should be appreciated that fading varies with path length, frequency, climate, and terrain. The rougher the terrain, the more reflections are broken up. Flat terrain, and especially paths over water, tends to increase the incidence of fading. For example, in dry, windy, mountainous areas the multipath fading phenomenon may be nonexistent. In hot, humid coastal regions a very high incidence of fading may be expected.

9.2.5 Diversity and Hot-Standby

Diversity reception means the simultaneous reception of the same radio signal over two or more paths. Each “path” is handled by a separate receiver chain and then combined by predetection or postdetection combiners in the radio equipment so that effects of fading are mitigated. The separate diversity paths can be based on space, frequency, and/ or time diversity. The simplest and preferred form of diversity for LOS microwave is space diversity. Such a configuration is illustrated in Figure 9.15.

The two diversity paths in space diversity are derived at the receiver end from two separate receivers with a combined output. Each receiver is connected to its own antenna, separated vertically on the same tower. The separation distance should be at least 70 wavelengths and preferably 100 wavelengths. In theory, fading will not occur on both paths simultaneously.

224 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.15 A space diversity configuration. The vertical distance between the upper and lower antennas is of key importance.

Frequency diversity is more complex and more costly than space diversity. It has advantages as well as disadvantages. Frequency diversity requires two transmitters at the near end of the link. The transmitters are modulated simultaneously by the same signal but transmit on different frequencies. Frequency separation must be at least 2%, but 5% is preferable. Figure 9.16 is an example of a frequency-diversity configuration. The two diversity paths are derived in the frequency domain. When a fade occurs one on frequency, it will probably not occur on the other frequency. The more one frequency is separated from the other, the less chance there is that fades occur simultaneously on each path.

Frequency diversity is more expensive, but there is greater assurance of path reliability. It provides full and simple equipment redundancy and has the great operational advantage of two complete end-to-end electrical paths. In this case, failure of one transmitter or one receiver will not interrupt service, and a transmitter and/ or a receiver can be taken out of service for maintenance. The primary disadvantage of frequency diversity is that it doubles the amount of frequency spectrum required in this day and age when spectrum is at a premium. In many cases it is prohibited by national licensing authorities. For example, the FCC does not permit frequency diversity for industrial users. It also should be appreciated that it will be difficult to get the desired frequency spacing.

The full equipment redundancy aspect is very attractive to the system designer. Another approach to achieve diversity improvement in propagation plus reliability improvement by fully redundant equipment is to resort to the “hot-standby” technique. On the receive end of the path, a space-diversity configuration is used. On the transmit end a second transmitter is installed, as in Figure 9.16, but the second transmitter is on hot standby. This means that the second transmitter is on but its signal is not radiated by the antenna. On a one-for-one basis, the second transmitter is on the same frequency as the first transmitter. On the failure of transmitter 1, transmitter 2 is switched in automatically, usually with no dropout of service at all.

Figure 9.16 A frequency-diversity configuration.

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Table 9.3 LOS Microwave Frequency Bands

2110–2130 MHz	18,920–19,160 MHz
3700–4200 MHz	19,260–19,700 MHz
5925–6425 MHz	21,200–23,600 MHz
6525–6875 MHz	27,500–29,500 MHz
10,700–11,700 MHz	31,000–31,300 MHz
17,700–18,820 MHz	38,600–40,000 MHz

9.2.6 Frequency Planning and Frequency Assignment

9.2.6.1 Introduction. To derive optimum performance from an LOS microwave system, the design engineer must set out a frequency-usage plan that may or may not have to be approved by the national regulatory organization. The problem has many aspects. First, the useful RF spectrum is limited from above dc (0 Hz) to about 150 GHz. The upper limit is technology-restricted. To some extent it is also propagation-restricted. The frequency ranges of interest for this discussion cover the bands listed in Table 9.3. The frequencies above 10 GHz could also be called rainfall-restricted, because at about 10 GHz is where excess attenuation due to rainfall can become an important design factor.

Then there is the problem of frequency congestion. Around urban and built-up areas, frequency assignments below 10 GHz are difficult to obtain from national regulatory authorities. If we plan properly for excess rainfall attenuation, nearly equal performance is available at those higher frequencies.

9.2.6.2 Radio-Frequency Interference (RFI): There are three facets to RFI in this context: (1) own microwave can interfere with other LOS microwave and satellite communication earth stations nearby, (2) nearby LOS microwave and satellite communication facilities can interfere with own microwave, and (3) own microwave can interfere with itself. To avoid self-interference (3), it is advisable to use frequency plans of CCIR (ITU-R organization) as set forth in the RF Series (Fixed Service). Advantage is taken of proper frequency separation, transmit and receive, and polarization isolation. CCIR also provides methods for interference analysis (coordination contour), also in the RF series. Another alternative is specialist companies, which provide a service of electromagnetic compatibility analysis.

The IEEE defines electromagnetic compatibility (EMC) as “The requirements for electromagnetic emission and susceptibility dictated by the physical environment and regulatory governing bodies in whose jurisdiction a piece of equipment is operated.” We’ll call electromagnetic emission (EMI) RFI, meaning the level of RF interference caused by a certain piece of equipment such as a microwave terminal. Susceptibility deals with how well a piece of equipment can operate in an RFI environment. EMC can be a real headache for a microwave engineer.

9.3 SATELLITE COMMUNICATIONS

9.3.1 Introduction

Satellite communications is an extension of LOS microwave technology covered in Section 9.2. The satellite must be within line-of-sight of each participating earth terminal. We are more concerned about noise in satellite communication links than we were with LOS microwave. In most cases, received signals will be of a much lower level. On satellite systems operating below 10 GHz, very little link margin is required; there is

226 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.17 Simplified functional block diagram of a transponder of a typical communication satellite.

essentially no fading, as experienced with LOS microwave. The discussion here only deals with geostationary orbit (GEO) communication satellites.

Satellite communications presents another method of extending the digital network (Chapter 6). These digital trunks may be used as any other digital trunks for telephony, data, the Internet, facsimile, and video. However, fiber optics has become a strong competitor of satellite communications. Only very small aperture terminal (VSAT) systems are showing any real growth in the GEO arena. A new type of communication satellite is being fielded. This is the LEO class of satellites, which we discuss in Chapter 16.

Three quarters of the satellite transponders over North America are used to provide entertainment services such as direct broadcast television, cable system headend feeds, and for private broadcaster connectivity.

9.3.2 Satellite

Most of the commercial communication satellites that are currently employed are RF repeaters. A typical RF repeater used in a communication satellite is illustrated in Figure 9.17. The tendency is to call these types of satellite bent pipe as opposed to processing satellites. A processing satellite, as a minimum, demodulates and regenerates the received digital signal. It may also decode and recode (FEC) a digital bit stream.10 It also may have some bulk switching capability, switching to crosslinks connecting to other satellites. Theoretically, three GEO satellites placed correctly in equatorial orbit could provide connectivity from one earth station to any other located anywhere on the surface of the earth (see Figure 9.18). However, high-latitude service is marginal and nil north of 808N and south of 808S.

9.3.3 Three Basic Technical Problems

As the reader can appreciate, satellite communication is nothing more than LOS microwave using one (or two) satellites located at great distances from the terminal earth stations, as illustrated in Figure 9.18.11 Because of the distance involved, consider the slant range from the earth station to the satellite to be the same as the satellite altitude above the equator. This would be true if the antenna were pointing at zenith (08 ele-

10FEC, forward error correction. (See Ref. 3, Chapter 4.)

11For voice communications, connectivity is limited to only one GEO satellite link because of the delay involved.

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Figure 9.18 Distances involved in satellite communications. One is looking down at or up at the equator (i.e., the circle).

vation angle) to the satellite. Distance increases as the pointing angle to the satellite decreases (elevation angle).

We thus are dealing with very long distances. The time required to traverse these distances—namely, earth station to satellite to another earth station—is on the order of 250 ms. Round-trip delay will be 2 × 250 or 500 ms. These propagation times are much greater than those encountered on conventional terrestrial systems. So one major problem is propagation time and resulting echo on telephone circuits. It influences certain data circuits in delay to reply for block or packet transmission systems and requires careful selection of telephone signaling systems, or call-setup time may become excessive.

Naturally, there are far greater losses. For LOS microwave we encounter free-space losses possibly as high as 145 dB. In the case of a satellite with a range of 22,300 mi operating on 4.2 GHz, the free-space loss is 196 dB and at 6 GHz, 199 dB. At 14 GHz the loss is about 207 dB. This presents no insurmountable problem from earth to satellite, where comparatively high-power transmitters and very-high-gain antennas may be used. On the contrary, from satellite to earth the link is power-limited for two reasons: (1) in bands shared with terrestrial services such as the popular 4-GHz band to ensure noninterference with those services, and (2) in the satellite itself, which can derive power only from solar cells. It takes a great number of solar cells to produce the RF power necessary; thus the downlink, from satellite to earth, is critical, and received signal levels will be much lower than on comparative radiolinks, as low as −150 dBW. A third problem is crowding. The equatorial orbit is filling with geostationary satellites. Radio-frequency interference from one satellite system to another is increasing. This is particularly true for systems employing smaller antennas at earth stations with their inherent wider beamwidths. It all boils down to a frequency congestion of emitters.

It should be noted that by the year 2000, we can expect to see several low earth-orbit (LEO) satellite systems in operation. These satellites typically orbit some 500 km above the earth.

228 CONCEPTS IN TRANSMISSION TRANSPORT

9.3.4 Frequency Bands: Desirable and Available

The most desirable frequency bands for commercial satellite communication are in the spectrum 1000 –10,000 MHz. These bands are:

3700–4200 MHz (satellite-to-earth or downlink); 5925–6425 MHz (earth-to-satellite or uplink); 7250 –7750 MHz (downlink);12 and

7900–8400 MHz (uplink).12

These bands are preferred by design engineers for the following primary reasons:

•Less atmospheric absorption than higher frequencies;

•Rainfall loss not a concern;

•Less noise, both galactic and man-made;

•Well-developed technology; and

•Less free-space loss compared with the higher frequencies.

There are two factors contraindicating application of these bands and pushing for the use of higher frequencies:

1. The bands are shared with terrestrial services.

2. There is orbital crowding (discussed earlier).

Higher-frequency bands for commercial satellite service are:

10.95–11.2 GHz (downlink);

11.45–12.2 GHz (downlink);

14.0 –14.5 GHz (uplink);

17.7–20.2 GHz (downlink); and

27.5–30.0 GHz (uplink).

Above 10 GHz rainfall attenuation and scattering and other moisture and gaseous absorption must be taken into account. The satellite link must meet a BER of 1 × 10−6 at least 99.9% of the time. One solution is a space-diversity scheme where we can be fairly well assured that one of the two antenna installations will not be seriously affected by the heavy rainfall cell affecting the other installation. Antenna separations of 4 –10 km are being employed. Another advantage with the higher frequencies is that requirements for downlink interference are less; thus satellites may radiate more power. This is often carried out on the satellite using spot-beam antennas rather than generalcoverage antennas.

9.3.5 Multiple Access to a Communication Satellite

Multiple access is defined as the ability of a number of earth stations to interconnect their respective communication links through a common satellite. Satellite access is classified (1) by assignment, whether quasi-permanent or temporary, namely, (a) pre-

12These two bands are intended mainly for military application.

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Table 9.4 INTELSAT VI, VII, and VIII Regular FDM/ FM Carriers, FDMA Voice-Channel

Capacity versus Bandwidth Assignments (Partial Listing)

Carrier capacity (number of

voice channels)	24.0	60.0	96.0	132.0	252.0	432.0	792.0
Top baseband
frequency (kHz)	108.0	252.0	408.0	552.0	1052.0	1796.0	3284.0
Allocated satellite
bandwidth (MHz)	2.5	2.5	5.0	10.0	10.0	15.0	36.0
Occupied bandwidth
(MHz)	2.00	2.25	4.5	7.5	8.5	12.4	32.4

Source: Ref. 4.

assigned multiple access or (b) demand-assigned multiple access (DAMA); and (2) according to whether the assignment is in the frequency domain or the time domain, namely, (a) frequency-division multiple access (FDMA) or (b) time-division multiple access (TDMA). On comparatively heavy routes (≥10 erlangs), preassigned multiple access may become economical. Other factors, of course, must be considered, such as whether the earth station is “INTELSAT” standard as well as the space-segment charge that is levied for use of the satellite. In telephone terminology, “preassigned” means dedicated circuits. DAMA is useful for low-traffic multipoint routes where it becomes interesting from an economic standpoint. Also, an earth station may resort to DAMA as a remedy to overflow for its FDMA circuits.

9.3.5.1 Frequency Division Multiple Access (FDMA). Historically, FDMA has the highest usage and application of the various access techniques considered here. The several RF bands available (from Section 9.3.4) each have a 500-MHz bandwidth. A satellite contains a number of transponders, each of which covers a frequency segment of the 500-MHz bandwidth. One method of segmenting the 500 MHz is by utilizing 12 transponders, each with a 36-MHz bandwidth. Sophisticated satellites, such as INTELSAT VIII, segment the 500-MHz bandwidth available with transponders with 36, 72, and 77 MHz in the 6/ 4 GHz frequency band pair and 72, 77, and 112 MHz in the 14/ 12 GHz frequency band pair.13

With FDMA operation, each earth station is assigned a segment or a portion of a frequency segment. For a nominal 36-MHz transponder, 14 earth stations may access in an FDMA format, each with 24 voice channels (two groups) in a standard CCITT modulation plan (analog) (see Section 4.5.2.3). The INTELSAT VIII assignments for a 36-MHz transponder are shown in Table 9.4, where it can be seen that when larger channel groups are used, fewer earth stations can access the same transponder.

9.3.5.2 Time Division Multiple Access (TDMA). Time-division multiple access operates in the time domain and may only be used for digital network connectivity. Use of the satellite transponder is on a time-sharing basis. Individual time slots are assigned to earth stations in a sequential order. Each earth station has full and exclusive use of the transponder bandwidth during its time-assigned segment. Depending on the bandwidth of the transponder, bit rates of 10–100 Mbps (megabits per second) are used.

With TDMA operation, earth stations use digital modulation and transmit with bursts

136/ 4 GHz frequency band pair, meaning 5925–6425 MHz uplink and 3700–4200 MHz downlink.