Biblio5

230 CONCEPTS IN TRANSMISSION TRANSPORT

Figure 9.19 Example of TDMA burst frame format.

of information. The duration of a burst lasts for the time period of the slot assigned. Timing synchronization is a major problem.

A frame, in digital format, may be defined as a repeating cycle of events. It occurs in a time period containing a single digital burst from each accessing earth station and the guard periods or guard times between each burst. A sample frame is shown in Figure 9.19 for earth stations 1, 2, and 3 to earth station N. Typical frame periods are 750 msec for INTELSAT and 250 msec for the Canadian Telesat.

The reader should appreciate that timing is crucial to effective TDMA operation. The greater N becomes (i.e., the more stations operating in the frame period), the more clock timing affects the system. The secret lies in the carrier and clock timing recovery pattern, as shown in Figure 9.19. One way to ensure that all stations synchronize to a master clock is to place a synchronization burst as the first element in the frame format. INTELSAT’s TDMA does just this. The burst carries 44 bits, starting with 30 bits carrier and bit timing recovery, 10 bits for the unique word, and 4 bits for the station identification code in its header.

Why use TDMA in the first place? It lies in a major detraction of FDMA. Satellites use traveling-wave tubes (TWTs) in their transmitter final amplifiers. A TWT has the undesirable property of nonlinearity in its input–output characteristics. When there is more than one carrier accessing the transponder simultaneously, high levels of intermodulation (IM) products are produced, thus increasing noise and crosstalk. When a transponder is operated at full power output, such noise can be excessive and intolerable. Thus input must be backed off (i.e., level reduced) by ≥3 dB. This, of course, reduces the EIRP and results in reduced efficiency and reduced information capacity. Consequently, each earth station’s uplink power must be carefully coordinated to ensure proper loading of the satellite. The complexity of the problem increases when a large number of earth stations access a transponder, each with varying traffic loads.

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On the other hand, TDMA allows the transponder’s TWT to operate at full power because only one earth-station carrier is providing input to the satellite transponder at any one instant.

To summarize, consider the following advantages and disadvantages of FDMA and TDMA. The major advantages of FDMA are as follows:

• No network timing is required; and

•Channel assignment is simple and straightforward. The major disadvantages of FDMA are as follows:

•Uplink power levels must be closely coordinated to obtain efficient use of transponder RF output power.

•Intermodulation difficulties require power back-off as the number of RF carriers increases with inherent loss of efficiency.

The major advantages of TDMA are as follows:

•There is no power sharing and IM product problems do not occur.

•The system is flexible with respect to user differences in uplink EIRP and data rates.

•Accesses can be reconfigured for traffic load in almost real time.

The major disadvantages of TDMA are as follows:

•Accurate network timing is required.

•There is some loss of throughput due to guard times and preambles.

•Large buffer storage may be required if frame lengths are long.

9.3.5.3 Demand-Assigned Multiple Access (DAMA). The DAMA access method has a pool of single voice channel available for assignment to an earth station on demand. When a call has been completed on the channel, the channel is returned to the idle pool for reassignment. The DAMA system is a subset of FDMA, where each voice channel is assigned its own frequency slot, from 30 kHz to 45 kHz wide.

The DAMA access method is useful at earth stations that have traffic relations of only several erlangs. DAMA may also be used for overflow traffic. It operates something like a telephone switch with its pool of available circuits. A call is directed to an earth station, where through telephone number analysis, that call will be routed on a DAMA circuit. With centralized DAMA control, the earth station requests a DAMA channel from the master station. The channel is assigned and connectivity is effected. When the call terminates (i.e., there is an on-hook condition), the circuit is taken down and the DAMA channel is returned to the pool of available channels. Typical DAMA systems have something under 500 voice channels available in the pool. They will occupy one 36-MHz satellite transponder.

9.3.6 Earth Station Link Engineering

9.3.6.1 Introduction. Up to this point we have discussed basic satellite communication topics such as access and coverage. This section covers satellite link engineering

232 CONCEPTS IN TRANSMISSION TRANSPORT

with emphasis on the earth station, the approach used to introduce the reader to essential path engineering. It expands on the basic principles introduced in Section 9.2, dealing with LOS microwave. As we saw in Section 9.3.2, an earth station is a distant RF repeater. By international agreement the satellite transponder’s EIRP is limited because nearly all bands are shared by terrestrial services, principally LOS microwave. This is one reason we call satellite communication downlink limited.

9.3.6.2 Satellite Communications Receiving System Figure of Merit, G/ T. The

figure of merit of a satellite communications receiving system, G/ T, has been introduced into the technology to describe the capability of an earth station or a satellite to receive a signal. It is also a convenient tool in the link budget analysis.14 A link budget is used

by the system engineer to size components of earth stations and satellites, such as RF
output power, antenna gain and directivity, and receiver front-end characteristics.
G/ T can be written as a mathematical identity:
G/ T c GdB − 10 log Tsys,	(9.18)

where G is the net antenna gain up to an arbitrary reference point or reference plane in the downlink receive chain (for an earth station). Conventionally, in commercial practice the reference plane is taken at the input of the low-noise amplifier (LNA). Thus G is simply the gross gain of the antenna minus all losses up to the LNA. These losses include feed loss, waveguide loss, bandpass filter loss, and, where applicable, directional coupler loss, waveguide switch insertion loss, radome loss, and transition losses.

Tsys is the effective noise temperature of the receiving system and

Tsys c Tant + Trecvr.

(9.19)

Tant or the antenna noise temperature includes all noise-generating components up to the reference plane. The reference plane is a dividing line between the antenna noise

component and the actual receiver noise component (Trecvr). The antenna noise sources include sky noise (Tsky) plus the thermal noise generated by ohmic losses created by all

devices inserted into the system. Trecvr or the actual receiver noise temperature, which has equivalence to the receiver noise figure. A typical earth station receiving system is illustrated in Figure 9.20 for a 12-GHz downlink. Earth stations generally have minimum elevation angles. At 4 GHz the minimum elevation angle is 58; at 12 GHz, 108. The elevation angle is that angle measured from the horizon (08) to the antenna main beam when pointed at the satellite. As the elevation angle decreases below these values, noise (sky noise) increases radically.

Take note that we are working with noise temperatures here. Noise temperature is another way of expressing thermal noise levels of a radio system, subsystem, or component. In Section 9.2 we used noise figure for this function. Noise figure can be related to noise temperature by the following formula:

NFdB c 10 log(1 + Te/ 290)

(9.20)

where Te is the effective noise temperature measured in kelvins. Note that the kelvin temperature scale is based on absolute zero.

14We called this path analysis in LOS microwave terminology.

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Figure 9.20 Model for an earth station receiving system.

Example. If the noise figure of a device is 1.2 dB, what is its equivalent noise temperature?

1.2 dB c 10 log(1 + Te/ 290) 0.2 c log(1 + Te/ 290).

Remember, 0.2 is the logarithm of the number. What is the number? Take the antilog of 0.2 by using the 10x function on your calculator. This turns out to be 1.58.

1.58 c 1 + Te/ 290

Te c 168.2 K.

Antenna noise (Tant) is calculated by the following formula:

Tant c	(la − 1)290 + Tsky	,	(9.21)
	la

where la is the numeric equivalent of the sum of the ohmic losses up to the reference plane and is calculated by

la c log−1	La	,	(9.22)
10	10

where La is the sum of the losses in decibels.

Sky noise varies directly with frequency and inversely with elevation angle. Some typical sky noise values are given in Table 9.5.

234	CONCEPTS IN TRANSMISSION TRANSPORT
	Table 9.5 Sky Noise Values for Several Frequencies and Elevation Angles
		Frequency (GHz)	Elevation Angle (8)	Sky Noise (K)
	4.0		5	28
	4.0		10	16
	7.5		5	33
	7.5		10	18
	11.7		10	23
	11.7		15	18
	20.0		10	118
	20.0		15	100
	20.0		20	80

Example. An earth station operating at 12 GHz with a 108 elevation angle has a 47-dB gain and a 2.5-dB loss from the antenna feed to the input of the LNA. The sky noise is 25 K developing an antenna noise temperature of 240 K. The noise figure of the LNA is 1.5 dB. Calculate the G/ T.

Convert the 2.5 dB noise figure value to its equivalent noise temperature. Use formula (9.20). For this sample problem Te c Trecvr.

1.5 dB c 10 log(1 + Te/ 290)

c 119.6 K c Trecvr

Tsys c Tant + Trecvr

c240 + 119.6

c359.6 K.

Now we can calculate the G/ T. Derive the net antenna gain (up to the reference plane—at the input of the LNA).

Gnet c 47 − 2.5 c 44.3 dB

G/ T c 44.3 dB − 10 log Tsys

c44.3 − 10 log 359.6

c+18.74 dB/ K or just +18.74 dB/ K

For earth stations operating below 10 GHz, it is advisable to have a link margin of 4 dB to compensate for propagation anomalies and deterioration of components due to aging.

9.3.6.3 Typical Downlink Power Budget. A link budget is a tabular method of calculating space communication system parameters. The approach is very similar to that used for LOS microwave links (see Section 9.2.3.4). We start with the EIRP of the satellite for the downlink or the EIRP of the earth station for the uplink. The bottom line is C/ N0 and the link margin, all calculated with dB notation. C/ N0 is the carrier-to-noise ratio in 1 Hz of bandwidth at the input of the LNA. (Note: RSL, or receive signal level, and C are synonymous.) Expressed as an equation:

	9.3 SATELLITE COMMUNICATIONS	235
C	c EIRP − FSLdB − (other losses) + G/ TdB/ K − k,	(9.23)
N0

where FSL is the free-space loss to the satellite for the frequency of interest and k is Boltzmann’s constant expressed in dBW. Remember in Eq. (9.9) we used Boltzmann’s constant, which gives the thermal noise level at the output of a “perfect” receiver operating at absolute zero in 1 Hz of bandwidth (or N0).15 Its value is −228.6 dBW/ Hz. “Other losses” may include:

•Polarization loss (0.5 dB);

•Pointing losses, terminal and satellite (0.5 dB each);

•Off-contour loss (depends on satellite antenna characteristics);

•Gaseous absorption loss (varies with frequency, altitude, and elevation angle); and

•Excess attenuation due to rainfall (for systems operating above 10 GHz).

The loss values in parentheses are conservative estimates and should be used only if no definitive information is available.

The off-contour loss refers to spacecraft antennas that provide a spot or zone beam with a footprint on a specific geographical coverage area. There are usually two contours, one for G/ T (uplink) and the other for EIRP (downlink). Remember that these contours are looking from the satellite down to the earth’s surface. Naturally, an offcontour loss would be invoked only for earth stations located outside of the contour line. This must be distinguished from satellite pointing loss, which is a loss value to take into account that satellite pointing is not perfect. The contour lines are drawn as if the satellite pointing were “perfect.”

Gaseous absorption loss (or atmospheric absorption) varies with frequency, elevation angle, and altitude of the earth station. As one would expect, the higher the altitude, the less dense the air and thus the less loss. Gaseous absorption losses vary with frequency and inversely with elevation angle. Often, for systems operating below 10 GHz, such losses are neglected. Reference 3 suggests a 1-dB loss at 7.25 GHz for elevation angles under 108 and for 4 GHz, 0.5 dB below 88 elevation angle.

Example of a Link Budget. Assume the following: a 4-GHz downlink, 58 elevation angle, EIRP is +30 dBW; satellite range is 25,573 statute miles (sm), and the terminal G/ T is +20.0 dB/ K. Calculate the downlink C/ N0.

First calculate the free-space loss. Use Eq. (9.4):

LdB c 96.6 + 20 log FGHz + 20 log Dsm

c96.6 + 20 log 4.0 + 20 log 25, 573

c96.6 + 12.04 + 88.16

c196.8 dB.

15Remember that geostationary satellite range varies with elevation angle and is minimum at zenith.

236 CONCEPTS IN TRANSMISSION TRANSPORT

EXAMPLE LINK BUDGET: DOWNLINK

EIRP of satellite	+30 dBW
Free-space loss	−196.8 dB
Satellite pointing loss	−0.5 dB
Off-contour loss	0.0 dB
Excess attenuation rainfall	0.0 dB
Gaseous absorption loss	−0.5 dB
Polarization loss	−0.5 dB
Terminal pointing loss	−0.5 dB
Isotropic receive level	−168.8 dBW
TerminalG/ T	+20.0 dB/ K
Sum	−148.8 dBW
Boltzmann’s constant (dBW)	−(−228.6 dBW)
C/ N0	79.8 dB

On repeatered satellite systems, sometimes called “bent-pipe satellite systems” (those that we are dealing with here), the link budget is carried out only as far as C/ N0, as we did above. It is calculated for the uplink and for the downlink separately. We then calculate an equivalent C/ N0 for the system (i.e., uplink and downlink combined). Use the following formula to carry out this calculation:

C

1

.

(9.24)

N0

c

1

/

(C

N0)(u) + 1

/

(C

N0)(d)

(s)

/

/

Example. Suppose that an uplink has a C/ N0 of 82.2 dB and its companion downlink has a C/ N0 of 79.8 dB. Calculate the C/ N0 for the system, (C/ N0)s. First calculate the equivalent numeric value (NV) for each C/ N0 value:

NV(1) c log−1(79.8/ 10) c 95.5 × 106 NV(2) c log−1(82.2/ 10) c 166 × 106 C/ N0 c 1/ [(10−6/ 95.5) + (10−6/ 166)]

c 1/ (0.016 × 10−6) c 62.5 × 106 c 77.96 dB.

This is the carrier-to-noise ratio in 1 Hz of bandwidth. To derive C/ N for a particular RF bandwidth, use the following formula:

C/ N c C/ N0 − 10 log BWHz.

Suppose the example system had a 1.2-MHz bandwidth with the C/ N0 of 77.96 dB. What is the C/ N ?

C/ N c 77.96 dB − 10 log(1.2 × 106)

c77.96 − 60.79

c17.17 dB.

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237

9.3.6.4 Uplink Considerations. A typical specification for INTELSAT states that the EIRP per voice channel must be +61 dBW (example); thus, to determine the EIRP for a specific number of voice channels to be transmitted on a carrier, we take the required output per voice channel in dBW (the +61 dBW in this case) and add logarithmically 10 log N, where N is the number of voice channels to be transmitted.

For example, consider the case for an uplink transmitting 60 voice channels:

+61 dBW + 10 log 60 c 61 + 17.78 c +78.78 dBW.

If the nominal 50-ft (15-m) antenna has a gain of 57 dB (at 6 GHz) and losses typically of 3 dB, the transmitter output power, Pt, required is

EIRPdBW c Pt + Gant − line lossesdB,

where Pt is the output power of the transmitter (in dB/ W) and Gant is the antenna gain (in dB) (uplink). Then in the example we have

+78.78 dBW c Pt + 53 − 3

Pt c +24.78 dBW

c 300.1 W.

9.3.7 Digital Communication by Satellite

There are three methods to handle digital communication by satellite: (1) TDMA, (2) FDMA, and (3) over a VSAT network. TDMA was covered in Section 9.3.5.2 and VSATs will be discussed in Section 9.3.8. Digital access by FDMA is handled in a similar fashion as with an analog FDM/ FM configuration (Section 9.3.5.1). Several users may share a common transponder and the same backoff rules hold; in fact they are even more important when using a digital format because the IM products generated in the satellite TWT high-power amplifier (HPA) can notably degrade error performance. In the link budget, once we calculate C/ N0 (Eq. (9.24)), we convert to Eb/ N0 with the following formula:

Eb/ N0 c C/ N0 − 10 log(bit rate).

(9.25)

The Eb/ N0 value can now be applied to the typical curves found in Figure 9.10 to derive the BER.

As mentioned previously, satellite communication is downlink limited because downlink EIRP is strictly restricted. Still we want to receive sufficient power to meet error performance objectives. One way to achieve this goal is to use forward error correction on the links where the lower Eb/ N0 ratios will still meet error objectives. Thus INTELSAT requires coding on their digital accesses. Some typical INTELSAT digital link parameters are given in Table 9.6. These parameters are for the intermediate data rate (IDR) digital carrier system. All IDR carriers are required to use at least R c 3/ 4 where R is the code rate.16 A detailed description of various FEC channel coding schemes is provided in Ref. 3.

16R c (information bit rate)/ (coded symbol rate). When R c 3/ 4 and the information rate is 1.544 Mbps, the coded symbol rate is 4/ 3 that value or 2,058,666 symbols a second. FEC coding simply adds redundant bits in a systematic manner such that errors may be corrected by the distant-end decoder.

238	CONCEPTS IN TRANSMISSION TRANSPORT
Table 9.6 QPSK Characteristics and Transmission Parameters for IDR Carriers
Parameter			Requirement
1. Information rate (IR)		64 kbit/ s to 44.736 Mbit/ s
2. Overhead data rate for carriers with IR ≥ 1.544 Mbit/ s		96 kbit/ s
3. Forward error correction encoding		Rate 3/ 4 convolutional
		encoding/ Viterbi decoding
4. Energy dispersal (scrambling)		As per ITU-R5.524-4
5. Modulation		Four-phase coherent PSK
6. Ambiguity resolution		Combination of differential
		encoding (1808) and FEC (908)
7. Clock recovery		Clock timing must be recovered
		from the received data stream
8. Minimum carrier bandwidth (allocated)		0.7 R Hz of [0.933 (IR + Overhead)]
9. Noise bandwidth (and occupied bandwidth)		0.6 R Hz or [0.8 (IR + Overhead)]
10. Eb/ N0 at BER (Rate 3/ 4 FEC)		10−3	10−7	10−8
a. Modems back-to-back		5.3 dB	8.3 dB	8.8 dB
b. Through satellite channel		5.7 dB	8.7 dB	9.2 dB
11. C/	T at nominal operating point	−219.9 + 10 log10 (IR + OH), dBW/ K
12. C/ N in noise bandwidth at nominal operating point		9.7 dB
(BER ≤ 10−7)		1 × 10−7
13. Nominal bit error rate at operating point
14. C/ T at threshold (BER c 1 × 10−3)		−222.9 + 10 log10 (IR + OH), dBW/ K
15. C/ N in noise bandwidth at threshold (BER c 1 × 10−3)		6.7 dB
16. Threshold bit error rate		1 × 10−3

Notes: IR is the information rate in bits per second. R is the transmission rate in bits per second and equals (IR + OH) times 4/ 3 for carriers employing Rate 3/ 4 FEC. The allocated bandwidth will be equal to 0.7 times the transmission rate, rounded up to the next highest odd integer multiple of 22.5 kHz increment (for information rates less than or equal to 10 Mbit/ s) or 125 kHz increment (for information rates greater than 10 Mbit/ s). Rate 3/ 4 FEC is mandatory for all IDR carriers. OH c overhead.

Source: IESS-308, Rev. 7, Ref. 5. Courtesy of INTELSAT.

The occupied satellite bandwidth unit for IDR carriers is approximately equal to 0.6 times the transmission rate. The transmission rate is defined as the coded symbol rate. To provide guardbands between adjacent carriers on the same transponder, the nominal satellite bandwidth unit is 0.7 times the transmission rate.

9.3.8 Very Small Aperture Terminal (VSAT) Networks

9.3.8.1 Rationale. VSATs are defined by their antenna aperture (diameter of the parabolic dish), which can vary from 0.5 m (1.6 ft) to 2.5 m (8.125 ft). Such apertures are considerably smaller than conventional earth stations. A VSAT network consists of one comparatively large hub earth terminal and remote VSAT terminals. Some networks in the United States have more than 5000 outlying VSAT terminals (a large drugstore chain). Many such networks exist.

There are three underlying reasons for the use of VSAT networks:

1. An economic alternative to establish a data network, particularly if traffic flow is to/ from a central facility, usually a corporate headquarters to/ from outlying remotes;

2. To by-pass telephone companies with a completely private network; and

3. To provide quality telecommunication connectivity where other means are substandard or nonexistent.

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Figure 9.21 Typical VSAT network topology. Note the star network configuration. The outlying VSAT terminals can number in the thousands.

Regarding reason 3, the author is aware of one emerging nation where 124 bank branches had no electrical communication whatsoever with the headquarters institution in the capital city.

9.3.8.2 Characteristics of Typical VSAT Networks. On conventional VSAT networks, the hub is designed to compensate for the VSAT handicap (i.e., its small size). For example, a hub antenna aperture is 5 m to 11 m (16 ft to 50 ft) (Ref. 12). Highpower amplifiers (HPAs) run from 100 W to 600 W output power. Low-noise amplifiers (LNAs), typically at 12 GHz, display (a) noise figures from 0.5 dB to 1.0 dB and (b) low-noise downconverters in the range of 1.5-dB noise figure. Hub G/ T values range from +29 dB/ K to +34 dB/ K.

VSAT terminals have transmitter output powers ranging from 1 W to 50 W, depending on service characteristics. Receiver noise performance using a low-noise downconverter is about 1.5 dB; otherwise 1 dB with an LNA. G/ T values for 12.5-GHz downlinks are between +14 dB/ K and +22 dB/ K, depending greatly on antenna aperture. The idea is to make a VSAT terminal as inexpensive as possible. Figure 9.21 illustrates the conventional hub/ VSAT concept of a star network. The hub is at the center.

9.3.8.3 Access Techniques. Inbound refers to traffic from VSAT(s) to hub, and outbound refers to traffic from hub to VSAT(s). The outbound link is commonly a time-divi- sion multiplex (TDM) serial bit stream, often 56 kbps, and some high-capacity systems reach 1.544 Mbps or 2.048 Mbps. The inbound links can take on any one of a number of flavors, typically 9600 bps.

More frequently VSAT systems support interactive data transactions, which are very short in duration. Thus, we can expect bursty operation from a remote VSAT terminal. One application is to deliver, in near real time, point-of-sale (POS) information, for-