9.3
SATELLITE COMMUNICATIONS
221
Table 9.4
INTELSAT VI, VII, and VII 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.
With TDMA operation, earth stations use digital modulation and transmit with bursts
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
µsec for
INTELSAT and 250
µsec 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.
Figure 9.19
Example of TDMA burst frame format.
Summary :
9.3 SATELLITE COMMUNICATIONS 221 Table 9.4 INTELSAT VI, VII, and VII 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.
Tags :
burst,frame,timing,tdma,bits,earth,stations,station,digital,period,format,figure,919