Info

gle network. In such situations, it is cost effective to provide a means of sharing a limited amount of satellite link capacity between users.

Figure 13-16 shows three basic techniques for sharing link capacity. Frequency division multiple access (FDMA) assigns a single carrier frequency for each input signal. Bandpass filters at the receiver separate the individual carriers from each other. Most communications satellite systems use this technique with a repeater transponder operating in a linear (back-off) mode. The ground station is less expensive because the transmitter peak power is lower than the TDMA ground station described below.

Time division multiple access (TDMA) assigns a single time slot in each time frame to a single input channel. A digitized input signal is sampled and stored in buffer memory. These samples are then transmitted as short bursts within the assigned time slots. The bit rate during the burst is high, therefore requiring a high peak transmitter power. At the receiver the samples are sorted and stored, and then read out at the original rate. These samples are then converted to an analog signal and smoothed to obtain a replica of die original input signal. Some systems, including Intelsat and DSCS-m, use this method for some of their users.

If the satellite uses a multiple-beam antenna, a switching matrix on the satellite may be used with TDMA to route each time slot burst to the desired downlink antenna beam. The NASA ACTS [Naderi and Kelly, 1988] uses this technique, known as Satellite Switched-TDMA (SS-TDMA). In some applications, the uplink uses the FDMA technique, and the downlink uses the TDMA [McElroy, 1988], This method requires onboard processing (demodulation and remodulation) of the signal in the satellite, as discussed below.

A third technique, code division multiple access (CDMA), consists of phase-modulating (BPSK or QPSK) a carrier with data, and then biphase-modulating the carrier with a pseudorandom noise (PN) code. (See Fig. 13-17.) The data rate is much lower than the PN code, or chip rate. Thus there are many code bits (or chips) per data bit. The receiver has a code generator which replicates the PN code of the desired signal on a carrier with frequency equal to the received carrier plus or minus an intermediate frequency (IF).

The PN codes are designed to have low cross-correlation properties (shown in Fig. 13-17B) so that two or more signals can be transmitted simultaneously at the same frequency with little mutual interference. Thus the received signals are essentially uncorrelated with the locally generated PN code and appear as noise to the receiver, except when identical received and locally-generated codes are aligned, or synchronized, in time. When this happens, the output of the mixer is a carrier containing only the narrow-band data modulation, which passes through the IF bandpass filter (BPF)

Technique

Time

Frequency

Advantages Disadvantages

Frequency

Frequency

Division

Multiple

Power

Frequency

• Continuous carrier transmission. Peak and average power equal. No complex timing.

• Modulation rates comparable to user data rates. Inexpensive ground stations.

Multiple users cannot share singletransmitter without mutual Interference or low power efficiency. Requires complex filter banks to separate channels. Difficult filtering to separate large power users from adjacent users. May require power control.

Power i

Time

Division

Multiple

Access

0 0

Post a comment