PSM-500/500L/500LT SCPC Satellite Modem
Description
PSM-500/500L/500LT - Rev. 0.91
Page 1-5
Figure 1-1, the shape of the configuration is drawn with the central “Hub” as the center of the star
and the remotes as points of the star. In both cases the transmit frequency and other parameters
may be shared by the receive of all the remotes.
The PSM-500L and LT are ideally suited for use at remote or small stations. Since the receive down-
converter requirement is significantly reduced in this version, requiring only that a data grade LNB
(Low Noise Block down-converter) be connected to the modem. The L-Band version modems can
even supply power and reference to the LNB if needed. In addition the PSM-500L and PSM-500LT
modems are designed for use with a Block Up Converter or “BUC” and can supply power and
reference signals on the transmit cable. Most BUCs today are designed to receive these signals on
the cable.
In Broadcast type systems where the remotes only require a receiver, the L or LT is very low cost
and the transmit modulator section can be simply turned off.
A “Star” network configuration is also commonly used with multiple point-to-point links where the hub
is common to every link. An example might be where each remote represents a house or building
with voice or data traffic all destined for a common switch located near the hub. Each link is then
usually dedicated to that customer and the link resources are wasted when no traffic is carried. That
loss is partially offset by being able to use smaller antennas and power at each of the remotes,
concentrating costs at the hub.
1.1.3.3 DAMA (Demand Assigned Multiple Access)
Suppose that we wanted to simulate a telephone network with a virtual switch between modems
carrying digitized voice information. We might use a central computer to assign a pair of frequencies
for any conversation and send this connection information to the proper sites to set up the
connection. Many systems of this type use “Star” network topology, but this has the disadvantage
that for a person at Remote A to talk to someone at Remote D the traffic must go through the hub.
The resulting delay through 2 satellite hops is just at the limit of what is tolerable for voice traffic.
In this application a new network configuration is usable. That is a “Mesh” network where any of the
voice modems at any site can be programmed to link with any other modem directly at any other
site. The resulting link diagram looks like a mesh of interconnects. Now there must be sufficient
antenna size and power at each remote to link to every other remote. The station costs can go up
significantly, and are multiplied by the number of stations.
Since the frequencies can be assigned on demand, the network is then called “Demand Assigned,
Multiple Access”, or DAMA. One important characteristic of a DAMA system used for voice
information is the lock-up time of the modem. At the low data rates used to digitize voice today (4.4
to 32 kbps) the modem receive acquisition method of sweeping results in lock-up times of tens of
seconds to minutes. The PSM-500 modem is uniquely designed to significantly reduce this time:
The fast acquisition digital signal processor used in the PSM-500 looks at the receive signals within
its acquisition range much like a person might view the same region using a spectrum analyzer. It
then “homes in” and locks to the most probable carrier. This acquisition mode can reduce the
receive acquisition time to approximately 1/3
rd
of a second at 9.6 kbps in QPSK mode over +/- 30
kHz, and less in BPSK mode.
1.1.3.4 TDMA (Time Division Multiple Access) Remote Site Application
In a TDMA network the central Hub continually transmits a stream of outbound data containing
information for multiple remote sites, while the remotes transmit back to the Hub on a timed basis.
Each of these remotes is said to “burst” its information back on a specific frequency. This may be the
same inbound frequency for all sites. Each of the remotes is responsible for accessing its own
information from the outbound data stream by reading the address assigned to specific parts of the
data.
The TDMA network usually looks like the Star network described above. The outbound (from the
Hub) data rate may be quite high to accommodate many remotes with low latency, while the inbound
data rate may be low to allow use of a small antenna and power amplifier at the many remote sites.
