User's Manual

AS4020 STs
25
RW Codes
Each RF channel is composed of 16 RW codes: Each RW uses can de either 128kbit/s or 2 X
64kbit/s
RW code 15 is used for initial ST link acquisition and for downlink transmission of the internal
CTS channel which is used to signal to STs the availability of channels to use for uplink
transmissions. The remaining RW codes are used for voice and/or data. The voice/data
boundary is dynamic with RW codes only being used for voice when this is required due to
voice calls in progress. This maximises the data carrying capacity of the system as RWs not
being used for voice are automatically made available for data.
Each ST has a permanent always-on RTS/CTS channel. When an ST wishes to send uplink
traffic, it signals this on the RTS channel and receives in return a reply from the AS4020 CT
on the CTS channel informing the ST of which RW(s) can be used for the uplink voice/data
transmission. The AS4020 is thus able to arbitrate between RTS requests from all STs and
decide on a priority basis which STs are allowed to make an uplink transmission.
For the maximum ST load of 480 STs per AS4020,One or two RWs are reserved in the uplink
direction for RTS channels. Note that the downlink part of each of these RWs is free to carry
user data traffic. This means that a data only AS4020 application has 15 RWs available in the
downlink and 13 or 14RWs available in the uplink for user data. The carrying capacity in bit/s
will depend on the modulation type and FEC code used for transmission to each ST.
A voice only AS4020 application has 13 or 14 RWs free in both the uplink and downlink
directions for user voice calls.
AS4020 delivers data services to individual STs at various data rates depending on the
modulation and forward error correction (FEC) mode it is operating in. The mode used is
dependant on the RF link quality which is measured as a ratio of received signal strength to
noise level (signal to noise ratio – SNR). The higher the SNR, the higher the mode of
operation and hence, the higher the data rate to the ST. Various factors influence the level of
noise seen by the system. These include:
· Thermal (background and intra-system) Noise
· Interference (co and adjacent frequency channels)
· Excessive Multipath (signals received from reflections)
AS4020 dynamically adjusts its mode of operation on a per ST basis depending on a
continually updated measure of SNR. This measure of SNR includes all factors as listed
above.