User Manual

SGD-SB2025NT-TUM, Part 2
Jan 12 Page 78 RECEIVER VOTING
10 RECEIVER VOTING
10.1 V OTING O VERVIEW
The process of selecting the best signal from a collection of signals that are being returned from
several Station NIs at remote radio sites is commonly referred to as “Voting”. This terminology will
be used throughout this section.
The voting process takes place in the TM unit at a digital level. In order that the TM can make a
selection of the best incoming signal, information needs to be available on which the voting
process can operate. That information is produced by the Station NI and, upon receipt of that
information, the TM will make a comparison of the value that is being returned from every active
site and, subject to criteria, select the site delivering the highest value.
10.2 S IGNAL Q UALITY I NFORMATION
Each and every Station NI will produce a value representative of the quality of any signal
originating from the co-sited base station Rx.
The source of this signal can be in one of two formats:
• Signal strength reading as an analogue voltage (RSSI).
• Variable tone representing the RSSI (either continuously variable or stepped).
The analogue voltage RSSI is taken to a dedicated input on the facilities connector, whereas the
variable tone is taken from the audio input that is being produced from the base station Rx.
Whatever the signal source selected (a user configurable feature), it is turned into a number that is
compared to preset boundary values (also configurable parameters), which define the minimum
threshold of operation and govern the range and resolution of the reported signal quality level. A
hard-wired signal indicating receiver mute open (COR) may also be used. The configuration
values of these parameters are likely to be the same at every radio site and that is a good way to
achieve initial setup. Subsequently, these may well be varied to suit individual site conditions, e.g.
sites with high background noise levels.
A value representing the signal quality is added to the packet of data containing the radio traffic
itself and passed back to the TM every 20 ms. This information is treated as a high priority part of
the Solar supervisory and signalling data subset information within the IP data packet.
10.3 V OTING P ROCESS
For each NI, the RTN traffic audio packets are assigned to slots in a queue (a buffer) according to
the packet number. This action ensures that the packet order is correct, so that when the Voter
process compares the RSSI of packets at the head of the queue they will all have the same time of
origin (the input from main Rx to Station NI). As this is done for every NI at the same time,
synchronisation of the RTN audio is achieved.
The Voting process consists of looking for the mute open status within each packet and extracting
the RSSI value if appropriate. The RSSI values for each active site are then compared and the
packet from the site offering the highest value is passed to the Central NI for decoding. Once a
site is selected, a site offering a higher RSSI value must exceed the current RSSI value of that
from the selected site by the ‘Voter Level Difference’ parameter before a change in selection will
occur.