For extended or improved coverage, multiple receivers can be installed on the same frequency. When the Subscriber Unit transmits, many (and possibly all) of the receivers may hear the transmission depending on the location of the Subscriber Unit. The voter will then determine and select the “best” received signal from all the signals received. The best signal is then rebroadcast from the base station transmitter enabling improved talk-back capability between mobiles in the fi eld. Each local receiver’s received signal is back-hauled to the voter and transmitter base station via IP links (Microwave or wired).
For Codan P25, each traffic packet received includes the Bit Error Rate (BER) detected by the receiver which is used by the voter to determine the relative quality of packets between the multiple streams received. This process is performed continuously as the quality of the received RF signal varies at each of the different receivers. An analog voice signal has no error detection; therefore the only measure of quality is the received signal strength indication (RSSI). The Codan Voter has a “good quality threshold” which defines the analog signal quality level above which no switching will be done to reduce unnecessary switching of channels.
P25 sIMuLCast
The Codan P25 Voting system also supports simulcast operation enabling a single P25 signal to be simultaneously broadcast using two or more geographically separated radio transmitters. For each transmission in the simulcast system, the Simulcast Controller sends the same signal with a timing offset with respect to a 1 Hz synchronization reference to each transmitter via an IP network.
Each Simulcast transmitter is confi gured with an additional timing offset that may then be applied to the transmitted signal, enabling tuning of the system by geographically shifting areas where the signals are destructively reinforcing. A network time protocol (NTP) from a GPS or an Internet connection is required for proper simulcast operation. The Codan P25 Voting/Simulcast system is based on the versatile and proven MT-4E conventional radio system. P25 Voting requires no changes to existing MT-4E radios and is supported in all the P25 frequency bands (VHF, UHF,
T-Band, 700 and 800 MHz).
