18 System Feature Overview
2.2.1.5 Capacity Plus Mode
When operating in Capacity Plus Mode, MOTOTRBO trunks the logical channels of multiple
MOTOTRBO repeaters (operating in digital repeater mode) at the same location. This allows the
radios to share the logical channels, resulting in less waiting time to access the system and
increased channel capacity for a given quality of service. Another advantage is that the probability
of all channels being busy at the same instant is low, therefore the probability of a call being
blocked is lower than when only one channel can be accessed.
Capacity Plus is a single site trunking configuration of the MOTOTRBO system. In a Capacity Plus
configuration, all the “idle” radios (i.e. radios neither receiving nor transmitting) are on an idle
channel called the Rest Channel. Therefore, a new call always starts on the Rest Channel. At the
start of a call, the Rest Channel repeater selects one of the idle channels as the new Rest
Channel, informs the radios on the current Rest Channel about the new Rest Channel, converts
the current Rest Channel to a traffic channel, and starts repeating the bursts sent by the radio. The
radios that are not participating in the call (i.e. destination of the call is not of their interest) move to
the new Rest Channel.
If the current Rest Channel is the last idle channel (i.e. all the other available channels are in use),
the current Rest Channel remains as the Rest Channel. The call starts on the channel and non-
participating radios stay on the channel. In this condition, non-participating radios indicate that the
channel is busy via its yellow LED. If all channels are busy and a radio user initiates a call, then the
radio generates a distinct tone to indicate that the system is busy. As soon as a channel becomes
free in the Capacity Plus system, the non-participating radios are informed, and move to the free
channel.
At the end of the call (i.e. after the call hang time), the repeater also broadcasts the status of all
other available channels. This triggers any radio on the channel to move to the current Rest
Channel or to a channel where a Group Call of interest is active.
The Capacity Plus system has no central controller to manage the Rest Channel. The Rest
Channel is managed collectively by all the trunked repeaters. A trunked repeater periodically
informs the status of its channels to other trunked repeaters whenever the status of its channels
change. When a new Rest Channel is selected, the selecting repeater informs all the other
repeaters. The new Rest Channel is selected based on the following conditions:
• At the start of a call, the repeater of the current Rest Channel selects the new Rest
Channel.
• On detection of interference or before starting CWID (i.e. BSI) transmission, the
repeater of the current Rest Channel selects the new Rest Channel.
• On detection of no Rest Channel (in the event of a failure of the current Rest Channel
repeater or the backend network), the repeater with the lowest ID selects the new
Rest Channel.
• When a call ends on a system, if a call is in progress on the current Rest Channel,
then the repeater of the current Rest Channel selects the new Rest Channel.
The Capacity Plus system does not require an exclusive control channel. The Rest Channel
changes on every call; in case of an interference or if the repeater becomes unavailable due to
failure. This results in the following advantages:
• Non-exclusive channels make it easier to satisfy regulator frequency coordination
(where exclusive use of channels is not possible).
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