System Feature Overview 5
68007024085 April 2011
2.1.2 Spectrum Efficiency via Two-Slot TDMA
2.1.2.1 Frequencies, Channels, and Requirements for Spectrum Efficiency
A radio communications channel is defined by its carrier frequency, and its bandwidth. The
spectrum of available carrier frequencies is divided into major bands (such as 800/900 MHz, VHF,
and UHF), and the majority of licensed channels in use today have widths of either 25 kHz or 12.5
kHz. As the airwaves have become increasingly crowded, new standards and technologies that
allow more radio users to share the available spectrum in any given area are needed. The demand
for greater spectral efficiency is being driven, in part, by regulatory agencies. In the U.S., for
example, the Federal Communications Commission (FCC) requires manufacturers to offer only
devices that operate within 12.5 kHz VHF and UHF channels by 2011. By the year 2013, all VHF
and UHF users are required to operate in 12.5 kHz channels.
The next logical step is to further improve the effective capacity of 12.5 kHz channels. While there
is no current mandate requiring a move to 6.25 kHz, such discussions are on-going at the FCC
and other agencies. It’s only a matter of time before the ability to carry two voice paths in a single
12.5 kHz channel, also known as 6.25 kHz equivalent efficiency, becomes a requirement in 800/
900 MHz, VHF, and UHF bands. Presently, FCC rules are in place to mandate manufacturers to
build radios capable of the 6.25 kHz efficiency for 800/900 MHz, VHF, and UHF bands, but the
enforcement of these rules are put on hold. In the meantime, MOTOTRBO offers a way to divide a
12.5 kHz channel into two independent time slots, thus achieving 6.25 kHz-equivalent efficiency
today.
2.1.2.2 Delivering Increased Capacity in Existing 12.5 kHz Channels
MOTOTRBO uses a two-slot TDMA architecture. This architecture divides the channel into two
alternating time slots, thereby creating two logical channels on one physical 12.5 kHz channel.
Each voice call utilizes only one of these logical channels, and each user accesses a time slot as if
it is an independent channel. A transmitting radio transmits information only during its selected
slot, and will be idle during the alternate slot. The receiving radio observes the transmissions in
either time slot, and relies on the signaling information included in each time slot to determine
which call it was meant to receive.
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