LMDS is a small-cell technology,
with each cell having about a 3- to-6-km radius. Small cells coupled with two-way transmission create a different set of architectural problems than MMDS. Figure
shows a schematic of LMDS service.
Content acquisition at the LMDS headend functions similarly to MMDS.
National television feeds are delivered by the programmer to a
production facility. In many cases, these national feeds come from
DBS, but the feeds also can come from other geosynchronous satellite
transmission or high-speed wired services, such as fiber-optic
networks.
Local content and advertising are acquired
over the air, encoded into MPEG, and multiplexed with the national
programming for local distribution. As in the case of MMDS, MPEG is
an important facilitator of LMDS because it enables digital
multiplexing.
Data services received from Web content
providers are already in digital format but would need additional
processing, such as encapsulation into MPEG and address resolution,
before being transmitted.
The program mix is delivered by satellite or
fiber to the LMDS broadcast tower. Generally, the LMDS headend and
the LMDS broadcast tower are not co-located because the headend
production facilities are normally shared among several towers.
An LMDS transmitter tower is erected in the
neighborhood, and traffic is broadcast to consumers using QPSK
modulation with forward error correction (FEC). It is possible to
use QAM modulation, but QPSK is chosen because it is more robust
than QAM 16 or QAM 64 and because bandwidth is so plentiful that
spectral efficiency is not an issue.
As shown in Figure
,
consumers receive the signal on a small dish about the size of a DBS dish or a flat-plate antenna. The dish is mounted outside the home and is connected by cable to a set-top converter, much the same way in which DBS connections are made. The signal is demodulated and fed to a decoder. Unlike DBS, LMDS is capable of two-way service, so both TV sets and PCs must be connected to the satellite dish. Furthermore, a two-way home networking capability must be supported instead of just the simple broadcast scheme of DBS.
In the return path, the customer transmits to
the carrier using the same dish with QPSK modulation. A MAC protocol
is required because the residences in the coverage area share the
return spectrum.
Architecturally, LMDS looks very much like
cable TV. Cable TV clusters serve 500. The MAC protocol is similar
to cable TV, as are the application-specific integrated circuits (ASICs)
for the customer premises modulators and demodulators. Upstream
users request data slots on a contention basis. After slots are
granted, the sender transmits in those slots, free of contention.
Ranging and power-level controls are also required, as is the case
with cable.