User Manual

ADCP-XX-XXX • November 2000 • Section 2: WMTS Description
Page 2-19
 2000, ADC Telecommunications, Inc.
5 BASE STATION RF COMPONENTS
Every headend or hub requires a transmitter or set of transmitters with the associated filters,
power combiner, waveguide or cabling and antenna to send the WMTS modulated signals to
the subscriber sites. Similarly, the headend requires an antenna, cabling, filtering, and a
receiver/downconverter to process the RF upstream signal from the client sites for the WMTS
demodulators. This is illustrated in Figure 2-9.
Figure 2-9. Typical Wireless Internet Access
The RF front end contains the downstream assembly, which transmits information from the
WMTS to the subscriber unit and the upstream assembly, which receives information from the
subscriber unit.
5.1 Downstream
The downstream path starts with data from the Internet or servers at the service provider or
base station through the C&F 100BaseT connector (or control signals generated by the
WMTS). These are passed to the appropriate downstream universal card, which sends the
signal (still in a digital format) to the downstream card. The data is then passed to the
appropriate downstream universal card, which sends the signal (still in a digital format) to the
downstream card. The downstream card converts it to the appropriate modulated IF format and
frequency (nominally 44 MHz). The IF signal leaves the WMTS chassis and is provided to the
RF transmitter, which converts it to the specified RF channel (or sub-channel) and amplifies it
to the transmit power level. Typically, multiple transmitters drive a common antenna and these
signals are frequency multiplexed in a power combiner. The output of the power combiner is
connected to the transmit antenna via a waveguide or coaxial cable or a combination of the
two.
There are many aspects to consider when specifying the RF components. One of the primary
concerns is that the transmitter be linear, have a flat response, low group delay, and low phase
noise suitable for the type of modulation that is being implemented.
The Axity system supports a single downstream channel per chassis, which may drive an
omni-directional (omni) antenna, or multiple downstreams. When multiple downstreams are
used, they may be allocated to different frequency channels or different antenna sectors or a
combination of the two. The optimal approach depends on what spectrum is available, and
what the distribution of the customer base is, taking such things as traffic loading, density, and
type of service being delivered into consideration.
There are three downstream modulations available: QPSK, which provides 8 Mbps total
throughput; 16QAM which supports 17 Mbps; and 64QAM, which supports 28 Mbps.
Downstream uses a packet broadcast-multiplexing scheme
.