User Manual

ADCP-XX-XXX • November 2000 • Section 3: WMTS Functional Description
Page 3-4
 2000, ADC Telecommunications, Inc.
3 TRAFFIC STRUCTURE
3.1 Overview
The traffic from the base station toward the WMU at the subscriber site is over a downstream channel
and back over an upstream channel. This arrangement provides full duplex wireless communication.
3.2 Frame Conversion
On the downstream, the WMTS encapsulates the IP frames into MPEG frames. Each MPEG frame
holds 188 bytes and the header holds the Packet Identifier (PID) to identify the original frame
(packet). The modem identifies the frames in order to decapsulate the original IP frames. On the
upstream the modem inserts IP data into the minislots allocated to it according to the options enlisted
in the MAP messages. A typical minislot contains 16 bytes, which is further broken up into ticks. The
burst consists of the header, data (payload), and FEC/CRC which is the error control information. The
header also contains the Service ID (SID) to implement versatile types of services to each modem.
The downstream channel is TDM multiplexed, and the upstream channel is TDMA multiplexed.
4 DOWNSTREAM
4.1 Overview
The forward path data is broadcast toward all the WMUs in a given sector, tuned to a particular
channel. Each WMU listens to the downstream and selects its relevant data and converts it to digital
data. The downstream PMD (Physical Media Dependent) protocol confirms to ITU--T J-83 Anex B.
4.2 Forward Error Correction (FEC)
The upstream channel is configurable to include user-specified FEC overhead, including none. The
flexible FEC coding enables the system operator to set the size of the error protected data blocks and
to set the number of correctable errors within each FEC. These changes can be made while the system
is operating normally. This flexible FEC coding option enables the system operator to choose for a
group of modems to stay on the same frequency by simply increasing the error protection on that
channel. Although, the additional few bytes of error protection reduces the channel information rate, it
makes the overall system capable of a much higher upstream spectrum utilization.
4.3 Downstream Transmission
The downstream signal from the ISP is converted by the WMTS to an IF signal which drives the
transmitter. It consists of the data, encapsulated with Forward Error Correction (FEC) plus control and
timing data. It is centered at the IF frequency of 44 MHz and directly drives a MMDS transmitter over
a 6MHZ band. There are three downstream modulations available; QPSK which provides 8 Mbps
total throughput, 16QAM which supports 17 Mbps, and 64QAM which supports 28 Mbps. The
advantage of the slower rates is that the required signal-to-noise ratio is lower and intersignal
interference may be reduced. Therefore, customers who have difficult locations in terms of distance,