User Manual

ADCP-XX-XXX • November 2000 • Section 3: WMTS Functional Description
Page 3-5
 2000, ADC Telecommunications, Inc.
fading, or multipath may be supported, albeit at a slower data rate. The MMDS/ITFS channels cover
the RF range of 2500 to 2686 MHz. Each channel is 6 MHz wide (except G4). The downstream
antenna is typically contoured toward customers physical residence location to support the customer
base. If the customers surround the transmission site, the antenna would typically be an omni type. If
the transmission site is near a mountain range or a lake or ocean, then the antenna chosen should have
a pattern directing the energy towards the customer base. This allows more of the transmitter power to
be directed at the customer base. If the customer base requires more data throughput than can be
supported with a single downstream channel, there are a couple of options. One is to assign more
MMDS/ITFS channels. The second approach requires multiple transmitters on the same channel and
directive antennas. The net data throughput could be quadrupled by using four 90 degree antennas and
four transmitters. The polarization of the antennas would be alternated between sectors to reduce
sector to sector interference. A third approach to increasing the total downstream bandwidth in a
territory is to use a cellularization approach. This would involve a number of lower powered
transmission sites, each covering a fraction of the territory serviced by the operator. Cellularization,
with sectorization, may be used to increase the system wide data capacity to any level required. The
practical trade-off, of physically establishing multiple transmission sites and supporting them is the
main limitation. At some point, alternative access methods become more practical. As with
MMDS/ITFS TV transmission, the RF signal requires a line of sight path.
4.4 Forward Path Modulation Format
The downstream physical layer is based on the International Telecommunication Union, ITU-T
Recommendation J.83 (04/97), Digital Transmission of television signals, Annex B (ITU-T J.83B).
This revision of ITU-TJ.83B includes not only the original 16 QAM modulation and a fixed depth
interleave used to deliver digital data, but also a 16 QAM for higher downstream channel data rates
as well as a variable depth interleaver. DOCSIS
PLUS TM
downstream channels may occupy a 6 MHz
band between 2500 MHz and 2680 MHz. The reliability of the QAM-modulated downstream
channels is improved by powerful concatenated FEC provided by the ITU-T J83.B specification.
Multiple layers of error detection and error correction, coupled with variable depth interleaving to
provide variable-length burst error resilience, delivers error rates ensuring customer satisfaction. The
high data rates together with the low error rates, provide a bandwidth efficient delivery mechanism
for digital data delivery. The WMU accepts a RF modulated signal with characteristics. The output
signal level of the WMTS is variable over the range of 50 - 61 dBmV.
4.5 Variable Depth Interleave
The forward path modulation supports a variable-depth interleave format with characteristics defined
according to DOCSIS. Variable depth interleave is a form of error protection. Based on the level of
the interleave, the data on the forward path is protected from noise burst lengths. Even with reduced
noise margins, the downstream channels are designed to deliver 16QAM signals with signal to noise
ratio of 23.5 dB. Interleaving improves protection from noise burst. It also adds latency in the
downstream channels. The process of interleaving the outgoing symbols and shuffling the position of
the symbols so that normally adjacent related symbols are now separated by unrelated symbols that
would otherwise be transmitted later, delays the delivery time of the related symbols. The benefit is
that a burst of noise damages only the unrelated symbols. The FEC then corrects the damaged symbols
once they are reshuffled back to their original order, as long as the burst damage did not span too
many related symbols. There is an intrinsic relationship between the depth of the interleaving and the