User Manual

ADCP-XX-XXX • November 2000 • Section 2: WMTS Description
Page 2-20
 2000, ADC Telecommunications, Inc.
5.2 Upstream
The upstream path starts with the signal being picked up by the antenna which is then passed
to the low noise amplifier (LNA) via a coaxial cable or a waveguide and filtering system
(which might include the power combiner). The signal is converted from the RF band to an IF
signal in the upstream receiver/downconverter. The IF is then presented to the upstream card
in the WMTS chassis.
Upstream traffic from the subscriber unit is burst mode.
The upstream downconverter, used to convert the RF to an IF suitable for the demodulator,
along with the LNA and filtering, must have good linearity, low distortion, low phase noise,
and have a low noise figure. The WMTS IF frequency from the demodulator is 5 to 65 MHz.
The RF frequency and the upstream downconverter determine the specific frequency used by a
given channel. This frequency should be selected with attention given to interference and
filtering considerations.
Upstream throughput is dependent on the sub-channel bandwidth (400 kHz, 800 kHz, and 1.6
MHz) and the modulation type (QPSK). Multiple channels and sub-channels from the same
antenna sector, if they are close enough in frequency to allow common filtering, may be
processed in the same LNA and upstream downconverter. Channels, which are offset in
frequency (for example, an MDS channel and a WCS channel), would require separate
upstream filters and downconverter. Different antenna sectors also require separate upstream
paths because combining multiple sectors reduces the sensitivity of the receiver.
5.3 Modulation
The modulation and symbol rates used are the primary determinant of the data transfer rate.
Wireless systems typically use QPSK upstream and 64 QAM downstream. The signal-to-noise
ratio at the receiver determines which modulation formats can be successfully used for a given
level of phase noise. The more sophisticated the modulation format, the better the signal-to-
noise ratio must be to accurately demodulate the signal.
QPSK is the most tolerant with a required S/N ratio of about 14 dB. 64 QAM passes about
three times as many bits per hertz of bandwidth, but requires 27 dB S/N for the same bit error
rate. The best modulation type for a system also depends on how much multipath and fading
immunity is required for the physical environment. The lower the modulation complexity, the
higher the immunity. But the lower the complexity, the lower the data rates that are possible if
the channel were perfect.
Immunity from fading and multipath are also determined by symbol rate. The lower the
symbol rate, the better the immunity. But again, the lower the theoretical data rate. Also, the
lower the symbol rate, the narrower the receiver's bandwidth. This effects the sensitivity and
therefore the range from the base station that is possible.
The downstream in the Axity system may be QPSK, 16 QAM, or 64 QAM and can be set by
the operator. The upstream bandwidth is also established by the operator. This allows the
system operator to configure for optimal format.