Hi SteveG:
In response to your questions:
a) I am not sure how WCII has implemented its point to point
system, but I suspect they assign a fixed amount of bandwidth
to each point to point link, independently of the amount of
traffic on this link. Point to multipoint transmission
has the ability to allocate bandwidth based on the actual
demand, so that it increases capacity.
b) By opposition with BPSK, which transmits only one bit
per baud, QPSK transmits two bits (one for each of the
orthogonally phase-shifted modulating waveforms) per baud,
so that QPSK is obviously preferable. Note that the version
of QPSK implemented by HP (now LU) is relatively spectrally
efficient, since it uses some raised cosine pulses
instead of rectangular pulses for the original form of
QPSK.
c) GMSK is a form of continuous-phase frequency modulation
used for GSM. It is more spectrally efficient than
BPSK, but it carries only one bit per baud. Its selection
for mobile applications comes from the fact that it has
constant amplitude, and allows power-efficient implementations
(needed to extend battery life). Power efficiency is not
a tremendously important consideration for fixed
wireless, since these systems are not running from batteries.
d) Orthogonal frequency division multiplexing (OFDM),
or more generally orthogonal multicarrier modulation,
which is the technique employed by the discrete
multitone (DMT) and discrete wavelet multitone
schemes for ADSL/VDSL are extremely interesting since
they come very close to achieving the theoretical
capacity of a communication channel. However their
circuit implementations are very complex, and I do
not expect they will be deployed in the near future
for high frequency wireless links.
e) Wireless ATM is one implementation of bandwidth
management. However, I am not sure that any standard
has yet been put in place (note that wireless ATM
arises in different contexts: LANs, fixed wireless
and satellite, and different standards are likely to
emerge for different applications).
Have a good weekend,
Bernard Levy |
