Hi SteveG:
Because of the relatively high attenuation of millimeter
waves with distance (or due to rain) the most widely
used digital modulation scheme for these frequencies
is quadrature-phase-shift-keying (QPSK) which yields
a spectrum efficiency of 2bits/Hz/sec. However, to
increase the size of cells, and to resist interferences
and attenuation, a form of forward error correction
based on Reed-Solomon codes is also employed. Assuming
that a code with rate 1/2 is employed (one bit out of two
is redundant) gives the 1 bit/Hz/sec you mentioned.
Note however that the above analysis is primarily true
for point to multipoint transmission. For point to point
transmission with a good line of sight between the
transmitter and a roof antenna, and a relatively short
distance, a more spectrally efficient modulation method
such as 16-QAM (which yield 4 bits/Hz/sec) could be
employed.
Assuming for the sake of argument that in a given
market WCII has 500MHz of bandwidth means that WCII
is able to deliver 500Mb/sec to all users within
a cell. Note that this rate is shared by all users.
A residential user would probably need about 2Mb/sec
(T1/E1-ADSL rate) for Internet access or 4Mb/sec
for video on demand. A large business would probably
need about 40-50Mb/sec (T3 type of rate). So
500 Mb/sec covers about 200 residential users per cell,
or about 10 large businesses. I am not an expert with
respect to current local loop fiber wiring, but large
businesses might well have local fiber connection.
However, medium and small businesses (WCII's prime
market) do not have access to fiber at the curbside.
T3 rates are very expensive, and even T1 rates ($1K/month)
are also expensive.
Because the bandwidth available to LMDS operators is shared
by users, I expect that over time, there will be a strong
need for efficient bandwidth management. For video on demand,
this would mean for example downloading movies in big bursts
by using some form of storage on customer premises. For
Internet connection, this would also mean using a demand based
bandwidth assignment scheme similar to the one used by
VSAT companies in the satellite industry.
Best regards,
Bernard Levy |
