Hi Dan:
All, right, let's start from the basics for the
anti-TERN argument. The objective of the Hi-PHY
standard is to increase upstream (and if possible
downstream) data rates. Increasing data rates can
only be accomplished by using modulation schemes
which are more spectrum efficient (high-order
QAM). However, S-CDMA offers no advantage in the
area of spectrum efficiency. Specifically,
orthogonal CDMA (such as S-CDMA) is robust against
narrowband frequency interferers, and impulsive
noise. However, CDMA has nothing to offer in the
area of spectral efficiency. QCOM's IS-95 spreads
a BPSK signal (reapeated on both quadrature components),
so that its spectral efficiency is 1bit/Hz/sec.
I believe (but am not sure) that cdma2000 spreads
QPSK, so that it has a spectrum efficiency of 2bits/Hz/sec
(these are coarse estimates).
For its Hi-PHY, I suspect that TERN was assuming that
they could spread high-order QAM signals. However,
a 2-year old report I saw which evaluated
TERN's attempt at spreading 16-QAM fell way short of
the expected 4 bits/sec/Hz efficiency. Basically, it
is extremely difficult to combine the complexity of QAM
transceivers with the complexity associated to S-CDMA.
So, S-CDMA can at best offer marginal improvements
in upstream and downstream rates. An alternative brute
force approach consists of course in subdividing
local user clusters as they get saturated (expensive
but certainly effective). I am also aware of
a solution proposed by CMTO 2 years ago (I have no
idea as to its effectiveness) which essentially
attempts to inroduce ``switching' in the shared
cable environment.
Now, let's look at it from the point of view of
cable companies. Suppose you are COX, or Time-Warner,
or Media One, and spent billions of dollars
upgrading you cable plant to 2-way HFC, and installing
DOCSIS compliant equipment. Are you going to
immediately embrace a new standard that offers only
marginal improvement, particularly when brute
force capacity increasing techniques (such as the
cluster splitting method) are available? Probably
not.
Now, suppose you are an operator who has not performed
the 2-way HFC upgrade. Certainly TERN"s equipment
is attractive. However, looking down the road, is
this going to stop you from upgrading your plant at
some point? No, since 2-way HFC makes possible selling
a wide range of services to consumers.
I never understood why the S-DMT proposal was not accepted
for Hi-PHY. S-DMT has all the noise robustness
features of S-CDMA, but it also comes much closer to
delivering the maximum data rate sustainable by
the channel. The one drawback is that it is very
complex and a power hog.
As I indicated in earlier posts TERN will have
100% of a market segment which ultimately will
shrink to zero. Its growth rate indicates TERN is
moving aggressively in filling in its market segment,
but the long run prospects are quite poor.
Best regards,
Bernard Levy |
