limtex:
I will try to find you a good book on wireless.
Ira Brodsky has a few wireless books which are
addressed to the general public, and which may
contain what you need (will try to be more specific soon).
I won't try to respond to all your questions at once,
but the one regarding TDMA and CDMA is simple(you would need
also to throw in frequency division multiple access--FDMA,
and space division multiple access --SDMA for good
measure). All concern how lots of users can share
a set of frequencies to send separate messages without
having all these messages mixed up into an unrecognizable
mess.
FDMA is the simplest and oldest: it divides the available
frequency spectrum into smaller frequency bands and says:
user A has the first band, user B has the second band, etc...
This is the way AM and FM radio stations can be separated.
TDMA does the same thing in the time domain: it divides
time into fixed intervals, and each interval is divided
into slots. During the first slot, user A can transmit,
and the other users are silent, then in the second slot,
it is the turn of user B, and so on. At the end of an interval,
it is time to restart: user A gets his/her turn again, and so
on.
CDMA is more complex, since all users transmit information all
the time and use the entire spectrum available. The trick is
that each user has a characteristic spreading sequence
(a key if you want), and superimposes the information
he/she wants to transmit on this sequence. The sequences used
by different users are orthogonal (or almost orthogonal)
to one another, which means that at the receiver, when
the superimposed signal is multiplied by a spreading
sequence, only the message associated to this sequence is
received.
SDMA is newer and uses the direction from which the signals
com from to separate users. Typically, it is used as a way
of improving TDMA or CDMA.
Right now WCII does not use either TDMA or CDMA, since it
is based on point to point technology: an antenna at the
base station sends a signal directly in the direction
of a receiving antenna on the roof of a customer. In
some sense, it is a mix of SDMA and FDMA (different
frequency bands can also be used to serve different
customers). All this will change with the rollout of
Nortel's point to multipoint technology, since it
includes a wireless ATM protocol, which I believe
is TDMA based (I have not yet done my homework on
Nortel's equipment).
Regarding accounting versus electrical engineering,
keep in mind that the grass is often greener on
the other side. However, I must admit that the last
20 years have been rather exciting for EEs. Because
of the microelectronics revolution, all sorts of
things have become possible which could only be
dreamt of in the past. For example, when the first
RAKE receiver (used to combine multipath signals
in CDMA) was built at Lincoln Labs in the late 50s
(early 60s?), it required a whole room. Now, you
will find it in each of QCOM's handsets.
Best wishes,
Bernard Levy |
