Features of CDMA
One of the amazing things
about CDMA is that many of its benefits occur in urban environments where they
are needed the most. One such feature is CDMA's use of a Rake correlator, which
actually allows CDMA implementations to benefit from multi-path signal
propagation, which often occurs when signals bounce off of and between
buildings. This is an important benefit because it allows CDMA phones to have
lower power output; this lengthens battery life but more importantly allows for
more cells to be crammed into urban areas where there is a greater need for
more capacity. A Rake correlator works by taking advantage of the way that
pseudo-random codes are almost orthogonal to slightly delayed versions of
themselves. This means that a receiver can take a matched filter and sift it
through the received signal to find where the signal peaks. Then the receiver
sums the signals from the various paths and thus the signal strength from each
of the multi-paths adds to the signal strength instead of the noise. In
traditional cellular implementations, the signals that are not direct usually
contribute to the combined noise and thus lower overall signal to noise ratio;
SNR actually improves for CDMA in a multi-path environment.
Although not intrinsically
tied to CDMA, most implemented versions of CDMA also feature a dynamically
variable power output. Basically what happens is the phone sends a predetermined
signal pattern to the tower. The tower then takes the inverse transform of the
signal it receives and transmits it to the handset, which applies it to its
power output. This causes the handset to have an extremely non-constant power
output. The result is that the received power by the tower which has gone
through the forward transform of the channel is now flat. The phone is in
essence trying to cancel out the effects of the channel by applying an inverse
transform first. This equalization of
power received from each handset allows a system to maximize the capacity of
each cell.
This brings us to another
major benefit of CDMA, higher frequency reuse. In general, most traditional
wireless implementations only allow each cell to use about 1/6 the total
bandwidth allotted to the wireless carrier. This is because a cell cannot use
the same frequencies as the any of the cells directly next to it because they
would interfere with the transmissions in the neighboring cell. Since CDMA
phones have lower power output and because CDMA uses orthogonal codes, even if
it does receive a signal from the neighboring cell in the same frequencies,
when it multiplies the input by its orthogonal chip, all the other signals
cancel out so it's as if they didn't exist. This allows for about twice the
frequency reuse rate of traditional implementations. That means about 1/3 the
total spectrum allotted to the carrier can be used in each cell.
An overall consequence of
the many benefits of CDMA is that there is not a sheer cutoff in the number of
users that it can support unlike TDMA or traditional cellular. In CDMA, the
limiting criteria on the number of users that can be supported, ultimately
depends on how much noise you're willing to tolerate. Since pseudo-random codes
are not in reality perfectly orthogonal, each additional signal added onto the
channel does contribute slightly to the noise level. Eventually if you add
enough calls, the noise level gets too high and impedes efficient
communication. The practical effect of this is the introduction of static into
the voice signal because of the increase in the bit error rate. This means that
the number of calls that can be handled is dependent on how much noise can be
tolerated. This is useful if a carrier decides that they would rather suffer
more static during peak hours in exchange for enabling their network to handle
more calls. This is also beneficial during handoffs from one tower to another.
With traditional cellular and TDMA, if the receiving tower does not have the
capacity for another call, the call must be dropped. With CDMA, the receiving
tower can decide to accept the call at a lower quality and then raise the
quality back up when it is handling fewer calls.
·
Vocoder
The vocoder in CDMA not
only allows for more efficient transmission of the voice signal by using an
adaptive bit rate. It also reduces the background noise in the process. This
occurs because the vocoder sets its data rate thresholds higher depending on
the background noise level. The higher the background noise level, the higher
the threshold is moved. This allows the vocoder to only go to the higher bit
rates when someone is talking into the phone as opposed to when the background
noise level increases. This also has the side benefit of removing a good deal
of ambient noise from the voice signal which improves voice quality.
·
Privacy
and Security
Now that we've seen how
CDMA can provide cheap, clear, and energy efficient wireless communication,
let's look at how CDMA can prevent others from listening in on our
conversation. CDMA by its very nature is more cryptic than both traditional
analog cellular and TDMA. Encoding and decoding CDMA is rather computationally
complex, especially considering that this encoding has to be done by a little
battery powered phone. When the idea of using orthogonal codes first was developed,
it was probably more realistic to view it as an encryption algorithm than a
transmission scheme. This is in stark contrast to analog cellular where all
that's needed is a broad range tuner that's available at Radio Shack. In order
to pick off a CDMA conversation, it is necessary to know the codes being used
which could probably be looked up in a book. Further more a computer is needed
to do the decoding. This might not seem like such a difficult task since it was
just mentioned that decoding is currently done with little battery powered
phones. However, it cannot be overlooked that CDMA handsets implement encoding
and decoding in HARDWARE and someone trying to pick off a CDMA conversation
would have to do the decoding on their home computer in SOFTWARE, hardware
being several orders of magnitude faster than software. As we discovered while
doing this project, although one might think an Ultra 10 is fast, it still
can't do CDMA encoding or decoding in real time (on Matlab, it might have been
able to do it if we wrote the programs in C). Furthermore, we have completely
overlooked the fact that in actuality, when CDMA is used, the digital signal
being transmitted is encrypted. This means that in order pick off a CDMA phone
call, one would have to some how find the encryption key in addition to doing
all the things we've just mentioned. The empirical evidence to the
effectiveness of CDMA's security is that currently, there are few if any
reports of people listening in on other people's conversations or of air-time
fraud like there was when analog cellular was most popular.
©2001 Kyle Bryson, Alison
Chen, and Allen Wan