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