CDMA and TDMA
Q: What is the difference between CDMA and TDMA?
A: These are two very different methodologies use to accomplish the same task, use frequency spectrum much more efficiently than the traditional dedicated fixed frequency transmitting system. The goal is to dramatically increase the number of essentially simultaneous users within a specific portion of radio spectrum. Both methods accomplish this.
CDMA is short for Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. CDMA does not assign a specific frequency for each user placing or receiving a call. Individual conversations are encoded with a pseudo-random digital sequence scheme. The receiving equipment must be able to decode the received signal by having the ability to replicate this pseudo-random digital sequencing.
CDMA is actually a military technology first used during World War II by the English allies to foil German attempts at jamming radio transmissions. The allies transmitted different parts of important information over several frequencies, instead of a single frequency, hence making it considerably more difficult for the Germans to pick up and assimilate the complete signal. Because Qualcomm, Inc., created the communication chips for CDMA technology, it had access to the classified information, and once the information became available to the public, they became the first to commercialize it.
TDMA is short for Time Division Multiple Access, a technology for delivering digital wireless service using time-division multiplexing (TDM). TDMA technology divides a radio frequency into time slots and then allocates these time slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. The receiving equipment must be able to decode the received signal by decoding the received signal and reconstitute it using the same time slot selection algorithm as was used when it was encoded and transmitted. TDMA is used by the GSM digital cellular system.
All rf wireless systems produce a decreasing signal as the distance between the transmitter and the receiver increase. In the simplest model, free space, the doubling the distance reduces the received signal level by the square of the ratio of change in distance. More sophisticated propagation modeling, which is widely used in the industry, will reveal that the relationship is far more volatile than the inverse of the square of the distance. With the use of CDMA and TDMA performance measurements exhibit that the degradation can be to the inverse of third or fourth order, making the proper selection of a desired receiver threshold even more important than with a conventional FM two-way radio modeling.
As an example, in a perfect world a CDMA phone might be able to provide adequate service with a received signal level of -106 dBm, where a phone employing TDMA mgut require -99 dBm of received signal level. However, it would be erroneous to infer that CDMA is always the preferred technology, because it can suitably operate with 7 dB less signal. The real life applications are not perfect. Building penetration will not be the same for both methods. Having the same signal level on both systems does NOT mean that the level of suitability of performance will be equal.
How does all this impact my received signal strength in order to have reliable wireless voice and data links when I use TAP for coverage predictions?
The bottom line is for a digital portable radio or cell phone to have satisfactory no matter what the encryption method, it must have a received signal level sufficient for the information to be preselected and decoded and hence reconstitute the originally transmitted data stream. This means that you must identify the required signal level in either dBm or microvolts that must be present at the receiver for reliable performance. This is the signal level that you must select and plot when producing a radio coverage map. In general the actual threshold needed will be different from a CDMA encoded radio from that which uses TDMA. Due to the characteristics of the encoding, both will likely (but not always) require a higher threshold than a conventional FM modulated radio system assuming everything is properly aligned and tuned. In any event the best method for selecting the level you wish to specify when you wish to plot radio coverage would be to use a threshold specified by the equipment manufacturer and perhaps increase that threshold slightly based on your own personal measurements on a system already in service using the particular technology.
When you decide to go into greater detail and plot different conditions of coverage based on traffic, you will need to specify the pertinent threshold you wish for that particular condition. TAP will allow you to plot multiple thresholds on the same plot and thereby even see where coverage will be degraded as nuances of signal degrade from time to time based on any conditions including traffic as long as you specify the desired threshold ranges you wish to plot.
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