There are three basic principles in multiple access, FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access). All three principles allow multiple users to share the same physical channel.
Time Division Multiple Access (TDMA)
Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using his own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity
Ref: http://www.all-satcom.com/gen2.aspx?mid=16&mcid=3&pt=TDMA%20Solutions
TDMA is utilized by Digital-Advanced Mobile Phone System (D-AMPS) and Global System for Mobile communications (GSM). However, each of these systems implements TDMA in a somewhat different and incompatible way.
Each user is allowed to transmit only within specified time intervals (Time Slots). Different users transmit in differents Time Slots. When users transmit, they occupy the whole frequency bandwidth (separation among users is performed in the time domain).
Advantages of TDMA
There are lots of advantages of TDMA in cellular technologies. It can easily adapt to transmission of data as well as voice communication. It has an ability to carry 64 kbps to 120 Mbps of data rates. This allows the operator to do services like fax, voice band data, and SMS as well as bandwidth-intensive application such as multimedia and
videoconferencing. Since TDMA technology separates users according to time, it ensures that there will be no interference from simultaneous transmissions. It provides users with an extended battery life, since it transmits only portion of the time during conversations
Disadvantages of TDMA
One major disadvantage using TDMA technology is that the users has a predefined time slot. When moving from one cell site to other, if all the time slots in this cell are full the user might be disconnected. Likewise, if all the time slots in the cell in which the user is currently in are already occupied, the user will not receive a dial tone.
Frequency Division Multiple Access (FDMA)
With Frequency Division Multiple Access (FDMA), different signals are assigned frequency channels. A channel is a frequency. FDMA is a basic technology in the analog Advanced Mobile Phone System (AMPS). With FDMA, each channel can be
assigned to only one user at a time.
Each user transmits with no limitations in time, but using only a portion of the whole available frequency bandwidth. Different users are separated in the frequency domain.
Frequency division is very simple: all transmitters sharing the medium have output power spectra in non-overlapping bands. Many of the problems experienced in TDMA due to different propagation delays are eliminated in FDMA.
Ref: http://comunicacionesvsat.wordpress.com/2010/11/11/analisis-telematico-de-la-red-vsat/
The major disadvantage of FDMA is the relatively expensive and complicated bandpass filters required. TDMA is realized primarily with much cheaper logic functions. Another disadvantage of FDMA is the rather strict linearity requirement of the medium.
Code Division Multiple Access (CDMA)
In Code Division Multiple Access (CDMA), each user is assigned a different pseudorandom binary sequence that modulates the carrier, spreading the spectrum of the waveform and giving each user a unique code pattern. CDMA refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. This technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands. The receiver, knowing the code sequence of the user, decodes the received signal and recovers the original data.
Ref: http://www.umtsworld.com/technology/cdmabasics.htm
If multiple users transmit a spread-spectrum signal at the same time, the receiver will still be able to distinguish between users, provided that each user has a unique code that has a sufficiently low crosscorrelation with the other codes.
Advantages of CDMA
One of the main advantages of CDMA is that dropouts occur only when the phone is at least twice as far from the base station. Thus it is used in the rural areas where GSM cannot cover. Another advantage is its capacity; it has a very high spectral capacity that it can accommodate more users per MHz of bandwidth.
Disadvantages of CDMA
One major problem in CDMA technology is channel pollution, where signals from too many cell sites are present in the subscriber’s phone but none of them is dominant. When this situation arises the quality of the audio degrades. Another disadvantage in this technology when compared to GSM is the lack of international
roaming capabilities.
Comparison of FDMA, TDMA and CDMA
Reference
http://acts.ing.uniroma1.it/courses/comelet/Slides/20071217_TEL_lecture_2.pdf
http://www.infosecwriters.com/text_resources/pdf/TDMA_versus_CDMA_SNatarajan.pdf
http://whitepapers.hackerjournals.com/wp-content/uploads/2010/04/FDMA-vs.-TDMA-vs.-CDMA.pdf
