Mobile Computing. Time Division Multiple Access (Tdma)

5292 words - 22 pages

* TIME DIVISION MULTIPLE ACCESS (TDMA)
Compared to FDMA, time division multiple access (TDMA) offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication, i.e., the receiver can stay at the same frequency the whole time. Using only one frequency, and thus very simple receivers and transmitters, Synchronization between sender and receiver has to be achieved in the time domain. Again this can be done by using a fixed pattern similar to FDMA
techniques, i.e., allocating a certain time slot for a channel, or by using a dynamic allocation scheme. Dynamic allocation schemes require an identification for each transmission as this is the ...view middle of the document...

These schemes typically have a reservation period followed by a transmission period. During the reservation period, stations can reserve future slots in the transmission period. While, depending on the scheme, collisions may occur during the reservation period, the transmission period can then be accessed without collision. Alternatively, the transmission period can be split into periods with and without
collision. In general, these schemes cause a higher delay under a light load (first the reservation has to take place), but allow higher throughput due to less collisions. One basic scheme is demand assigned multiple access (DAMA) also called reservation Aloha, a scheme typical for satellite systems.
Multiple accesses with collision avoidance (MACA) present a simple
Scheme that solves the hidden terminal problem, With MACA, A does not start its transmission at once, but sends a request to send (RTS) first. B receives the RTS that contains the name of sender and receiver, as well as the length of the future transmission. This RTS is not heard
by C, but triggers an acknowledgement from B, called clear to send (CTS). The CTS again contains the names of sender (A) and receiver (B) of the user data, and the length of the future transmission. This CTS is now heard by C and the medium for future use by A is now reserved for the duration of the transmission. After receiving a CTS, C is not allowed to send anything for the duration indicated in the CTS toward B. A collision cannot occur at B during data transmission, and the hidden terminal problem is solved Still, collisions can occur during the sending of an RTS. Both A and C could send an RTS that collides at B. RTS is very small compared to the data transmission, so the probability of a collision is much lower. B resolves this contention and acknowledges only one station in the CTS (if it was able to recover the RTS
at all). No transmission is allowed without an appropriate CTS.
Polling: Where one station is to be heard by all others polling schemes (known from the mainframe/terminal world) can be applied. Polling is a strictly centralized scheme with one master station and several slave stations. The master can poll the slaves according to many schemes: round robin randomly, according to reservations etc. The master could also establish a
list of stations wishing to transmit during a contention phase. After this phase, the station polls each station on the list.
* Explain Code division multiple access (CDMA)??
Code division multiple access (CDMA) systems use exactly these codes to separate different users in code space and to enable access to a shared medium without interference. The main problem is how to find “good” codes and how to separate the signal from noise generated by other signals and the environment. A code is called good code if it has good autocorrelation and orthogonal properties. Two vectors are called orthogonal if their inner product is 0, as is the case for the...

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