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Multiple access technique Multiple Access is the use of multiplexing techniques to provide communication service to multiple users over a single channel. It allows for many users at one time by sharing a finite amount of spectrum.
Typology: Lecture notes
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(one direction only)
(one direction at a time)
(both directions anytime)
The available bandwidth is subdivided into a number of narrower band channels. Each user is allocated a unique frequency band in which to transmit and receive on.
Wavelength Division Multiple Access (WDMA) is a related to FDMA. Wavelength is used in optical fiber communication systems to partition channels. All wavelengths propagate within a single optical fiber.
Two time slots are shown
time
frequency
power
How is synchronization achieved in TDMA?
GSM uses a combination of both TDMA and FDMA techniques. The FDMA element divides the assigned frequency of 25 MHz bandwidth into 124 carrier frequencies , all spaced 200 kHz apart. The carriers are also divided in time using TDMA. Different users of each RF channel are allocated different time slots (there are 8 time slots per channel).
Time slots
f 1 f 2 f 3 f 4 ๏ ๏ ๏
6 5
BDMA uses multiple radiators to form multiple beams simultaneously in a mobile telephone cell, thus, providing for multiple access.
Also known as โ Spatial Division Multiple Access โ (SDMA)
1/3 of cell
3-sector Base Station Antenna
The CDMA standard was originally designed by Qualcomm in the U.S. and is primarily used in the U.S. and portions of Asia by other carriers.
Salient Features of CDMA:
CDMA is based on the spread spectrum technique
CDMA is the common platform on which 3G technologies are built. It is used by Verizon, and Sprint.
A duplex method whereby the Uplink and the Downlink transmissions use two separate frequency bands โ Uplink โ 1920 MHz to 1980 MHz Downlink โ 2110 MHz to 2170 MHz Bandwidth โ Each carrier located at center in 5 MHz band
CDMA allows up to 61 concurrent users in a 1.2288 MHz channel by processing each voice packet with its PN code. There are 64 Walsh codes available to differentiate between calls. Operational limits and quality issues will reduce the maximum number of calls somewhat lower than this value.
In fact, many different "signals" baseband with different spreading codes can be modulated on the same carrier to allow many different users to be supported. Using different orthogonal codes, interference between the signals is minimal. Conversely, when signals are received from several mobile stations, the base station is capable of isolating each because they have different orthogonal spreading codes.
To continue with CDMA we next discuss spread spectrum techniques.
Spread spectrum is Wideband Modulation and uses a PN code
Primary benefits:
Two approaches:
During World War II, Hedy Lamarr and composer George Antheil realized that radio-controlled torpedoes, which could be important in the naval war, could easily be jammed, thereby causing the torpedo to go off course. With the knowledge she had gained about torpedoes from her first husband, and using a method similar to the way piano rolls work, they drafted designs for a new frequency-hopping, spread-spectrum technology that they later patented.