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Multiple Access Techniques
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.
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Wireless And COMMUNICATION, Lecture notes of Wireless Networking
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
2019/2020
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Multiple Access Techniques
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.
Simplex, Half Duplex and Full Duplex
Simplex
(one direction only)
Half Duplex
(one direction at a time)
Full Duplex
(both directions anytime)
Frequency Division Multiple Access
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.
Time Division Multiple Access
Two time slots are shown
time
frequency
power
How is synchronization achieved in TDMA?
GSM Cellular Uses Both FDMA and 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 ๏ ๏ ๏
GSM Cellular Example: Number of Users Supported by GSM
The uplink band in GSM has a total of 25 MHz of bandwidth and
each radio channel has an assigned bandwidth of 200 kHz. The
number of radio channels ( FDMA ) is
6 5
25 10 Hz
125 channels
channels 2 10 Hz/channel
N
Actually, in practice GSM uses 124 channels (not 125 channels).
Each channel is divided into 8 time slots, so 8 users are allowed
per radio channel ( TDMA ). The maximum number of users is then
Nusers ๏ฝ 124 channels ๏ด 8 users/channel ๏ฝ992users
GSM also has a corresponding downlink band for sending signals to
the mobile phone. For GSM the uplink and downlink frequency
bands are 890 to 915 MHz and 935 to 960 MHz , respectively.
(Europe)
Beam Division Multiple Access (BDMA)
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
Beam Steering in a Phased Array Antenna
Code Division Multiple Access (CDMA) โ I
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
- In CDMA, every channel uses the full available spectrum.
- Individual conversations are encoded with a pseudo- random digital sequence and then transmitted.
- CDMA consistently provides better capacity for voice and data communications, allowing more subscribers to connect at any given time.
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.
Code Division Multiple Access (CDMA) โ II
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.
Why Use Spread Spectrum?
1. Reduced crosstalk and interference
2. Better voice quality/data integrity
3. Lower susceptibility to multipath fading
4. Much improved security with minimum complexity
5. Allows for co-existing signals over a wide bandwidth
6. Within ISM band one can have greater signal power
๏ greater distance
7. Hard to detect it presence
8. Hard to intercept and/or spoof
9. Harder to jam a spread spectrum signal
Spread Spectrum In General
Spread spectrum is Wideband Modulation and uses a PN code
Primary benefits:
- Provides data or message security
- Resistant to interference and jamming
- It allows for band sharing
Two approaches:
- Frequency Hopping Spread Spectrum ( FHSS ) Data is constant but Frequency is pseudo random
- Direct Sequence Spread Spectrum ( DSSS ) Data is randomized Frequency band is constant
- Time Hopping Spread Spectrum ( THSS ) Not as widely used โ not covered here (But most efficient use of bandwidth)
Basic Concept of Frequency Hopping Spread Spectrum
FHSS First Proposed by Hedy Lamarr & G. Antheil
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.