Channel access method
In
A channel access method is based on multiplexing, that allows several data streams or signals to share the same communication channel or transmission medium. In this context, multiplexing is provided by the physical layer.
A channel access method may also be a part of the multiple access protocol and control mechanism, also known as
Fundamental schemes
Several ways of categorizing multiple-access schemes and protocols have been used in the literature. For example, Daniel Minoli (2009) categorize the protocols into Conflict-free access protocols, Aloha protocols, and Carrier Sensing protocols.
The Telecommunications Handbook (Terplan and Morreale, 2000)[4] identifies the following MAC categories:
- Fixed assigned: TDMA, FDMA+WDMA, CDMA, SDMA
- Demand assigned (DA)
- Reservation: DA/TDMA, DA/FDMA+DA/WDMA, DA/CDMA, DA/SDMA
- Polling: Generalized polling, Distributed polling, Token Passing, Implicit polling, Slotted access
- Random access (RA): Pure RA (ALOHA, GRA), Adaptive RA (TRA), CSMA, CSMA/CD, CSMA/CA
Channel access schemes generally fall into the following categories.[1][5][6]
Frequency-division multiple access
The
A related technique is wavelength division multiple access (WDMA), based on wavelength-division multiplexing (WDM), where different data streams get different colors in fiber-optical communications. In the WDMA case, different network nodes in a bus or hub network get a different color.[7]
An advanced form of FDMA is the orthogonal frequency-division multiple access (OFDMA) scheme, for example, used in 4G cellular communication systems. In OFDMA, each node may use several sub-carriers, making it possible to provide different quality of service (different data rates) to different users. The assignment of sub-carriers to users may be changed dynamically, based on the current radio channel conditions and traffic load. Single-carrier FDMA (SC-FDMA), a.k.a. linearly-precoded OFDMA (LP-OFDMA), is based on single-carrier frequency-domain-equalization (SC-FDE).
Time-division multiple access
The time-division multiple access (TDMA) channel access scheme is based on the time-division multiplexing (TDM) scheme. TDMA provides different time slots to different transmitters in a cyclically repetitive frame structure. For example, node 1 may use time slot 1, node 2 time slot 2, etc. until the last transmitter when it starts over. An advanced form is dynamic TDMA (DTDMA), where an assignment of transmitters to time slots varies on each frame.
Code-division multiple access and spread spectrum multiple access
The code-division multiple access (CDMA) scheme is based on spread spectrum, meaning that a wider radio channel bandwidth is used than the data rate of individual bit streams requires, and several message signals are transferred simultaneously over the same carrier frequency, utilizing different spreading codes. Per the Shannon–Hartley theorem, the wide bandwidth makes it possible to send with a signal-to-noise ratio of much less than 1 (less than 0 dB), meaning that the transmission power can be reduced to a level below the level of the noise and co-channel interference from other message signals sharing the same frequency range.
One form is
Another form is
Other techniques include OFDMA and multi-carrier code-division multiple access (MC-CDMA).
Space-division multiple access
Power-division multiple access
Power-division multiple access (PDMA) scheme is based on using variable transmission power between users in order to share the available power on the channel. Examples include multiple
Packet mode methods
Packet mode channel access methods select a single network transmitter for the duration of a packet transmission. Some methods are more suited to wired communication while others are more suited to wireless.[1]
Common statistical time-division multiplexing multiple access protocols for wired multi-drop networks include:
- Carrier-sense multiple access with collision detection (CSMA/CD), used in Ethernet and IEEE 802.3
- Multiple Access with Collision Avoidance(MACA)
- Multiple Access with Collision Avoidance for Wireless (MACAW)
- Carrier-sense multiple access (CSMA)
- Carrier-sense multiple access with collision avoidance and resolution using priorities (CSMA/CARP)
- Bitwise Arbitration based on constructive interference as used on CAN bus
- Token bus(IEEE 802.4)
- Token Ring (IEEE 802.5)
- FDDI
- Dynamic time-division multiple access(Dynamic TDMA)
Common multiple access protocols that may be used in packet radio wireless networks include:
- WiFi, potentially using a distributed coordination function
- ALOHA and slotted ALOHA, used in ALOHAnet
- Reservation ALOHA(R-ALOHA)
- Mobile Slotted Aloha (MS-ALOHA)
- Code-division multiple access (CDMA)
- Orthogonal frequency-division multiple access (OFDMA)
- Orthogonal frequency-division multiplexing (OFDM)
Duplexing methods
Where these methods are used for dividing forward and reverse communication channels, they are known as
- Time-division duplex(TDD)
- Frequency-division duplex(FDD)
- Echo cancellation
Hybrid application examples
Note that hybrids of these techniques are frequently used. Some examples:
- The GSM cellular system combines the use of frequency-division duplex (FDD) to prevent interference between outward and return signals, with FDMA and TDMA to allow multiple handsets to work in a single cell.
- dynamic TDMAscheme for transferring the actual data.
- CSMA/CAfor shared channel access within a network.
- CSMA/CAfor multiple access within the cell.
- packet scheduling.
- CSMA/CARPto allow multiple devices to share the medium.
Application-specific definitions
Different channel access constraints and schemes apply to different applications.
Local and metropolitan area networks
In
Satellite communications
In
Cellular networks
In
The origins of CDMA can be traced back to the 1940s where it was patented by the United States government and used throughout World War II to transmit messages. However, following the war the patent expired and the use of CDMA diminished and was widely replaced by TDMA.[9] That was until Irwin M. Jacobs an MIT engineer, and fellow employees from the company Linkabit founded the telecommunications company Qualcomm.[10] At the time Qualcomm was founded, Jacobs had already been working on addressing telecommunications problems for the military using digital technology to increase the capacity of spectrum.[11] Qualcomm knew that CDMA would greatly increase the efficiency and availability of wireless, but the wireless industry having already invested millions of dollars into TDMA was skeptical.[11] Jacobs and Qualcomm spent several years improving infrastructure and performing tests and demonstrations of CDMA. In 1993, CDMA became accepted as the wireless industry standard. By 1995, CDMA was being used commercially in the wireless industry as the foundation of 2G.[9]
See also
- Diversity scheme
- Dynamic bandwidth allocation
- Radio resource management for inter-base station interference control
References
- ^ ISBN 978-1107143210.
- ISBN 978-1-4200-7868-8. Retrieved 1 June 2012.
- .
- ISBN 978-0-8493-3137-4. Retrieved 1 June 2012.
- ^ "Fundamentals of Communications Access Technologies: FDMA, TDMA, CDMA, OFDMA, AND SDMA". Electronic Design. 2013-01-22. Retrieved 2014-08-28.
- ISBN 9781420037401.
- ^ Sadique, Abubaker. "Multiple Access Techniques in communication: FDMA, TDMA, CDMA". Archived from the original on 2019-10-09.
- ^ Elinav, Doron; Rubin, Mati E.; Brener, Snir (Mar 6, 2014), Power Division Multiple Access, retrieved 2016-06-29
- ^ a b c Qualcomm, Qualcomm. "The world-changing technology that almost wasn't". Qualcomm.
- ISSN 0099-9660. Retrieved 2019-12-03.
- ^ ISBN 978-0-8144-2858-0.