24-09-2012, 12:23 PM
Multiple Access Techniques
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Multiple access techniques and protocols
A limited amount of bandwidth is allocated for wireless services. A wireless system is required to accommodate as many users as possible by effectively sharing the limited bandwidth. Therefore, in the field of communications, the term multiple access could be defined as a means of allowing multiple users to simultaneously share the finite bandwidth with least possible degradation in the performance of the system. There are several techniques how multiple accessing can be achieved. They are four basic schemes.
3.2 Type of multiple access techniques
1. Frequency division multiple access (FDMA)
2. Time division multiple access (TDMA)
3. Code division multiple access (CDMA)
In the case of frequency division multiple accesses (FDMA), different Earth stations are able to access the total available bandwidth in satellite transponder(s) by virtue of their different carrier frequencies, thus avoiding interference among multiple signals. The term Should not be confused with frequency division multiplexing (FDM), which is the process of grouping multiple base band signals into a single signal so that it could be transmitted over a single communication channel without the multiple base band signals interfering with each other. Here, multiple base band signals modulate different carrier frequencies called subcarrier frequencies and the multiplexed signal then modulates a common relatively higher frequency carrier, which then becomes the signal to be transmitted from the Earth station. Similarly, other stations may also have similar frequency division multiplexed signals with a different final carrier frequency. These multiplexed signals, by virtue of their different final carrier frequencies, are able to access the satellite simultaneously. In the case of time division multiple access (TDMA), different Earth stations in satellite’s Foot print make use of the transponder by using a single carrier on a time division basis. Again it should not be confused with time division multiplexing (TDM), which is the technique used at a given Earth station to simultaneously transmit digitized versions of multiple base band signals over a common communication channel by virtue of their separation on the timescale. The composite time multiplexed signal modulates a high frequency carrier using any of the digital carrier modulation techniques. Multiple time multiplexed signals from other stations having the same carrier frequency are then able to access the satellite by allowing each station to transmit during its allotted time slot. In the case of code division multiple access (CDMA), the entire bandwidth of the transponder is used simultaneously by multiple Earth stations at all times. Each transmitter spreads its signal over the entire bandwidth, which is much wider than that required by the signal otherwise. One of the ways of doing this is by multiplying the information signal by a pseudorandom bit sequence. Interference is avoided as each transmitter uses a unique code sequence. Receiving stations recover the desired information by using a matched decoder that works on the same unique code sequence used during transmission.
Demand-Assigned FDMA
In the demand-assigned mode of operation, the transponder frequency bandwidth is subdivided into a number of channels. A channel is assigned to each carrier in use, giving rise to the single-channel - per carrier mode of operation discussed in the preceding section. As in the Pre assigned access mode, carriers may be frequency modulated with analog information signals, these being designated FM/SCPC, or they may be phase modulated with digital information signals, these being designated as PSK/SCPC. Demand assignment may be carried out in a number of ways. In the Polling method, a master earth station continuously polls all the earth stations in sequence, and if a call request is encountered, frequency slots are assigned from the pool of available frequencies. The polling delay with such a system tends to become excessive as the number of participating earth stations increases. Instead of using a polling sequence, earth stations may request calls through the master earth station as the need arises. This is referred to as centrally controlled random access. The requests go over a digital order wire, which is a narrowband digital radio link or a circuit through a satellite transponder reserved for this purpose. Frequencies are assigned, if available, by the master station, and when the call is completed, the frequencies are returned to the pool. If no frequencies are available, the blocked call requests may be placed in a queue, or a second call attempt may be initiated by the requesting station. As an alternative to centrally controlled random access, control may be exercised at each earth station, this being known as distributed control random access
Spade System
The word Spade is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment equipment. Spade was developed by Comsat for use on the INTELSAT satellites (see, e.g., Martin, 1978) and is compatible with the INTELSAT SCPC pre assigned system. However, the distributed-demand assignment facility requires a common signaling channel (CSC) this is shown in Fig. 3-2 .The CSC bandwidth is 160 kHz, and its center frequency is 18.045 MHz below the pilot frequency, as shown in Fig. 3-2 to avoid interference with the CSC, voice channels 1 and 2 are left vacant, and to maintain duplex matching, the corresponding channels 1 and 2 are also left vacant. Recalling from Fig. 3-3 that channel 400 also must be left vacant; this requires that channel 800 be left vacant for duplex matching. Thus six channels are removed from the total of 800, leaving a total of 794 one-way or 397 full-duplex voice circuits, the frequencies in any pair being separated by 18.045 MHz, as shown in Fig. 3-2. (An alternative arrangement is shown in Freeman, 1981.)
Bandwidth-Limited and Power-Limited TWT Amplifier Operation
A transponder will have a total bandwidth BTR, and it is apparent
that this can impose a limitation on the number of carriers that can access the transponder in an FDMA mode. For example, if there are K carriers each of bandwidth B, then the best that can be achieved is K _ BTR/B. Any increase in the transponder EIRP will not improve on this, and the system is said to be bandwidth-limited. Likewise, for digital systems, the bit rate is determined by the bandwidth, which again will be limited to some maximum value by BTR
Time Division Multiple Access (TDMA)
In the time division multiple access (TDMA) scheme the time axis is divided into time slots, pre assigned to the different users. Every user is allowed to transmit freely during the slot assigned to it, that is, during the assigned slot the entire system resources are devoted to that user. The slot assignments follow a predetermined pattern that repeats itself periodically; each such period is called a cycle or a frame. In the most basic TDMA scheme every user has exactly one slot in every frame .More general TDMA schemes in which several slots are assigned to one user within a frame, referred to as generalized TDMA, are considered in the next section. Note that for proper operation of a TDMA scheme, the users must be synchronized so that each one knows exactly when and for how long he can transmit.
[attachment=34450]
Multiple access techniques and protocols
A limited amount of bandwidth is allocated for wireless services. A wireless system is required to accommodate as many users as possible by effectively sharing the limited bandwidth. Therefore, in the field of communications, the term multiple access could be defined as a means of allowing multiple users to simultaneously share the finite bandwidth with least possible degradation in the performance of the system. There are several techniques how multiple accessing can be achieved. They are four basic schemes.
3.2 Type of multiple access techniques
1. Frequency division multiple access (FDMA)
2. Time division multiple access (TDMA)
3. Code division multiple access (CDMA)
In the case of frequency division multiple accesses (FDMA), different Earth stations are able to access the total available bandwidth in satellite transponder(s) by virtue of their different carrier frequencies, thus avoiding interference among multiple signals. The term Should not be confused with frequency division multiplexing (FDM), which is the process of grouping multiple base band signals into a single signal so that it could be transmitted over a single communication channel without the multiple base band signals interfering with each other. Here, multiple base band signals modulate different carrier frequencies called subcarrier frequencies and the multiplexed signal then modulates a common relatively higher frequency carrier, which then becomes the signal to be transmitted from the Earth station. Similarly, other stations may also have similar frequency division multiplexed signals with a different final carrier frequency. These multiplexed signals, by virtue of their different final carrier frequencies, are able to access the satellite simultaneously. In the case of time division multiple access (TDMA), different Earth stations in satellite’s Foot print make use of the transponder by using a single carrier on a time division basis. Again it should not be confused with time division multiplexing (TDM), which is the technique used at a given Earth station to simultaneously transmit digitized versions of multiple base band signals over a common communication channel by virtue of their separation on the timescale. The composite time multiplexed signal modulates a high frequency carrier using any of the digital carrier modulation techniques. Multiple time multiplexed signals from other stations having the same carrier frequency are then able to access the satellite by allowing each station to transmit during its allotted time slot. In the case of code division multiple access (CDMA), the entire bandwidth of the transponder is used simultaneously by multiple Earth stations at all times. Each transmitter spreads its signal over the entire bandwidth, which is much wider than that required by the signal otherwise. One of the ways of doing this is by multiplying the information signal by a pseudorandom bit sequence. Interference is avoided as each transmitter uses a unique code sequence. Receiving stations recover the desired information by using a matched decoder that works on the same unique code sequence used during transmission.
Demand-Assigned FDMA
In the demand-assigned mode of operation, the transponder frequency bandwidth is subdivided into a number of channels. A channel is assigned to each carrier in use, giving rise to the single-channel - per carrier mode of operation discussed in the preceding section. As in the Pre assigned access mode, carriers may be frequency modulated with analog information signals, these being designated FM/SCPC, or they may be phase modulated with digital information signals, these being designated as PSK/SCPC. Demand assignment may be carried out in a number of ways. In the Polling method, a master earth station continuously polls all the earth stations in sequence, and if a call request is encountered, frequency slots are assigned from the pool of available frequencies. The polling delay with such a system tends to become excessive as the number of participating earth stations increases. Instead of using a polling sequence, earth stations may request calls through the master earth station as the need arises. This is referred to as centrally controlled random access. The requests go over a digital order wire, which is a narrowband digital radio link or a circuit through a satellite transponder reserved for this purpose. Frequencies are assigned, if available, by the master station, and when the call is completed, the frequencies are returned to the pool. If no frequencies are available, the blocked call requests may be placed in a queue, or a second call attempt may be initiated by the requesting station. As an alternative to centrally controlled random access, control may be exercised at each earth station, this being known as distributed control random access
Spade System
The word Spade is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment equipment. Spade was developed by Comsat for use on the INTELSAT satellites (see, e.g., Martin, 1978) and is compatible with the INTELSAT SCPC pre assigned system. However, the distributed-demand assignment facility requires a common signaling channel (CSC) this is shown in Fig. 3-2 .The CSC bandwidth is 160 kHz, and its center frequency is 18.045 MHz below the pilot frequency, as shown in Fig. 3-2 to avoid interference with the CSC, voice channels 1 and 2 are left vacant, and to maintain duplex matching, the corresponding channels 1 and 2 are also left vacant. Recalling from Fig. 3-3 that channel 400 also must be left vacant; this requires that channel 800 be left vacant for duplex matching. Thus six channels are removed from the total of 800, leaving a total of 794 one-way or 397 full-duplex voice circuits, the frequencies in any pair being separated by 18.045 MHz, as shown in Fig. 3-2. (An alternative arrangement is shown in Freeman, 1981.)
Bandwidth-Limited and Power-Limited TWT Amplifier Operation
A transponder will have a total bandwidth BTR, and it is apparent
that this can impose a limitation on the number of carriers that can access the transponder in an FDMA mode. For example, if there are K carriers each of bandwidth B, then the best that can be achieved is K _ BTR/B. Any increase in the transponder EIRP will not improve on this, and the system is said to be bandwidth-limited. Likewise, for digital systems, the bit rate is determined by the bandwidth, which again will be limited to some maximum value by BTR
Time Division Multiple Access (TDMA)
In the time division multiple access (TDMA) scheme the time axis is divided into time slots, pre assigned to the different users. Every user is allowed to transmit freely during the slot assigned to it, that is, during the assigned slot the entire system resources are devoted to that user. The slot assignments follow a predetermined pattern that repeats itself periodically; each such period is called a cycle or a frame. In the most basic TDMA scheme every user has exactly one slot in every frame .More general TDMA schemes in which several slots are assigned to one user within a frame, referred to as generalized TDMA, are considered in the next section. Note that for proper operation of a TDMA scheme, the users must be synchronized so that each one knows exactly when and for how long he can transmit.