Wireless Networking Fundamentals

05

Multiple Access Methods

As it was shown in previous sections, usually simultaneous data transmission by several subscribers leads to collisions, so the time-frequency resource must be somehow distributed among the devices. For this purpose several access methods to the environment can be used:

  • Frequency Division Multiple Access (FDMA);
  • Time Division Multiple Access (TDMA);
  • Spatial Division Multiple Access (SDMA);
  • Code Division Multiple Access (CDMA);
  • Orthogonal Frequency Division Multiple Access (OFDMA);
  • Single Carrier Frequency Division Multiple Access (SC-FDMA);
  • Carrier Sense Multiple Access (CSMA) (particularly, in wireless networks is used carrier sense multiple access with collision detection CSMA/CA).

FDMA

In FDMA systems all devices transmit signals simultaneously. The subscribers isolation is provided by using different frequencies: first subscriber use F1 frequency, second - F2 and so on.

Figure 1 FDMA operation scheme

TDMA

In time division multiple access, each subscriber is allocated a time slot for data transmission. Slots from all subscribers are combined into super-slots, which are cyclically repeated. Thus, each of the subscribers is guaranteed to have access to the media within each super-slot. The advantage of this method is a low jitter, which is important for real-time data transmission, for example, a voice. Such systems are dependent on synchronization because each subscriber must transmit only within the limits of his slot, without using resources of other subscribers. Synchronization can be performed by periodically sending previously known service sequences within one or more super-slots. The best solution is to use external synchronization systems, for example, based on GPS or GLONASS, but this solution requires the installation of additional modules responsible for communication with the external system.

Figure 2 TDMA operation scheme

In case the number of slots exceeds the number of subscribers, the extra time resource can be allocated to subscribers with intensive traffic - a similar case is shown in Figure 3.

Figure 3 Unfilled time slot allocation for subscribers with an intensive data stream

SDMA

As it is shown in the 3. Antennas section, in addition to the frequency and time there is a spatial resource, access to which depends on the characteristics and location of antennas. Thus, due to the spatial selectivity of antennas, multiple access can be organized without additional subscriber diversity by frequency or time.

To make antennas diversity, it is necessary to take into account that their radiation patterns, including radiation of side and back lobes, should not overlap. The location of the antennas in practice depends on the characteristics of site used: mounted on a mast antennas can be spaced-apart vertically, mounted on a building - at different roof corners, etc.

Figure 4 SDMA operation scheme

CDMA

In code division systems, subscriber isolation is not performed at the physical level, as in previous methods, but on the logical: instead of "0" or "1", a direct or inverted sequence is transmitting, which is assigned to each subscriber. Such manipulation expands the original signal spectrum. Unlike systems with time and frequency division, each subscriber receives signals addressed to all subscribers and take out from them one that is addressed to it, using the fixed code sequence. On the receiving side, the signal can be detected, since sequences assigned to each subscriber have a pronounced correlation function.

Figure 5 CDMA operation scheme

OFDMA

In orthogonal frequency division multiple access system, the frequency range is divided into several equal width channels, equal time intervals - timeslots are allocated on the timeline. This frequency-time plane is similar to a chessboard. After that, each subscriber can be allocated separate cells, which are a timeslot in one frequency channel. In case if free resources and intensive subscriber traffic appear, a particular subscriber can be allocated several frequency channels within the time slot. The allocation of resources between subscribers is organized through the exchange of service information, which is carried out in a dedicated frequency channel, or at specific timeslots of a dedicated frequency channel. Thus, each subscriber exchanges service information in the assigned frequency channel / time slot and additionally uses the resources allocated to it to exchange user data.

Figure 6 OFDMA operation scheme

SC-FDMA

In the frequency division multiple access method, same as OFDMA, a grid of time-frequency resources is built on the same carrier, but subscribers are allocated entire timeslots on all subcarriers. Thus, the subscribers are isolated in time. The difference between this multiple access scheme and TDMA is that a low modulation level is used in each frequency channel, that increases the system energy efficiency while maintaining the overall transmission rate due to the plurality of frequency channels.

Figure 7 SC-FDMA operation scheme

CSMA/CA

The carrier sense multiple access with collision detection method is a variation of TDMA, but there is no one resource management center in this system and access to the media is random. The following principles is used to get access to the media:

  • principle of preliminary listening of a carrier (before the transmission starts the device listens to the environment if some stations transmits data, it postpones the transmission process to a random time interval);
  • using a jam-signal (in case the environment is free, the station sends a jam-signal, which signals to other stations about the forthcoming data transmission. After a jam-signal sending the station waits for a time period during which it can receive a jam-signal from another station, and starts transmission);
  • jam-signals detection (in case the station detected a jam-signal from another station during data transmission, it stops the transmitting process).

Polling

Polling is the marker access technology allowing subscriber terminals connect to the medium for data transmission to the base station sector. Unlike CSMA / CA, the principle of polling systems operation is based on the BS sector behaviour which centrally distributing service packs - markers, between subscribers. The sector consistently polls subscriber devices on the availability of data for transmission, by sending them service requests. A subscriber unit can not start data transmission until it has received a service marker. Thus, using of polling allows to increase stability and bandwidth of the channel in conditions of high load and unbalanced signal levels of subscriber terminals: centralized environment management allows to avoid collisions and dynamically allocate resources between subscribers depending on their needs.

Figure 8 Polling operation scheme

In Figure 8 is shown the polling operation scheme of the BS sector with two subscriber terminals - CPE 1 and CPE 2. Firstly, the BS sector sends out a request to subscribers, and receives in response requests for reservation of air-time from the CPE, which have data in the transmission queue. Secondly, the BS sector decides which subscriber terminals should transmit data primarily, according to a number of factors, such as load, quality of communication, etc. In the example, CPE 1 responded first, but the first marker was transmitted to CPE 2, and only then to CPE 1. Note, that markers regulate only the transmission in the uplink, there is no need in marker to transmit data from the sector to subscriber terminals.

In addition the resource can be distributed between the uplink and downlink channels in various proportions, depending on subscribers requirements. This function is available on systems with TDMA and OFDMA:

Figure 9 Source distribution between the uplink and downlink channels

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