Introduction

Wireless communication systems are multiuser systems in which information is transmitted by means of radio waves. The fundamental consideration is that the radio channel is basically a broadcast medium, hence the transmitted signal is available almost to all receivers. This inherent high connectivity causes high level of interference to the desired signal and some sort of access coordination has to be exercised for successful communication. Channel access coordination and methods of sharing a radio channel among multiple users are essential to the successful operation of the wireless network of any topology. Access coordination can be achieved either by insulating the signals coming from various users of the same access medium or by allowing the users to compete for the access medium. Combining these two approaches is also possible.

Signal insulation is easily attainable in fixed-assignment access method where the channel resources (frequency or time, or both) are allocated to each user on a predetermined basis. The three such access methods are FDMA, TDMA and CDMA. For stream, steady-flow traffic, the available channel resources are very efficiently utilized if allotted on a fixed access basis.

The frequency-division multiple-access (FDMA) technique is built upon the well known frequency-division multiplexing (FDM) scheme. The system signal domain is sliced into frequency bands. Each user has use of an assigned band for an unlimited amount of time. The time-division multiple-access (TDMA) scheme is based on a time-division multiplexing (TDM). Signal insulation in the time domain is achieved by allowing one user to use the frequency band during a specific period of time named time slot. Nonoverlapping time slots constitute the orthogonal channels.

In wireless systems it is often desirable to accomplish simultaneously the forward and reverse directions of communication (e.g. to talk and listen simultaneously). Duplexing may be done using frequency (FDD) or time (TDD) domain duplexing techniques. There are possible several combinations of FDMA, TDMA and FDD, TDD techniques.

Signal insulation can also be achieved by assigning each signal a different code word (e.g. PN sequence) almost orthogonal to code words assigned to other signals. With Code Division Multiple Access (CDMA), multiple users operate simultaneously over the entire time and frequency signal domain. The two CDMA techniques discussed in this course are direct sequence CDMA (DS-CDMA) and frequency hopping CDMA (FH-CDMA).

For intermittent, burst (random) traffic a good efficiency can be achieved if the available resources are seized on a random access basis. Fixed-assignment access methods can result in communication resources being wasted much of the time. The random access methods make use of a contention scheme in which no (or little) coordination is provided. Typically the terminal sending the packet waits for the confirmation of the successful reception of it. This acknowledgment mechanism is used to detect collision. The most commonly used random access methods are pure ALOHA, slotted ALOHA, CSMA (nonslotted, slotted, 1-persistent, non-persistent, p-persistent) and CSMA/CA. CSMA is sometimes called listen-before-talk protocol.

The use of controlled-access methods is recommended if the traffic is random (burst), a high incidence of packet collision occurs and we want to impose a discipline on the network of the independent users. The access methods in this category make use of a deterministic or contentionless strategy in which a control signal gives permission to send to terminals individually. Only one terminal is allowed to access the medium at a time, when a controlled-access method is applied. In this category two access schemes described here are reservation-ALOHA (R-ALOHA) and polling.

The R-ALOHA scheme is a combination of slotted ALOHA and TDM protocols. In the reserved mode the time axis is divided into equal-length slots of two kinds. There are message slots and much shorter reservation subslots. Reservation of the message slots for a particular user can take place only in reservation subslots. They are vulnerable to collisions. If there are no reserved message slots the system switches to the unreserved mode in which there are only short reservation subslots.

The polling technique utilizes very centralized control. One station in the network periodically polls all the other stations to determine if they have data to transmit. Polling requires the constant exchange of control message between the controller and the terminals.