1 k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f c t time frequency code users 1) Frequency Division Multiple Access (FDMA) Total frequency band is divided into sub-frequency bands, each user uses the frequency band allocated for it If the allocated frequency band is not used, bandwidth is wasted and throughput decreases Not appropriate for bursty trafffics that have varying bandwidth requirements The FDMA is usually implemented in narrowband systems. The FDMA is a continuous (analog) transmission scheme The FDMA mobile unit uses duplexers The FDMA requries tight RF filtering to minimize adjacent channel inteference The number of channels that can be simultenously supported is Example: Example: AMPS B t : uplink : MHz, downlink: MHz. B c : each duplex channel BW:60KHz. total 666 duplex channels.

2 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 Each user is assigned same frequency but different time slots either for transmit or receive Different number of time slots can be assigned to each user Due to multipath time delay, guard time is added to the transmission of each user TDMA systems transmit data in buffer-and-burst method. TDMA systems use digital modulation techiniques. N time slots comprise a frame time frequency code kullanıcılar 2) Time Division Multiple Access (TDMA) m: maximum number of TDMA users supported on each radio channel

3 One TDMA Frame Preamble Information Message Trail Slot 1 Slot 2 … … Slot N-1 Slot N Trail bitsSynch. bitsInformation bitsGuard bits efficiency of TDMA: b OH is number of overhead bits per frame b T is total number of bits per frame TDMA frame structure

4 # of reference bursts perframe # of overhead bits per burst # of traffic bursts perframe # of overhead bits per presample in each slot # of equivalent bits in each guard time inteval Bit rateFrame duration Example: Example: GSM system uses a frame structure, 8 time slots, each time slot contains bits R= kbps. a)Time duration of a bit b)Time duration of a slot c)Time duration of a frame d)A user has to wait ms the arrival time of a new frame for its next transmission

5 k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c time frequency code kullanıcılar Mobile users share the same frequency and time Each mobile user’s signal is spread over a large spectrum by multiplication with pseduo-noise (PN) signal No bandwidth limitation. But, CDMA has a soft capacity limit,i.e., capacity is limited by interference level(SINR). There is no absolute limit on the number of users CDMA user complex receiver compared to FDMA and TDMA. RAKE receiver can be used to improve reception by combining multipath signals coherently Self-jamming due non-orthogonality (due to multipath) of PN spreading codes may occur Near-far problem: wrong reception of high power unwanted user’s signal instead of desired user’s signal. So, power control is crucial in CDMA systems Direct sequence spreading or frequency hopped spreading techniques may be used 3) Code Division Multiple Access (CDMA)

6 4) Space Division Multiple Access (SDMA) Mobile users can transmit / receive in the same time, frequency, and code Distinct spatial locations of users are utilized by multiple antennas at the base station Implementation of fully adaptive smart antennas Capacity is larger compared to CDMA,TDMA, and FDMA Frequency Code Time

7 Pure ALOHA Originally developed for ground-based packet radio communications in 1970 Goal: let users transmit whenever they have something to send 1. Transmit whenever you have data to send 2. Listen to the broadcast Because broadcast is fed back, the sending host can always find out if its packet was destroyed just by listening to the downward broadcast one round-trip time after sending the packet 3. If the packet was destroyed, wait a random amount of time and send it again The waiting time must be random to prevent the same packets from colliding over and over again The Pure ALOHA Algorithm Random Access Methods for Mobile Data Networks

8 Note that if the first bit of a new packet overlaps with the last bit of a packet almost finished, both packets are totally destroyed. t : one packet transmission time Vulnerable period: 2t t0t0 t 0 +tt 0 +2tt 0 +3t t Vulnerable Due to collisions and idle periods, pure ALOHA is limited to approximately 18% throughput in the best case Throughput: Pure ALOHA Where G is total offered traffic

9 Slotted ALOHA Slotted ALOHA cuts the vulnerable period for packets from 2t to t. This doubles the best possible throughput from 18.4% to 36.8% Throughput: How? Time is slotted. Packets must be transmitted within a slot The base station transmits a beacon signal for timing, and all MSs synchronize their time slots to this beacon signal The Slotted ALOHA Algorithm 1. If a host has a packet to transmit, it waits until the beginning of the next slot before sending 2. Listen to the broadcast and check if the packet was destroyed 3. If there was a collision, wait a random number of slots and try to send again Example: In GSM system the initial contact between the MS and BS to establish a traffic channel for TDMA voice communication is performed through a random access channel using slotted ALOHA protocol. Other voice oriented systems adopy a similar approach as first step in the registration process of an MS

10 In ALOHA, users do not take into account what other users are doing when they attempt to transmit data packets and there are no mechanisms to avoid collisions A simple method to avoid collisions is to sense the channel before transmission of a packet If there is another user transmitting on the channel, it is obvious that a terminal should delay the transmission of the packet Protocols employing this concept are referred to as CSMA or listen- before-talk (LBT) protocols Using back-off algorithm, a mobile terminal delays its transmission for a later time, therefore reducing packet collision probability significantly compared with ALOHA protocol. However, collision can not be eliminated entirely When two terminals sense the channel busy, they reschedule their packets for a later time Large propagation time between the terminals reduces the effectiveness of carrier sensing in preventing collisions As a result, CSMA is applied in local area applications while ALOHA is preferred for wide area applications Carrier Sense Multiple Access (CSMA) Wireless Random Access Protocols

11 CSMA/CA (Carrier Sense Multiple Access with Collision Avoidence) CSMA/CA (Carrier Sense Multiple Access with Collision Avoidence ) Derived from CSMA/CD (Collision Detection), which is the base of Ethernet. Collision detection is easily performed on a wired network, simply by sensing voltage levels against threshold (with a wire all transmissions have approximately the same strength). Such a simple scheme is not readily applicable to wireless channels because of fading and other propagation channel chracteristics. On a wireless channel the transmitting terminal’s own signal dominates all other signals received in its vicinity, and thus the receiver may fail to recognize the collision and simply retrieve its own signal. So, the protocol can't directly detect collisions like with Ethernet and only tries to avoid them.

12 The protocol starts by listening on the channel (this is called carrier sense), and if it is found to be idle, it sends the first packet in the transmit queue. If it is busy (either another node transmission or interference), the node waits the end of the current transmission and then starts the contention (wait a random amount of time). When its contention timer expires, if the channel is still idle, the node sends the packet. The node having chosen the shortest contention delay wins and transmits its packet. The other nodes just wait for the next contention (at the end of this packet). Because the contention is a random number and done for every packets, each node is given an equal chance to access the channel (on average - it is statistic). RTS: Request To SendCTS: Clear To Send ack: Acknowledgement CSMA/CA (Carrier Sense Multiple Access with Collision Avoidence) CSMA/CA (Carrier Sense Multiple Access with Collision Avoidence )

13 RTS/CTS and hidden nodes in CSMA/CA The hidden node problem comes from the fact that all nodes may not hear each other because the attenuation is too strong between them. Because transmissions are based on the carrier sense mechanism, those nodes ignore each other and may transmit at the same time. before sending a packet, the transmitter sends a RTS and wait for a CTS from the receiver (see figure below). The reception of a CTS indicates that the receiver is able to receive the RTS, so the packet (the channel is clear in its area).