Covers various multiple access schemes in use in mobile systems, mobile phones and local networks.

1. IT 802DMobile CommunicationsPart 2 Debasis Das

2. Some Definitions Base Station: A fixed station that communicates with mobile stations. Located on the center or the edge of a coverage region it has transmitter, a set of allocated radio channels, receiver antenna all on a tower. Control channel: Radio channel for call set up, call request, call initiation and other beacon or control purposes. Froward channel: radio channel for transmission from the base stations to the mobile stations. Handoff: Process of transferring a mobile station from one channel or base station to another. 2/28/2011 Debasis Das Mallabhum Institute of Technology 2

3. Definitions-1 Mobile Station: Stations in use while in motion in a mobile cellular system. Could be installed in a vehicle or a personal unit. Mobile Switching center: Switching center that coordinates the routing of calls in a large service area in a cellular radio system. The MSC connects the cellular base stations and the mobiles to the PSTN. MSCs are also known as mobile telephone switching office or MTSO. Page: A brief message broadcast over the entire service area service area by many base stations. 2/28/2011 Debasis Das Mallabhum Institute of Technology 3

4. Definitions-2 Reverse Channel: Channel for transmission from the mobile stations to the base station. Roamer: A mobile station operating in an area other than the area where services have been subscribed Subscriber: A user paying subscription charges for using the mobile communication services. Transceiver: A device capable of simultaneously transmitting and receiving radio signals. Simplex: One way communication system Half Duplex: System that communicates the same radio channel for forward as well the reverse communication, one at a time. Full Duplex: Systems that can communicate both ways simultaneously. 2/28/2011 Debasis Das Mallabhum Institute of Technology 4

5. Cellular System 2/28/2011 Debasis Das Mallabhum Institute of Technology 5 PSTN MSC/MTSO

6. Call Set Up-1 5% of all channel allocations over a given service area are control channels. All mobiles scan the forward control channels to determine the strongest one and hangs onto that until the signal drops altogether, when it scans again for a strong signal. When a mobile is called the MSC send dispatches the request to all base stations. Mobile Identification number(MIN) of the called number is broadcast as a page over all forward control channels. The called mobile responds by identifying itself over a reverse control channel. The base station relays the ack inform MSC. 2/28/2011 Debasis Das Mallabhum Institute of Technology 6

7. Call Set up-2 MSC instructs the base station to move the call to an unused voice channel within the channel. Typically 10 to 60 voice channels and one control channel are used at a base station. Base station asks the mobile to switch to unused voice forward and reverse channels. Data message, called an alert, is sent over the forward voice channel is sent to make the mobile ring. When the call is in progress the MSC adjusts the transmitted power of the mobile and the switches channel, as needed, to maintain call quality as the subscriber moves through the various cells. Special control signaling is used to voice channels so that the mobile unit may be controlled by the base station and the MSC during call progress. 2/28/2011 Debasis Das Mallabhum Institute of Technology 7

8. Call Set Up-3 When a mobile originates a call, a call initiation request is sent on a reverse control channel The mobile also transmits its telephone number(MIN), the ESN, electronic serial number and the telephone number of the called party. The mobile also transmits a service class mark (SCM) which is an indicator of what the maximum power the mobile can handle. The base station forwards the data to the MSC which in turn makes the connection and asks the base station to switch to unused pair of frequencies 2/28/2011 Debasis Das Mallabhum Institute of Technology 8

9. Roaming The mobile is registered as a “roamer” in the new service area. Every roamer camps on a FCC. Every few minutes the MSC asks over all FCCs all unregistered roamers to provide MIN/ESN information and register via the RCC. On registration MSC requests billing status from the HLR, home location register When status is valid, all call in and out are recorded in the home location for billing 2/28/2011 Debasis Das Mallabhum Institute of Technology 9

10. Reference Model 2/28/2011 Debasis Das Mallabhum Institute of Technology 10 End system End system Application Application Transport Base station Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical Wireless

11. Layer Functionality-1 Physical: data bits converted to signals for the medium, For wireless frequency selection, carrier generation, signal detection, data modulation, encryption. Data Link: accessing the medium, multiplexing data streams, correction of transmission errors, synchronization, responsible for reliable point to point and point to multi point connections. Network: routing of packets, establishing two entity connections, (addressing, routing, device location, handover, etc.) 2/28/2011 Debasis Das Mallabhum Institute of Technology 11

12. Layer Functionality-2 Transport: establish end to end connection, QoS, flow and congestion control, Application: service location, support for multi-media applications, adaptive applications taking care of variations in transmission media, wireless to web access. 2/28/2011 Debasis Das Mallabhum Institute of Technology 12

13. Medium Access Control 2/28/2011 Debasis Das Mallabhum Institute of Technology 13

14. Need for Specialized MAC The well known CSMA/CD does not work It is the transmitter that is trying to detect collision It would not detect collisions at receiver on wireless medium 2/28/2011 Debasis Das Mallabhum Institute of Technology 14

15. Hidden & Exposed Terminals 2/28/2011 Debasis Das Mallabhum Institute of Technology 15 Hidden terminals can cause collisions, while exposed terminals cause delays A C B A is hidden from C C is hidden from A

16. Near and Far Terminals 2/28/2011 Debasis Das Mallabhum Institute of Technology 16 C A B B being near C, will drown out signal from A at C

17. SDMA Each user operates from a different space with different frequencies With mobiles MAXC layer has to decide which base station may be the best for a mobile station Decides which base station is best based on which frequencies, time slot or code is the available SDMA is always used along with some other scheme. Cells with Sectorized antennas could be a SDMA scheme 2/28/2011 Debasis Das Mallabhum Institute of Technology 17

18. FDMA 2/28/2011 Debasis Das Mallabhum Institute of Technology 18 Downlinks are from 96o MHz To 935.2 MHz, 200 KHz deviation per channel, corresponding channel is automatically allocated 124 915 to 890.2 MHz, uplinks 3 2 1

19. TDMA Several schemes possible In a fixed scheme, receivers have fixed time slot, not flexible Dynamic schemes will need MAC addressing as receiver needs to be identified 2/28/2011 Debasis Das Mallabhum Institute of Technology 19

20. TDMA Schemes 2/28/2011 Debasis Das Mallabhum Institute of Technology 20

21. Fixed TDM 2/28/2011 Debasis Das Mallabhum Institute of Technology 21 417 micro Sec 11 12 1 2 3 11 12 1 2 3 Up links Downlinks

22. Fixed RDM Characteristics Fixed allocation of time slots to a channel. MAC is simple, crucial thing is to detect the right time slot at the right moment Fixed pattern can be assigned by a base station. Many digital system use this scheme; IS-34, IS-136, GSM, DECT, PACS, PHS Like most fixed allocation , inefficient use of bandwidth. 2/28/2011 Debasis Das Mallabhum Institute of Technology 22

23. Classical Aloha Neither coordinates medium access, nor does it resolve contentions on MAC. When simultaneous transmissions occur, collisions happen and data is destroyed. Higher levels need to take care of this situation. Maximum throughput is achieved for 18% load 2/28/2011 Debasis Das Mallabhum Institute of Technology 23

24. Slotted Aloha Transmissions are permitted only at fixed times( beginning of a time slot). All stations need to be synchronized Reduces chances of collision Throughput increase from 18% to 36% 2/28/2011 Debasis Das Mallabhum Institute of Technology 24

25. Carrier Sense Multiple Access Transmit only when there is no carrier(no transmission). Probability of collision reduces. Hidden terminals can still cause collisions. Persistent schemes allow transmission with a probability p if the medium is quiet. Back off times can be made proportional to waiting time to introduce some fairness. CSMA/CA and EY-NMPA9as used in HIPERLAN1) 2/28/2011 Debasis Das Mallabhum Institute of Technology 25

26. Demand Assigned Multiple Access Reservation is contention based. No data loss during actual data transmission. DAMA is an explicit reservation scheme Needs periodic synchronization 2/28/2011 Debasis Das Mallabhum Institute of Technology 26 Aloha Reserved

27. Packet Reservation Multiple Access(PRMA) Fixed number of slots form a frame A frame can be reserved Base station transmits status of the slots in a frame to all mobile stations Attempts to use the free slots can be Aloha style Once reservation succeeds, all future slots are for use by the station that got the reservation, until relinquished Guaranteed data rates can be ensured. 2/28/2011 Debasis Das Mallabhum Institute of Technology 27

28. Reservation TDMA N mini slots are followed by N8k data slots Each station is assigned a mini slot for reservation of up to k slots Each station gets a guaranteed data rate and fixed delay The free slots can be used by any station in a round robin scheme or Aloha access Best effort traffic can be sent but without any guarantees. 2/28/2011 Debasis Das Mallabhum Institute of Technology 28

29. Multiple Access with Collision Avoidance(MACA) Is not affected by hidden terminals Senders send “Request to Send(RTS) to the recipient Recipient sends a CTS RTS collisions can still occur, these packets being small, probabilities are lower. Hidden terminal do not cause a problem as the receiver decides which data to accept. Exposed terminals also do not cause problems 2/28/2011 Debasis Das Mallabhum Institute of Technology 29

30. Polling One master stations queries others if they have data to be transmitted. Raise hands and ask question type polling A contention phase followed by polling of the stations that have data to deliver 2/28/2011 Debasis Das Mallabhum Institute of Technology 30

31. Inhibit Sense Multiple Access Also known as Digital sense multiple access Base station send out a busy/idle indicator Any station can transmit when busy tone is not present Not receiving positive ACK indicates a collision. Back off and retransmissions are used in case of collisions. 2/28/2011 Debasis Das Mallabhum Institute of Technology 31

32. CDMA Schemes 2/28/2011 Debasis Das Mallabhum Institute of Technology 32

33. CDMA Requirements The code, chipping sequence, should have very high autocorrelation The codes should be orthogonal Power adjustments need to be made Often at a rate of 1000 times per second Is a problem when the members in the network can be arbitrary, as in ad-hoc networks 2/28/2011 Debasis Das Mallabhum Institute of Technology 33

34. Spread Aloha Multiple Access(SAMA) In the normal course stations access the medium in the narrowband spectrum In case of collisions stations use the same chipping code with a slight phase shift, which brings down the auto-correlation Probability of collision is quite low when number of transmitters are low 2/28/2011 Debasis Das Mallabhum Institute of Technology 34

35. Comparisons 2/28/2011 Debasis Das Mallabhum Institute of Technology 35