E716_lec06

7th Year, Integrated Technical Education Cluster AlAmeeria‎
lec#6, Mobile Communication Systems

Transcript

1. Lecture #6 Basic Concepts of Cellular Transmission (p3) Instructor: Dr. Ahmad El-Banna November 2014 E-716-A Mobile Communications Systems Integrated Technical Education Cluster At AlAmeeria‎ © Ahmad El-Banna
2. Agenda Duplexing Techniques Call Steps in Cellular Systems Handoff Strategies 2 E-716-A, Lec#6 , Nov 2014 © Ahmad El-Banna
3. DUPLEXING TECHNIQUES 3 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
4. Simplex vs. Duplex • Simplex communication refers to communication that occurs in one direction only. • Duplex communication system is a point-to-point system composed of two connected devices that can communicate with one another in both directions. • There are two types of duplex communication systems: • half-duplex • provides communication in both directions, but only one direction at a time (not simultaneously). • full-duplex • allows communication in both directions to happen simultaneously. 4 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
5. Duplexing Methods • As channel access methods are used in point-to-multipoint networks (such as cellular networks) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods, such as time- division duplexing and frequency-division duplexing. • Time-division duplexing (TDD) is the application of time-division multiplexing to separate outward and return signals. • It emulates full duplex communication over a half duplex communication link. • Frequency-division duplexing (FDD) means that the transmitter and receiver operate at different carrier frequencies. 5 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
6. CALL STEPS IN CELLULAR SYSTEMS 6 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
7. Overview of Cellular System 7 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014 PTSN MTSO BTS MS • Base Station (BS) – includes an antenna, a controller, and a number of receivers • Mobile telecommunications switching office (MTSO) – connects calls between mobile units • Two types of channels available between mobile unit and BS • Control channels • Setting up and maintaining calls • Establish relationship between mobile unit and nearest BS • Traffic channels • Carry voice and data
8. Typical Call in Single MTSO Area • Mobile unit initialization • Scan and select strongest set up control channel • Automatically selected BS antenna of cell • Usually but not always nearest (propagation anomalies) • Handshake to identify user and register location • Scan repeated to allow for movement • Change of cell • Mobile unit monitors for pages • Mobile originated call • Check set up channel is free • Monitor forward channel (from BS) and wait for idle • Send number on pre-selected channel • Paging • MTSO attempts to connect to mobile unit • Paging message sent to BSs depending on called mobile number • Paging signal transmitted on set up channel 8 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
9. Typical Call in Single MTSO Area.. • Call accepted • Mobile unit recognizes number on set up channel • Responds to BS which sends response to MTSO • MTSO sets up circuit between calling and called BSs • MTSO selects available traffic channel within cells and notifies BSs • BSs notify mobile unit of channel • On-going call • Voice/data exchanged through respective BSs and MTSO • Handoff • Mobile unit moves out of range of cell into range of another cell • Traffic channel changes to one assigned to new BS • Without interruption of service to user 9 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
10. Call Stages 10 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
11. Other Functions • Call blocking • During mobile-initiated call stage, if all traffic channels busy, mobile tries again • After number of fails, busy tone returned • Call termination • User hangs up • MTSO informed • Traffic channels at two BSs released • Call drop • BS cannot maintain required signal strength • Traffic channel dropped and MTSO informed • Calls to/from fixed and remote mobile subscriber • MTSO connects to PSTN • MTSO can connect mobile user and fixed subscriber via PSTN • MTSO can connect to remote MTSO via PSTN or via dedicated lines • Can connect mobile user in its area and remote mobile user 11 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
12. HANDOFF STRATEGIES 12 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
13. Handoff/Handover Types • In cellular telecommunications, the term handover or handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. • Types: • Inter-cell handover: the source and the target are different cells (even if they are on the same cell site). • Intra-cell handover: the source and the target are one and the same cell and only the used channel is changed during the handover. • Hard handover: the channel in the source cell is released and only then the channel in the target cell is engaged. • Soft handover: the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. 13 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
14. Handoff Performance Metrics • Cell blocking probability – probability of a new call being blocked • Call dropping probability – probability that a call is terminated due to a handoff • Call completion probability – probability that an admitted call is not dropped before it terminates • Probability of unsuccessful handoff – probability that a handoff is executed while the reception conditions are inadequate • Handoff blocking probability – probability that a handoff cannot be successfully completed • Handoff probability – probability that a handoff occurs before call termination • Rate of handoff – number of handoffs per unit time • Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station • Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur 14 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
15. Handoff Strategies Used to Determine Instant of Handoff • Relative signal strength • Relative signal strength with threshold • Relative signal strength with hysteresis • Relative signal strength with hysteresis and threshold • Prediction techniques 15 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014 • The handoff decision is complicated by the use of power control techniques.
16. Power Control • Design issues making it desirable to include dynamic power control in a cellular system • Received power must be sufficiently above the background noise for effective communication • Desirable to minimize power in the transmitted signal from the mobile • Reduce cochannel interference, alleviate health concerns, save battery power • In spread spectrum systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS • Types of Power Control: • Open-loop power control • Depends solely on mobile unit • No feedback from BS • Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength • Closed-loop power control • Adjusts signal strength in reverse channel based on metric of performance • BS makes power adjustment decision and communicates to mobile on control channel 16 © Ahmad El-Banna E-716-A, Lec#6 , Nov 2014
17. • For more details, refer to: • Chapter 4, J. Chiller, Mobile Communications, 2003. • Chapter 10, W. Stallings, Wireless Communications and Networks, 2005. • The lecture is available online at: • https://speakerdeck.com/ahmad_elbanna • For inquires, send to: • ahmad.elbanna@feng.bu.edu.eg 17 E-716-A, Lec#6 , Nov 2014 © Ahmad El-Banna