Communication Switching Techniques

Transcription

1 Communication Switching Techniques UNIT 5 P.M.Arun Kumar, Assistant Professor, Department of IT, Sri Krishna College of Engineering and Technology, Coimbatore.

2 PRINCIPLES OF CELLULAR NETWORKS TOPICS TO BE COVERED CELLULAR NETWORK ORGANISATION (FREQUENCY REUSE) OPERATION OF CELLULAR SYSTEMS2 MOBILE RADIO PROPOGATION EFFECTS HANDOFF POWER CONTROL TRAFFIC ENGINEERING CHANNEL ASSIGNMENT STRATEGIES References 1. Wireless communication and networks william stallings, Pearson education,

3 Mobile Radio Environment

4 Mobile Radio Environment The transmissions over the wireless link are in general very difficult to characterize. EM signals often encounter obstacles, causing reflection, diffraction, and scattering. Mobility introduces further complexity. We have focused on simple models to help gain basic insight and understanding of the wireless radio medium. Three main components: Path Loss, Shadow fading, Multipath fading (or fast fading).

5 Limitations of Wireless Channel is unreliable Spectrum is scarce, and not all ranges are suitable for mobile communication Transmission power is often limited Battery Interference to others

6 Early Mobile Telephone Services First introduced in the U.S. by AT&T (1946) Used to interconnect mobile users (in automobiles) to telephone networks. A single powerful transmitter from the BS to cover up to approx. 50 miles radius. Few channels for many people Early Bell Mobile Phone service in New York had 12 channels, serving 543 customer, waiting list of 3,700 and market of 10 million!! - CAPACITY LIMITED Advanced systems for their time but very inefficient, and service was terrible (blocking probabilities as high as 65%).

7 Advent of Cellular Systems Noting from the channel model, we know signal will attenuated with distance and have no interference to far users. In the late 1960s and early 1970s, work began on the first cellular telephone systems. The term cellular refers to dividing the service area into many small regions (cells) each served by a low-power transmitter with moderate antenna height.

8 Cellular Networks

9 Principles of Cellular Networks Underlying technology for mobile phones, personal communication systems, wireless networking etc. Developed for mobile radio telephone Replace high power transmitter/receiver systems Typical support for 25 channels over 80km Use lower power, shorter range, more transmitters

10 Cellular Network Organization Multiple low power transmitters 100w or less Area divided into cells Each with own antenna Each with own range of frequencies Served by base station Transmitter, receiver, control unit Adjacent cells on different frequencies to avoid crosstalk

11 Shape of Cells Square Width d cell has four neighbors at distance d and four at distance d 2 Better if all adjacent antennas equidistant Simplifies choosing and switching to new antenna Hexagon Provides equidistant antennas Radius defined as radius of circum-circle Distance from center to vertex equals length of side 3 Distance between centers of cells radius R is R Not always precise hexagons Topographical limitations Local signal propagation conditions Location of antennas

12 Cellular Geometries

13 Frequency Reuse

14 Frequency Reuse The Objective is to use the same frequency band in multiple cells at some distance from one another. the reuse of frequencies is what enables a cellular system to handle a huge number of calls with a limited number of channels.

15 Frequency reuse

16 Frequency Reuse Power of base transceiver controlled Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Use same frequency for multiple conversations frequencies per cell E.g. N cells all using same number of frequencies K total number of frequencies used in systems Each cell has K/N frequencies Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 frequencies per cell on average

17 Characterizing Frequency Reuse D = minimum distance between centers of cells that use the same band of frequencies (called cochannels) R = radius of a cell d = distance between centers of adjacent cells (d = R) N = number of cells in repetitious pattern Reuse factor Each cell in pattern uses unique band of frequencies Hexagonal cell pattern, following values of N possible N = I 2 + J 2 + (I x J), I, J = 0, 1, 2, 3, Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, D/R= D/d = 3N N

18 Frequency Reuse Patterns

19 Frequency Reuse Frequency Reuse is the core of cellular mobile radio systems. A radio channel using a Frequency f 1 in a Cell with a Radius R can be reused at Distance D. Users in both cells can use the same frequency simultaneously. Improper system planning & design can cause unacceptable level of Co-channel Interference.

20 Frequency Reuse Concept R f 1 undesired signal f 1 co-channel interference D R desired signal with the concept of Frequency Reuse comes the term Co-channel Interference

21 Ideal Cells Formation Base Station

22 Real Cell Formation Base Station

23 Fictitious Cells Formation Base Station

24 Frequency Reuse Pattern N = Note: some textbooks use K instead of N

25 Frequency Reuse Pattern N=

26 Frequency Reuse Pattern N=

27 Frequency Reuse Pattern N=

28 Cellular Coverage Representation

29 3-cell reuse pattern (i=1,j=1)

30 4-cell reuse pattern (i=2,j=0)

31 7-cell reuse pattern (i=2,j=1)

32 12-cell reuse pattern (i=2,j=2)

33 19-cell reuse pattern (i=3,j=2)

34 Cellular geometry Cluster Size Co channel distance in unit radius Co channel reuse ratio i2+j2+i*j sqrt (i2+j2+i*j) D/R=sqrt (3*N) where I and j are integers i j cluster size Co channel reuse ratio

35 19-cell reuse example (N=19) To find the co channel interference: Method for locating co channel Cell

36 Relationship between Q and N

37 Increasing Capacity (1) Add new channels Not all channels used to start with Frequency borrowing Taken from adjacent cells by congested cells Or assign frequencies dynamically Cell splitting Non-uniform distribution of topography and traffic Smaller cells in high use areas Original cells km 1.5 km limit in general More frequent handoff More base stations

38 Increasing Capacity (2) Cell Sectoring Cell divided into wedge shaped sectors 3 6 sectors per cell Each with own channel set Subsets of cell s channels Directional antennas Microcells Move antennas from tops of hills and large buildings to tops of small buildings and sides of large buildings Even lamp posts Form microcells Reduced power Good for city streets, along roads and inside large buildings

39 Cell Splitting

40 Operation of Cellular Systems

41 Operation of Cellular Systems Base station (BS) at center of each cell Antenna, controller, transceivers Controller handles call process Number of mobile units may in use at a time BS connected to mobile telecommunications switching office (MTSO) One MTSO serves multiple BS MTSO to BS link by wire or wireless MTSO: Connects calls between mobile units and from mobile to fixed telecommunications network Assigns voice channel Performs handoffs Monitors calls (billing) Fully automated

42 Overview of Cellular System

43 Channels Control channels Setting up and maintaining calls Establish relationship between mobile unit and nearest BS Traffic channels Carry voice and data

44 Typical Call in Single MTSO Area (1) 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 (see below) 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

45 Typical Call in Single MTSO Area (2) 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 Ongoing 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

46 Call Stages

47 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

48 Mobile Radio Propagation Effects

49 Mobile Radio Propagation Effects Signal strength Strength of signal between BS and mobile unit strong enough to maintain signal quality at the receiver Not strong enough to create too much cochannel interference Noise varies Fading Automobile ignition noise greater in city than in suburbs Other signal sources vary Signal strength varies as function of distance from BS Signal strength varies dynamically as mobile unit moves Even if signal strength in effective range, signal propagation effects may disrupt the signal

50 Fading Time variation of received signal Caused by changes in transmission path(s) E.g. atmospheric conditions (rain) Movement of (mobile unit) antenna

51 Multipath Propagation Reflection Surface large relative to wavelength of signal May have phase shift from original May cancel out original or increase it Diffraction Edge of impenetrable body that is large relative to wavelength May receive signal even if no line of sight (LOS) to transmitter Scattering Obstacle size on order of wavelength Lamp posts etc. If LOS, diffracted and scattered signals not significant Reflected signals may be If no LOS, diffraction and scattering are primary means of reception

52 Reflection, Diffraction, Scattering

53 Effects of Multipath Propagation Signals may cancel out due to phase differences Intersymbol Interference (ISI) Sending narrow pulse at given frequency between fixed antenna and mobile unit Channel may deliver multiple copies at different times Delayed pulses act as noise making recovery of bit information difficult Timing changes as mobile unit moves Harder to design signal processing to filter out multipath effects

54 Types of Fading Fast fading Rapid changes in strength over distances about half wavelength 900MHz wavelength is 0.33m 20-30dB Slow fading Slower changes due to user passing different height buildings, gaps in buildings etc. Over longer distances than fast fading Flat fading Nonselective Affects all frequencies in same proportion Selective fading Different frequency components affected differently

55 Handoff (or) Handover

56 Handoff When a mobile user travels from one area of coverage or cell to another cell within a call s duration the call should be transferred to the new cell s base station. Otherwise, the call will be dropped because the link with the current base station becomes too weak as the mobile recedes. Indeed, this ability for transference is a design matter in mobile cellular system design and is call handoff.

57 Handoff

58 Handoff Types- Hard Handoff Soft Handoff

59 Hard Handoff the link to the prior base station is terminated before or as the user is transferred to the new cell s base station the mobile is linked to no more than one base station at a given time Initiation of the handoff may begin when the signal strength at the mobile received from base station 2 is greater than that of base station 1

60 Hard Handoff Hard handoff is used by the systems which use timedivision multiple access (TDMA) and frequency division multiple access (FDMA) such as GSM and General PacketRadio Service (GPRS)

61 Handover types in GSM Intra Cell Handover Inter Cell, intra BSC handover Inter BSC, Intra MSC handover Inter MSC handover

62 Handoff in GSM

63 Handoff in GSM Intra Cell Handover : This happens when within a cell, when narrowband interference could make transmission at a certain frequency impossible. The BSC could then decide to change the carrier frequency. (1) Inter Cell, intra BSC handover : This type of handover is a typical handover within the GSM system and occurs when the MS moves from one BTS to another but stays within the control of same BSC. The BSC performs the handover and assigns a new radio channel in the new BTS, then releases the old BTS. (2)

64 Inter BSC, Intra MSC handover : Since a BSC controls a limited number of BTSs, the GSM system has to perform handovers between BSCs. This form of handover is controlled by the MSC. (3) Inter MSC handover : A handover could also be required etween two BTSs that belong to two different MSCs, now both MSCs perform the handover together.(4)

65 Soft Handoff CDMA uses soft handoff improves performance by using macro diversity In a CDMA system with soft handoff, each mobile user is connected to two or more base stations at a time.

66 Soft Handoff The base station with the highest relative strength seen from the mobile is given the control of the mobile user s call. Also, because a user in soft handoff is connected to several adjacent base stations, probability of a lost call is reduced.

67 Handoff Performance Metrics 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

68

69 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

70 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 SS systems using CDMA, it s desirable to equalize the received power level from all mobile units at the BS

71 Traffic Engineering Ideally, available channels would equal number of subscribers active at one time In practice, not feasible to have capacity handle all possible load For N simultaneous user capacity and L subscribers L < N nonblocking system L > N blocking system

72 Channel assignment strategies

73 Channel assignment strategies Fixed Dynamic Fixed channel assignment Each cell is allocated a predetermined set of voice channels. Any call attempt within the cells can only be served by unused channels in that particular cell. If all the channels in the cell are occupied, the call is blocked and the subscriber does not receive service. Variation includes a borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied. This is supervised by the MSC. Dynamic channel assignment The voice channels are not allocated to different cells permanently. instead each time a call request is made, the serving base station request a channel from the mobile switching center. Dynamic channel assignment is more complex (real time), but reduces likelihood of blocking.

74 Channel assignment What is channel allocation? A given radio spectrum is to be divided into a set of disjointed channels that can be used simultaneously while minimizing interference in adjacent channel by allocating channels appropriately (especially for traffic channels). Channel allocation schemes can be divided in general into Fixed Channel Allocation schemes (FCA schemes); Dynamic Channel Allocation schemes (DCA schemes); Hybrid Channel Allocation schemes (HCA schemes: combining both FCA and DCA techniques);

75 Fixed Channel Allocation (FCA) In FCA schemes, a set of channels is permanently allocated to each cell in the network. If the total number of available channels in the system S is divided into sets, the minimum number of channel sets N required to serve the entire coverage area is related to the frequency reuse distance D as follows: N = D 2 / 3R 2 Due to short term fluctuations in the traffic, FCA schemes are often not able to maintain high quality of service and capacity attainable with static traffic demands. One approach to address this problem is to borrow free channels from neighboring cells.

76 Simple Borrowing (CB) Schemes In CB schemes, cell (acceptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls. Borrowing can be done from an adjacent cell which has largest number of free channels (borrowing from the richest) Select the first free channel found for borrowing using a search algorithm (borrow first available scheme) Return the borrowed channel when channel becomes free in the cell (basic algorithm with reassignment) To be available for borrowing, the channel must not interfere with existing calls, as shown in the next figure.

77 Simple Channel Borrowing (CB) Schemes Donor Cell for Sector X 1 Cell 3 2 X Y Z A call initiated in the sector X of cell 3 can borrow a channel from adjacent cells 1 or 2.

78 Impact of Channel Borrowing in Sectored Cell-based Wireless System A 7 c a A 2 b c b a A 6 c b a A 1 c a x A 3 b c b a A 5 c b a A 4 c b a X borrows some channels from a

79 Dynamic Channel Allocation (DCA) In DCA schemes, all channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system. After each call is completed, the channel is returned to the central pool. It is fairly straightforward to select the most appropriate channel for any call based simply on current allocation and current traffic, with the aim of minimizing the interference. DCA scheme can overcome the problem of FCA scheme. However, variations in DCA schemes center around the different cost functions used for selecting one of the candidate channels for assignment.

80 Dynamic Channel Allocation (DCA) DCA schemes can be centralized or distributed. The centralized DCA scheme involves a single controller selecting a channel for each cell; The distributed DCA scheme involves a number of controllers scattered across the network (MSCs). Centralized DCA schemes can theoretically provide the best performance. However, the enormous amount of computation and communication among BSs leads to excessive system latencies and renders centralized DCA schemes impractical. Nevertheless, centralized DCA schemes often provide a useful benchmark to compare practical decentralized DCA schemes.

81 Centralized DCA For a new call, a free channel from the central pool is selected that would maximize the number of members in its co-channel set. Minimize the mean square of distance between cells using the same channel.

82 Distributed DCA Schemes Based on one of the three parameters: Co-channel distance -co-channel cells in the neighborhood not using the channel - sometimes adjacent channel interference taken in to account Signal strength measurement - anticipated CIR above threshold Signal to noise interference ratio - satisfy desired CIR ratio

83 Comparison between FCA and DCA FCA Performs better under heavy traffic Low flexibility in channel assignment Maximum channel reusability Sensitive to time and spatial changes Not stable grade of service per cell in an interference cell group High forced call termination probability Suitable for large cell environment Low flexibility DCA Performs better under light/moderate traffic Flexible channel allocation Not always maximum channel reusability Insensitive to time and time spatial changes Stable grade of service per cell in an interference cell group Low to moderate forced call termination probability Suitable in microcellular environment High flexibility

84 Comparison between FCA and DCA FCA Radio equipment covers all channels assigned to the cell Independent channel control Low computational effort Low call set up delay Low implementation complexity Complex, labor intensive frequency planning Low signaling load Centralized control DCA Radio equipment covers the temporary channel assigned to the cell Fully centralized to fully distributed control dependent on the scheme High computational effort Moderate to high call set up delay Moderate to high implementation complexity No frequency planning Moderate to high signaling load Centralized, distributed control depending on the scheme

85 Other Channel Allocation Schemes Based on different criterion being used as a potential way of optimizing the performance, many other channel allocation schemes have been suggested. Hybrid Channel Allocation (HCA) Flexible Channel Allocation (FCA) Handoff Channel Allocation (HCA)

86 References 1. Wireless communication and networks william stallings, Pearson education,