Ch3. The Cellular Concept Systems Design Fundamentals. From Rappaport s book

Transcription

1 Ch3. The Cellular Concept Systems Design Fundamentals. From Rappaport s book Instructor: Mohammed Taha O. El Astal LOGO

2 Early mobile systems The objective was to achieve a large coverage area by using a single high power transmitter with an antenna mounted on a tall tower. It has a very good coverage area but just for a few # of users. i.e. Bell Mobile system supported 12 users simultaneously.

3 3.1 Introduction: Cellular Concept

4 CONT. The Cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum allocation without any major technical changes. So, the increasing demand can be faced by reuse the channels through Tx. with low power

5 3.2 Frequency Reuse Cellular System rely on an intelligent allocation and reuse of channels. Each B.S. is allocated a group of radio channels to be used within a cell. B.S. in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells. To prevent large interference, optimum antenna radiation pattern to coverage just cell area must be designed. channel group

6 CONT. The real coverage of B.S. is called footprint. Since it is amorphous in nature, we need to regular shape to use it in the systematic design process. 1 st choice is the circle shape, but?? Adjacent circle can not be overlaid upon a map without leaving gaps or creating overlap region.

7 CONT. Thus, when considering a geometric shapes which cover an entire region without overlapping and with equal areas, there are 3 choices : square, equilateral triangle, hexagon since : the cell must serve the weakest mobile with the footprint, so the hexagon has the largest area if the farthest distance from center is fixed, the fewest number of shape can cover specific geographic area. the hexagon closely approximate the circular radiation

8 Two schemes Center-excited cell antenna type?? Edge-excited cell antenna type??

9 CONT. S is the total number of duplex channel for the system from the regulation body. K is the # of channel per cell. if there are N of cells, then : S=KN. if the area covered by M clusters then : C=MS=MKN. where C is he total capacity of the system. M++ C++ & as M-- C--. cell area fixed: N-- M++ C++ but co=channel interference ++. cell area fixed: N++ M-- C-- but co=channel interference --. Frequency reuse factor= 1/N

10 number of cell/ cluster The number of cell per cluster (N) can only have value which satisfy : move i cells in any direction and then move j cells in clockwise direction

11 CONT. EXAMPLE 2.1: If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 khz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (a) four-cell reuse, (b) seven-cell reuse, and (c) 12-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems. Simply locate the same number of channels in each cell wherever possible for N=4; #c=5;#v=160 for N=7;4 -#c=3-#v=92// another (4*91+3*92) 2 #c=3-#v=90// 1-#c=2-#v=92// for N=12 ; 8 #c=2;#v= 53 4 #c=1;#v=54

12 2.3 Channel Assignment strategies: channel assignment strategies Fixed Dynamic Objective: Maximize the System Capacity while Minimizing the Interference [A Constrained Optimization Problem] Each Cell is Assigned a Predetermined Set of V. Ch (x) Any request for a new call initialization or HO beyond x will be blocked or waited. Borrowing Strategy Reserve some channels for Handoff. No Permanent Assignment of V. Ch. to any Cell Any request for a new call/ HO will be met by a dynamic allocation of a channel from the central pool of av. ch. by MSC that takes into Account: probability of future blocking, frequency of use of candidate channels. reuse distance of the Channel

13 CONT. Advantages of Dynamic Channel Allocation but Reduction of blocking probability Reduction of call drop probability during Hand Off Improvement of System Trunking Capacity [Traffic Intensity/Channel]- All Channels are Accessible by all Cells Storage and Computational Load on MSC. MSC must Collect real-time Channel Occupancy Data. Traffic Distribution Information Radio Signal Strength Indications (RSSI) of all the Channels

14 2.4 Handoff Definition Definition : When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station Important task in any cellular radio system Must be performed successfully, infrequently, and imperceptible to users. Identify a new base station Channel allocation in new base station with a high priority than initiation request.

15 CONT. Let : P r,min is the minimum acceptable signal to maintain the call. since HO be performed successfully, infrequently, and imperceptible to users, so P r,handoff, is the signal power where to start handoff process, must be slightly greater than P r,min. Then : = P r,handoff - P r,min usable is the design parameter. must choose carefully, not too large and not too small, why??

16 2.4 Handoff Styles: Network Controlled Handoff (NCHO) In first generation cellular system each MSC base station constantly monitors signal strength from mobiles in its cell Cell A fa Cell B Based on the measures, MSC decides if handoff necessary RSL dbm RSL A RSL B Mobile plays passive role in process Distance Heavy Burden on MSC B G F A C D MSC E

17 CONT. Mobile Assisted Handoff (MAHO) Present in 2 nd generation systems Mobile measures received power from surrounding base stations and report to serving base station. During the idle time periods, the M.S. can tune to other radio ch. frequencies and measure their signal strengths. Handoff initiated when power received from a neighboring cell exceeds current value by a certain level or for a certain period of time Faster, why??

18 HO Types: The Handoff types: Hard HO : is one in which the channel in the source cell is released and only then the channel in the target cell is engaged. known as (break-before-make). Soft HO: is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. known as (make-before-break).

19 Intersystem Handoff: When? Handoff can take place Mobile is at the Border of the system Call Type what will happened? Roaming is Allowed or Not? MSC of the Serving Cell Talks to the Compatibility Issues [Standards] MSC of the Neighboring System or User Authenticity and Call Charges Vice Versa Issues

20 2.4.1 Prioritizing Handoff: Guard Channel Method A fraction of the total available channels is reserved for Handoffs In case of fixed channel assignment, it affects system capacity [C = M k N] Good in case of dynamic channel assignment Queuing Handoff Request Method Any Handoff request, if can not be tackled immediately, it will be placed in a queue Does not Guarantee 100% Success for all Handoff Requests

21 2.4.2 Practical HO Consideration: High Speed Vehicles: Umbrella cell approach to tackle the issue. Large Umbrella Cell for Hi peed Traffic Small MicroCells for Low Speed Traffic Cell Dragging: HO threshold and coverage design parameters must be chosen very carefully.

22 Site Configuration In Jawwal Workshop. Tuesday 13/12/ :00 AM

23 CONT. Antenna: is a device that transmits/receives EM signals simultaneously Antenna Categories: Passive Antenna: RP Controlled by Type and Construction of the Device, Using Mechanical Means we can Guide the Signal Active Antenna: RP Controlled by Type and Construction as well as DSP Technique of the Device General Classification on RP Omni Directional Antenna: Equal Radiation in all Directions Directional Antenna: More Radiation in a Certain Direction

24 CONT. Bore Sight GAIN Main Antenna Parameters : Antenna Directivity. Antenna Gain Antenna Beam Width Isotropic Antenna Front-to-Back Ratio Frequency Response 0 db - 3 db GAIN Typical Antenna Parameters Type Frequency Response BW M. Antenna MHz 70 MHz D. Antenna MHz 25 MHz Mob. Ant MHz 70 MHz fl fo fh FREQUENCY

25 3.5 Interference and System Capacity Interference: is unwanted signal which affects the signal quality. It is the major limiting factor in the performance of cellular radio systems. Interference sources in cellular systems: another mobile in the same cell. a Call in Progress in the Neighboring Cell. Other base stations using the same frequency band. Some non-cellular device/system leaking energy in the cellular band. Interference in voice channel cause a crosstalk whereas in control channel cause missed or blocked call due to errors in digital signaling

26 CONT. Interference is more severe in urban area due to : greater noise floor. high number of B.S. A major bottle-neck in system capacity: a trade-off between system capacity and speech quality. Two Major Ones are: Co-Channel Interference Adjacent-Channel Interference

27 3.5.1 Co-Channel Interference & system capacity : Co-channel Cell : are the cells using the same set of frequencies in the given coverage area. The signals of the co-channel cells interfere with each other, the effect called co-channel interference. Unlike thermal noise, co-channel interference can't reduced by increasing SNR, so what the solution?? Co-channel cells must be physically separated by a minimum distance to provide sufficient isolation, (D) if cells size are same, and also have same Tx. power, then the co-channel interference become a function of R & D only.

28 CONT. A B C G F E D A B C G F E D A B C G F E D A B C G F E D D The Co-channel reuse factor Q is defined as : Q = D / R = (3 N) As Q++ co-channel I -- but ( N++ C--) As Q-- co-channel I ++ but ( N-- C++) that s mean there is a tradeoff between C & I.

29 CONT. A B C G F E D A B C G F E D A B C G F E D A B C G F E D D The Co-channel reuse factor Q is defined as : Q = D / R = (3 N) As Q++ co-channel I -- but ( N++ C--) As Q-- co-channel I ++ but ( N-- C++) that s mean there is a tradeoff between C & I.

30 CONT. Exercise : If the power of propagating signal in cellular system follow : P r (dbm)=p 0-10 n log(d/d 0 ), where P 0 is the Tx power, and n is the path exponent, then find the following : 1. The general equation that describe SIR at cell boundary (assume all at D distance ). 2. simplified the equation if the interference just form first co-channel cell layer and the distance between cells center are same. 3. if SIR must be equal or above of 18 db and n=4 then find N. N=7.

31 Exact SIR: Suppose N from feasible values. calculate Q. Calculate S/I. if larger than required then decrease N and recalculate until obtained minimum SIR verify the condition. if smaller then increase N until have a minimum SIR verify the condition.

32 CONT. Example 3.2 : If the power of propagating signal in cellular system follow : P r (dbm)=p 0-10 n log(d/d 0 ), where P 0 is the Tx power, and n is the path exponent, then find the following : 1. The general equation that describe SIR at cell boundary (assume all at D distance ). 2. simplified the equation if the interference just form first co-channel cell layer and the distance between cells center are same. 3. if SIR must be equal or above of 18 db and n=4 then find N. N=7.

33 3.5.2 Channel planning for Wireless Systems: Cellular systems spectrum : Voice channels. Control channels. Voice channels can face interference more than control channels can do, why? In system have N=7 for voice channels, it must use N=21 for control channels.

34 3.5.3 Adjacent Channel Interference: Adjacent-Channel Interference [ACI]: An interference arising from energy spillover between two adjacent channels ACI results from the imperfect behavior of the Rx filters allowing nearby frequencies to leak into the pass-band filter ch1 ch2 ch3 It is more critical when near-far scenario, which called near-far effect.

35 CONT. ACI minimizing methods : Careful filter design: determine the required decay slope to have an acceptable ACI for the worst case Intelligent Channel assignment: F 6,13,20,2 7 D 4,11,18,2 5 A 1,8,15,22 B 2,9,16,23 by keeping the inter-channel frequency difference as large as possible C 3,10,17,2 4 E 5,12,19,2 6 G 7,14,21,2 8 Example : if a M.S. at 20 times close to B.S. than other M.S. which operate at adjacent channel (n=4), then by 20dB/oct filter you need to six times separation to have SIR=0

36 Example 3.3: CONT.

37 3.5.4 Power control for reducing Interference: The serving B.S. control the Tx. power of M.S. to maintain just smallest required power. this help to : prolong battery life for the subscriber unit. reduced the reverse channel SIR in the system.

38 3.6 Trunking and Grade of Service: The Concept of Trunking : allow large # users share small # of Ch. by serve the user only on demand from a pool of available Ch., then release the channel to the pool, based on specific GOS. Developed by Danish mathematician, A.K. Erlang, based on a statistical behavior of users. Traffic intensity unit : Earlang (dimensionless) which equal one hour call per hour or 1 minute call per minute.

39 CONT. Exercise : compute traffic intensity of a radio channel occupied for 30 minutes during one hour?? The definition of GOS : is a measure of the ability of a user to access a trunked system during the busiest hour. The GOS is a benchmark used to define the desired performance of a particular trunked system.

40 Trunk terminology : Call Set up Time: Time Required to Allocate a Trunked Radio Channel to a Requesting User Blocked/Lost Call: A Call that can not be Completed at the Time of the Request, due to Congestion Holding Time: Average Duration of a typical Call, denoted by H Request Rate: Average Number of Call Requests/Unit Time [denoted by ] Traffic Intensity : Average Channel Occupancy, measured in Earling.

41 Trunk System Types: 1 st type offers no queuing for call requests: Immediate access to a channel if one is available. Else, the request is blocked and is free to try again later Also, called blocked calls cleared (BCC). Pr / A/ C, 3 critical parameters. Erlang B formula / curve. 2nd type offers queuing for call requests: Immediate access to a channel if one is available. Else, the request is queuing for waiting time. Also, called blocked calls delayed (BCD). Erlang C formula / curve.

42 CONT. the probability of queuing the call for period greater than t : The average delay time, D : Gives Relationships among GOS, C, and A for BCD Trunking Radio Systems

43 Erlang B chart : Gives Relationships among GOS, C, and A for BCD Trunking Radio Systems

44 Erlang C : Gives Relationships among GOS, C, and A for BCC Trunking Radio Systems

45 Examples

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53 Trunk efficiency : Trunk efficiency: is a measure of the number of users which can be offered a particular GOS with a particular configuration of fixed channels. The way in which channels are grouped can substantially alter the number of users handled by a trunked system. C=10, GOS=1% A=4.46 where C=5,GOS=1% A=1.36 which doesn t equal A=4.46/2 that s mean you should have as large as C in the same cell.(60% increase than 2*5)

54 3.7 Improving coverage & C : Review: What is System Capacity? C = M k N. N is the Cluster Size. k is the number of Channels used per Cell. and M is the Replications of the Cluster in the given System Coverage Area. In terms of Traffic Intensity, A = U A u as U ++ A ++, so the traffic offered to the system will increase which leading to congestion [Blockage of the Calls]. Given an Allocated Spectrum [S = k N ] which is Fixed, We have to use some Cellular Design Techniques to Improve System Capacity [ C or A as A is a Function of C].

55 1.Cell Splitting : Objective : D C E To increase M with keeping cluster size N and G D co-channel reuse ratio, Q = D/R = sqrt(3n) constant. How :(In general) By reducing the cluster coverage area (in other words cell area). C G F E B D C D F E A B F G B C G F E It Maintains S/I by reducing the base station Tx power, antenna height, and antenna down- Tilting Mechanism.

56 CONT. How much reduction of Tx. power? P r (old Cell Boundary) P t1 R -n P r (new Cell Boundary) P t2 (R/2) -n C Equating them, we get P t2 = P t1 / 2 n D E Example : There is existing system and their G D engineer noticed there are increased blocking rate in cell A. How : By reducing the cell area: R R/2 which C G F E D C D E A B F G B C F E result 6 new microcells, so there is additional # of channels per unit area. B F G As R R/2, also D D/2, so R/D remain constant. if n=4, the new power must equal p t1 /16

57 CONT. Practical consideration in splitting : The # HO ++. C At initial phase, which power will have to use? D E Suggested solutions: Two group of channels are created, one of them for the large cell and the another for the small cells. The size of these group is determined by in any C G F E D C G D E A B F G B C D F E phase the system reside. B F G

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59 2.Cell Sectoring : Objective : To decrease N (which increase M) with keeping co-channel reuse ratio small. How : Reduce N : reassign S channel to new N, which increase M. Since it plays with N, thus, changing Q and S/I To avoid co-channel interference by using directional Antennas It improves system capacity and S/I but at the cost of decreased system trunking efficiency.

60 CONT.

61 Self Reading

62 LOGO