Cellular Mobile Communication Unit-2.pptx

cmc unit2

1. 1 CELLULAR AND MOBILE COMMUNICATIONS by Mrs.B.Priyanka Associate Professor Dept of ECE Raghu Engineering College

2. TEXT BOOKS 2 Mobile Cellular Telecommunications – W.C.Y. Lee, Tata McGraw Hill, 2 nd Ed., 2006. Principles of Mobile Communications – Gordon L. Stuber, Springer International 2 nd Ed., 2007.

3. REFERENCES 3 Wireless Communications – Theodore. S. Rapport, Pearson education, 2 nd Ed., 2002. Wireless and Mobile Communications – Lee. McGraw Hill, 3 rd Ed., 2006. Mobile Cellular Communication – G. Sasibhushana Rao, Pearson Wireless Communication and Networking – John W. Mark and Weihua Zhqung , PHI, 2005. Wireless Communication Technology – R. Blake, Thomson Asia Pvt. Ltd., 2004.

4. Cellular Concepts 4

5. 5 Cellular Concept Replacing a single, high power transmitter (large cell) with many low power transmitters (small cells). Each providing coverage to only a small portion of the service area. Each base station is allocated a portion of the total number of channels available to the entire system Nearby base stations are assigned different groups of channels All the available channels are assigned to a relatively small number of neighboring base stations.

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7. 7 Neighboring base stations are assigned different groups of channels. The interference between base stations (and the mobile users under their control) is minimized. Frequency spectrum may be reused as many times by systematically spacing base stations and their channel groups in a given geographic area.

8. General description of the problem 8 How many customers can we serve in s busy hour? How many subscribers can we take into our system? How many frequency channels do we need?

9. General description of the problem 9 Based on the concept of efficient frequency spectrum utilization, the cellular mobile radio system design can be divided into following elements. The concept of frequency reuse channels The co channel interference reduction factor The desired carrier to interference ratio The handoff mechanism

10. 10 E a ch c ell u lar b ase s t ati o n is all o cated a g ro u p o f r a d io c h a n n els w it h in a s m all g e o g r a p h ic a r ea c a lled a c e l l . N e i gh bo r i n g c e lls a r e as s i gn ed d i f f e r e n t c h a n n el g r o u p s . B y li m iti n g t h e c o v e r a g e a r ea to w it h in t h e bo un d a r y o f t h e c e ll, t h e c h a n n el g ro u p s m ay b e r e us ed to c o v er d i f f e r e n t cell s . K e eps i n te r f e r e n ce le v els w it h in t o le r a b le Frequency reuse

11. Frequency reuse 11 The figure represents the concept of cellular frequency reuse, where cells labelled with same letters use same frequency group. Hexagon cell shape is universally used for representing a cell because they closely approximated to circle also the permit easy and manageable analysis of cellular systems. These cells can be of two types Centre excited cell-BS are placed at centre Edge excited cell-BS are placed at the edges of the cell

12. Number of channels in a cellular system 12 C o n s i d er a c e ll u lar s y s tem w h ich h as a t o tal o f S d u p lex c h a n n els. E a ch c ell is a llo c ated a g ro u p o f k c h a n n el s , k < S and S c h a n n els a r e d i v i d ed a m o n g N c e ll s . T h e t o tal nu m b er o f a v aila b le r a d io c h a n n els S = kN T h e N cells w h ich u s e t h e c o m p lete set o f c h a n n els is c alled cl u s te r T h e cl us ter c a n b e r e p e a ted M ti m es w it h in t h e s y s te m . T h e t o tal nu m b er o f c h a n n els, C , is u s ed as a m e a s u r e o f c a p a c ity C = MkN = MS T h e c a p a c ity is d ire c tly p r opo r ti o n al to t h e nu m b er o f r e p licati o n M .

13. 13 1 3 2 1 4 3 2 2 7 5 4 3 1 6 3-cell cluster 4-cell cluster 7-cell cluster T h e cl us ter s ize, N , is t y p ically e q u al to 3, 4 , 7 , 12 or 19 . S m all N is d esir a b le to m a x i m ize c a p a c it y . No frequency reuse is done within a cluste r Large cluster size indicates ratio b/w R and D are small Small cluster size indicates the co-channel cells are located much closer to each other

14. 14 He x a g o n al g e o m et r y h as E x ac t l y six e qu i d ista n c e n e i g hbo rs T h e l i n e s j o i n i n g t h e ce n ters o f a n y ce ll a n d eac h o f i t s n e i g hbo rs a re s e p a r a ted b y m u l t i p les o f 6 d e g r ee s. O n ly c er tain cl us ter s izes a n d c e ll la y o u t a r e po ss i b le. T h e nu m b er o f c e lls p er cl us te r , N , c a n o n ly h a v e v al u es w h ich s ati s f y C o- c h a n n el n ei g h bo r s o f a p a r tic u lar c e ll, e x : i =3 a n d j= 2  

15. Channel assignment Strategies 15 For efficient utilization of the radio spectrum ,a frequency reuse scheme that is consistent with the objectives of increasing capacity and minimizing interference is required . Channel assignment strategies can be divided into Fixed channel assignment Dynamic channel assignment Fixed channel assignment: Each cell allocated with predetermined set of voice channels Any attempt to place call should done within allotted channel If all channels occupied, new calls will be blocked Borrowing strategy->MSC supervises this without interference Dynamic channel assignment: Not permanently allotted MSC assigns channel when needed by following considerations.. Future blocking within cell Frequency of use of the candidate channel Reuse distance of the channel Cost functions

16. Frequency Reuse Distance 16 The minimum distance which allows the same frequency to be reused will depend on the following factors Number of cochannel cells Type of geographical terrain contour Antenna Height Transmitted power at each cell site The frequency reuse distance D can be determined from Where N is the Cluster size  

17. Frequency Reuse Distance 17

18. Types of Interferences S o u r c e s o f I n te r f e r ence A n o t h er m ob ile in t h e s a m e c e ll A ca ll in p r o g re s s i n t h e n e i g hbo r i n g ce ll O t h er b a s e s t at i on s op er a ti n g in t h e s a m e f re qu e n cy b a n d A Non- ce l l u lar s y stem le a k s e n e r g y i n to t h e ce l l u lar f re q u e n c y b a n d Drawback: Channel capacity is reduced T w o m a j o r c e ll u lar i n te r f e r ences are: C o -c h a n n el I n ter f er e n ce Non Co-c h a nn e l I n ter f e re n c e

19. C o -c h a n n el I n ter f er e n ce T h e r e a r e s e v e r al c e lls t h at u s e t h e s a m e s et o f f r e q u e n cies called as c o - c h a n n el c e lls and the interference caused between these co-channel cells is known as C o - c h a n n el I n te r f e r e n ce T o r e d u ce c o - c h a n n el i n te r f e r e n c e, c o - c h a n n el c e lls m u s t b e s e p a r ated b y a m i n i m u m d i s ta n c e W h en t h e s ize o f t h e c e lls are a pp r o x i m ately t h e s a m e, c o - c h a n n el i n te r f e r e n ce is i n d e p e n d e n t o f t h e tra ns m itted po w er C o - c h a n n el i n te r f e r e n ce is a f un cti o n o f R : R a d i u s o f t h e c e ll D : D i s ta n ce to t h e ce n ter o f t h e n ea r est c o - c h a n n el cell

20. C o - ch a n nel I n t er f ere n c e Reduc ti on Fa c t o r 20 C o - cha n n e l i nt e r f e r e nce is a f unction of a pa r am e t e r q d e f i n e d as T he p a r am e t e r q is t h e c o - cha n n e l i n t e r f e r e nce r e duction f ac t o r . When q increases, cochannel interference decreases. T he s e pa r a tio n D in a b o v e equ a tio n is a function of K I a n d C/I. W h e r e is t h e nu m b er o f c o- c hann el i n t er f eri n g c ells in t h e f i r s t t ier an d is t h e re c ei v ed c a r r ie r ‐ t o ‐ i n t er f ere n c e r a t io a t t h e d esi r ed m ob ile r e c ei v er.  

21. 21 In a f ully e qu i pp e d h e x a go n a l s ha p e d c e llul a r s y s te m , t h e re are alwa y s s ix c o - cha n n e l i nter fe ring c e lls in t he fir s t tier Hence, C/I is given as, For First tier,   S i x e ff ec t i v e i n t er f ering cells o f cell 1

22. 22 C /I c an th e n b e e xp r e ss e d as T he s ix c o - ch a nn e l i n t e r f e ring c e l ls i n the s e c o n d ti e r cau s e w e a k e r i n t e r f e r e n c e t h an t h o s e i n t h e f ir s t t i e r . T h e r e f ore , t h e c o - cha n n e l i n t e r f e r e nce f r om t h e s e c o n d tier of i n t e r f e ring c e lls is n e gligible.  

23. Cochannel Interference from Six interferers 23 Receiving at the cell site Receiving at the mobile unit

24. Co-Channel Measurement When customer demand increases, the limited channels have to be repeatedly used in different areas which provides many cochannel cells. This increases the system capacity but results in co-channel interference The received voice quality is affected by both the grade of coverage and the amount of cochannel interference. For detection of co-channel interference areas in a cellular system, two tests were suggested Find the co-channel Interference area from a mobile receiver Find the co-channel Interference area which effects a cell site

25. Test1- Find the co-channel Interference area from a mobile receiver Measure co-channel interference by selecting any one channel and transmitting on that channel at all co-channel sites at night, while the mobile receiver is traveling in one of the cochannel cells. Detect any change in the field strength recorded at the mobile unit and compare the data with the condition of no co-channel sites being transmitted. This test must be repeated as the mobile unit travels in every co-channel cell.

26. One channel records the signal level, another channel records the interference level while the third channel ,which is not in use, records the noise level. C/I ratio can be obtained by and C/N ratio can be obtained by Four conditions used to compare the results: If C/I > 18dB throughout the cell, then the system is properly designed If C/I < 18dB and C/N > 18dB in some areas, then there is co-channel interference If C/I < 18dB, C/N < 18dB and C/I ≈ C/N in a given area, then there is a coverage problem If C/I < 18dB, C/N < 18dB and C/I < C/N in a given area, then there is a coverage problem and co-channel interference  

27. Test 2 - Find the co-channel Interference area which affects a cell site Record the signal strength at every co-channel cell site while a mobile unit is travelling either in its own cell or in one of the cochannel cells Find the areas in the interfering cell in which top 10 percent of the signal transmitted from the mobile unit is received at the J th cell. The mobile units now travels in all six interfering cells, calculating the top 10 percent signal strength

29. Desired C/I from a normal case in an Omnidirectional Antenna system As long as the received C/I ratio at both mobile unit and cell site are same, the system is called a balanced system. We know that, Assuming all the are same, and Thus, and  

30. C/I of 18db is measured by the acceptance of voice quality from cellular mobile receivers and considering =4 for mobile radio environment, But we know that For q=4.41, N=7, indicates that a seven cell reuse pattern is needed for a C/I of 18dB. The value of q may not be large enough to maintain a C/I ratio of 18dB, which is particularly true in worst case.  

31. Design of an Omnidirectional Antenna system in the worst case The w or st ca se is at the lo ca tion w her e the mobi l e unit would r e ce ive the w e a k e st si g n a l f r om i t s own c e ll si t e but strong inte r fere n c e s fr om a ll inte r fer ing ce ll si t e s. A N=7 ce llpat t e rn do e s not p r o v ide a suffici e nt f r e qu e n cy r e use dis t an c e even when the ideal condition of flat terrain is assumed. The distances from all six cochannel interfering sites are shown in the figure: Two distances of , Two distances of , and Two distances of  

32. For q= 4 . 6, C /I = 1 7d B , which is l o w er t h a n 1 8 dB. I f w e use t he sho rt e s t d i s t a nce , t hen  

33. The r e f o r e, in an Omn i - d i r e c t ion a l - c e ll s y s t e m , N =9 or N = 1 2 w ould be a c o r r e c t choice. Then t he v a l ues of q a r e: N=9 and N=12 Cell patterns are used when the traffic is light. Each cell covers an adequate area with adequate number of channels to handle the traffic.  

34. Cell Structures 34 Wireless cells can be categorized as: Macro cell: 10km Micro cell: 1 km – Shopping centres, airports etc. Pico cells: 50 – 300 m – Inside building Femto cells: 10 – 50 m – Inside rooms

35. Macro Cell 35 Cell size is quite large, typically 1-10 km Covering large areas, e.g. suburban areas Small number of base stations - 30-45 m height to cover a wider coverage area (e.g. 500 m or more). Lower capacity High power: 20 – 160 W (Typical 60 W) Poor service at the cell edge which includes a large percentage of the cell area.

36. Micro Cell 36 Cell size: 300m -1km Shopping centres, airports etc. Quality of service – Leads to improved throughput - i.e., higher capacity, which is 80-98% higher than Macro cells Too many base station – 15-25 m height to cover a limited area (e.g., 200 m) to provide capacity to a hot spot or coverage in a dead zone. Lower delays – Faster down loads Reduced transmit power: 2 – 20 W (Typical 5 W) Large number of handovers Require accurate power control to reduce interference

37. Pico Cells 37 50 – 300 m : Inside building Quality of service - Leads to improved throughput - i.e., higher capacity, which is 80-98% higher than Macro cells Flexibility High number of base station - 10-15 m height to cover a limited area (e.g., 100 m) to provide capacity to a hot spot or coverage in a dead zone. Lower transmit power: 250 mW – 2 W Lower delays – Faster down loads Better cell-edge performance, particularly for the uplink than large cells Higher level of handover Require accurate power control to reduce interference

38. Femto Cells 38 10 – 50 m : Inside rooms In-building coverage: small cells provide better outdoor-to-indoor coverage. Considering that 40% of mobile traffic originates from home and 25% from work, this can represent a significant source of revenue for network operators Better cell-edge performance, particularly for the uplink than large cells Low cost

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42. Cell Splitting 42 Cell Splitting is process of subd i viding a c o n g ested ce l l into sm aller cel l s ea c h wi th its o wn base station (with co r resp o n d ing re d uction in antenna hei g ht and Tx p o w er) Increases the capacity of ce l lu l ar system since it increases the number of t i mes the channel are reused By install i ng new c el l s whi c h have smaller radius than original ce l ls betwe e n existing cel l s These sm al l er cel l s are called MICROCEL L S

43. 43 Spl i t con g es t ed c e ll into smaller c e ll s . P r ese r v e f r e qu e n c y r e u se plan. R e d uc e transmission p o w e r . mi c ro ce ll R e d u c e R to R/2

44. 44 T ransmis s ionp ow er r e d u c tionfrom to E x aminingther e c e i v i ngp ow eratthenew and old cellbounda r y If w eta k e = 4 (pathloss)andset ther e c e i v edp ow er e qualto e a c hother T het r ansmitp ow er m ustbe r e d u c ed b y12 d Binorderto fillin theoriginalc o v e r a g earea. If onlypa r tof the c ells a r espl i tt e d, then Diffe r ent cell sizesw i lle x istsi m ultan e ously Handoff issues : h i ghspeedand l o wspeed tr a fficcan besi m ultaneouslyac c ommodat e d  

45. Cell Sectoring 45 D e c r ease the co - channel i nter f e r ence and k e e p the c ell ra d ius R un c han g ed R e p la c ing single om ni -dir e c tio n al an ten n a b y se v e ral dir e c tio n al a n ten n as Radiatin g within a s p ec i f i e d se c t o r X A C B Placing directional trans m itte r s at corners w h ere t hree adjacent cel l s m eet

46. Sector i ng by Antenna Design 46 f 60 o (a ) O m ni ( b ) 1 20 o sec t or (e ) 6 o sec t or (c ) 1 20 o sec t or (al t er n a t e) c 120 o a b c 120 o a b ( d ) 9 o sec t or d 90 o a b c a b c d e

47. 47 Illustration of how 120° sectoring reduces interference from co-channel cells. Out of the 6 co-channel cells in the first tier, only 2 of them interfere with the center cell. If omni-directional antennas were used at each base station, all 6 co-channel cells would interfere with the center cell.