Wireless Communications Principles and Practice 2 nd Edition Prentice-Hall. By Theodore S. Rappaport

Transcription

1 Wireless Communications Principles and Practice 2 nd Edition Prentice-Hall By Theodore S. Rappaport

2 Chapter 3 The Cellular Concept- System Design Fundamentals

3 3.1 Introduction January, 2004 Spring 2011

4 3.1 Introduction January, 2004 Spring 2011

5 3.2 Frequency Reuse January, 2004 Spring 2011

6 Linear Cells as an Example of Frequency Reuse Total Band width (BW) is divided into three adjacent bands f 1, f 2 and f 3 Such that BW = f 1 +f 2 +f 3 f 1 f 2 f 3 f 1 f 2 f 3 P I Cell 1 Cell 2 Cell 3 Cell 1 Cell 2 Cell 3 Region 1 Region 2 For acceptable voice quality Signal to Interference ratio P/I > 50 (17dB)

7 3.1 Introduction (Contd.) Different Type of Possible Cell shapes R R R For the same Cell Radius (R) (distance from center to Cell boundary) Area of Hexagon is the largest

8 Cluster Size 3 Cell Cluster 4 Cell Cluster

9 Frequency Reuse for Area Coverage 3 4 Base Station (BS)

10 Hexagonal Cells for Area Coverage

11 In area coverage using Hexagonal Cells the distance between any two Cells can be expressed as a linear combination of two Basis Vectors v 1 and v 2 B v 2 60 o v 1 A C

12 Magnitude of the Basis Vectors v 2 v 1 = 2 x R cos(30 o ) = 3 R v 2 = 2 x R cos(30 o ) = 3 R 60 o v 1 30 o R

13 A Parallelogram can be defined by Basis Vectors v 1 and v 2 v 2 60 o v 1 Area of the Parallelogram = v 1 x v 2 = ( 3R)( 3R)sin(60) = 3R 2 sin(60).

14 The Parallelograms defined by v 1 and v 2 have the same transitional periodicity as the hexagonal Cells Therefore, if M hexagonal cells are required to cover an area then we will have M Parallelograms covering the same area Hence the area of the Parallelogram is equal to the area of the Hexagon

15 Area of Parallelogram is equal to the area of the Hexagon?? Area of the Parallelogram = v 1 x v 2 v 2 v 1

16 Cellular Concept January, 2004 Spring 2011

17 Capacity January, 2004 Spring 2011

18 Capacity January, 2004 Spring 2011

19 Frequency Reuse Vectors U 1 and U 2 The region of Frequency reuse can be composed of any integer number N of contiguous (adjacent) cells All other regions are obtained by translation of the defining region through a linear combination of the frequency reuse vectors U 1 and U 2 In the Figure N = 7. In the Figure same colour represents a frequency reuse region U 2 v 2 60 o 1 v U

20 Frequency Reuse Vectors U 1 and U 2 The displacement between any two cells using the same frequency can also be expressed as a linear combination of the two reuse vectors. e.g., Displacement AB = U 1 +U 2 Displacement AC = 2U 2 U 2 A C U 1 B

21 Parallelogram defined by Frequency Reuse Vectors U 1 and U 2 The Parallelogram defined by U 1 and U 2 has the same transitional periodicity as the frequency reuse region Hence the area of the frequency reuse region is equal to the area of the Parallelogram Area = U 1 x U 2 = N time area of a single cell U 1 x U 2 = N. v 1 x v 2 U 2 A U 1

22 Frequency Reuse Vectors U 1 and U The Frequency reuse vectors/displacement vectors can be expressed in terms of the Basis Vectors (v 1 & v 2 ) as U 1 = k 1 v 1 + m 1 v 2 U 2 = k 2 v 1 + m 2 v 2 Where the constants k 1, k 2, m 1 and m 2 are integers. e.g., in the present case U U 1 v 2 60 o 1 5 U 1 = 2v 1 + v v U 2 = -v 1 + 3v 2 7 6

23 Area of Parallelogram defined by Vectors U 1 and U U 1 xu 2 = k 1 m 2 k 2 m 1 v 1 x v 2 As discussed earlier Area = U 1 x U 2 = N time area of a single cell U 1 x U 2 = N. v 1 x v 2 N = k 1 m 2 k 2 m U U 1 v 2 60 o v

24 Calculation for the Value of N for symmetrically located co-channel cells 3 4 For symmetric reuse patterns only certain values of N are allowed. To find these values k 2 and m 2 are expressed in terms of k 1 and m 1 under the condition that 1. Magnitude of U 2 and U 1 is same 2. U 2 is rotated 60 o counter clockwise w.r.t U U 2 v 2 60 o v U

25 Calculation for the Value of N for symmetrically located co-channel cells U 2 If we represent U 1 as a linear combination of v 1 and v 2 as, U 1 = k 1 v 1 + m 1 v 2 -v 1 60 o Similarly, we can represent U 2 as a linear combination of v 2 and (v 2- v 1 ) as, v 2 -v 1 v 2 U 1 v 1 U 2 = k 1 v 2 + m 1 (v 2 -v 1 )

26 Calculation for the Value of N for symmetrically located co-channel cells U 2 = k 1 v 2 + m 1 (v 2 -v 1 ) rearranging U 2 = -m 1 v 1 +(k 1 + m 1 )v (1) Previously it was defined as U 2 = k 2 v 1 + m 2 v (2) Comparing (1) and (2) we get k 2 = -m 1 and m 2 = k 1 + m 1 Therefore, N = k 1 m 2 k 2 m 1 = m 12 + m 1 k 1 + k 1 2

27 Locating Co-Channel Cells January, 2004 Spring 2011

28 Locating Co-Channel Cells January, 2004 Spring 2011

29 3.3 Channel Assignment Strategies Channel Assignment Strategies are used for Efficient Utilization of Radio Spectrum with the main Objectives of: Increasing Capacity Minimizing Interference Classification of Channel Assignment Strategies Fixed channel assignment strategy Dynamic channel assignment strategy

30 3.3 Channel Assignment Strategies (Contd.)

31 3.3 Channel Assignment Strategies (Contd.) Fixed Channel Assignment Strategy Each cell allocated a predetermined set of voice channels If all channels are occupied then calls are blocked * Several variations of the fixed assignment strategy exist such as Borrowing strategy to tackle call blockage * MSC supervises this borrowing ensuring that borrowing of channels does not disrupt or interfere with any other calls Or Reserves some channels for handoff.

32 3.3 Channel Assignment Strategies (Contd.) Dynamic Channel Assignment Strategy voice channels are not allocated to different cells permanently at a call request the serving base station requests the MSC for a channel MSC allocates the channel following an algorithm that takes into account, * Likelihood of future blocking with in the cell * Frequency of use of the candidate channel * Reuse distance, etc.

33 3.3 Channel Assignment Strategies (Contd.)

34 3.3 Channel Assignment Strategies (Contd.) Dynamic Channel Assignment Strategy Advantages Reduces the likelihood of blocking, which increases the trunking capacity of the system Disadvantages MSC has to collect real-time data on channel occupancy and radio signal indications (RSSI), hence, increases storage and computational load

35 3.3 Channel Assignment Strategies (Contd.)

36 3.4 Handoff Strategies Handoff (HO) 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 The HO operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station

37 3.4 Handoff Strategies

38 3.4 Handoff Strategies

39 3.4 Handoff Strategies (Contd..)

40 3.4 Handoff Strategies (Contd..) Many HO strategies prioritize HO requests over call initiation requests when allocating unused channels HO must be performed Successfully, infrequently and should be imperceptible to the users Threshold signal level = P r handoff P r minimum usable Pr handoff specifies the optimum signal level at which to initiate handoff Pr minimum usable minimum usable signal for acceptable voice quality at the base station receiver If to large: unnecessary HO s If to small: calls may be lost due to insufficient time for HO

41 3.4 Handoff Strategies (Contd..)

42 3.4 Handoff Strategies (Contd..)

43 3.4 Handoff Strategies (Contd..)

44 3.4 Handoff Strategies (Contd..) In 1G analog cellular systems, signal strengths measurements are made by BS and supervised by MSC. BS measures signal strength of all Reverse Voice Channels RVC s In each BS a spare receiver or locator receiver measures signal strengths of channels in neighboring cells Based on the information from all the locator receivers the MSC decides if HO is necessary or not

45 3.4 Handoff Strategies (Contd..) In 2G cellular systems that uses digital TDMA HO decisions are mobile assisted; mobile assisted handoff (MAHO) In MAHO every mobile measures the received power from the surrounding BS s and continually reports the results to the serving BS MAHO allows much faster HOs, as the HO measurements are made by each mobile and the MSC no longer constantly monitors the signal strength MAHO is particularly suited for microcellular environments where HOs are more frequent

46 3.4 Handoff Strategies (Contd..) Intersystem Handoff During the course of a call if a mobile moves from one cellular system to a different cellular system controlled by a different MSC and intersystem handoff becomes necessary Conditions for Intersystem HO When a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress Issues to be addressed when Implementing Intersystem HO A local call may become long distance call (billing issue) Compatibility between the two MSCs

47 3.4 Handoff Strategies (Contd..)

48 3.4.1 Prioritizing Handoffs Different systems have different policies and methods for managing handoff requests. Some systems give priority to HO over call initiation others deal them at same priority

49 3.4.1 Prioritizing Handoffs Guard channel concept Disadvantage reduces total carried traffic Advantage efficient spectrum utilization with dynamic channel assignment Queuing of Handoff requests Possible because a finite time interval between the time the signal drops below the HO threshold and the time the call is terminated due to insufficient signal level * There is a tradeoff between the decrease in probability of forced termination and total carried traffic

50 3.4.1 Prioritizing Handoffs

51 3.4.2 Practical Handoff Considerations Umbrella Cells

52 3.4.2 Practical Handoff Considerations

53 3.4.2 Practical Handoff Considerations Cell Dragging

54 Cont.

55 3.4.2 Practical Handoff Considerations IS-95 CDMA Soft Handoff This technique exploits macroscopic space diversity provided by the different physical locations of the base stations and allows the MSC to make soft decision as to which version of the user s signal to pass along to the PSTN at any instance.