2.1 Concept of Cellular Communications

Transcription

1 2.1 Concept of Cellular Communications In the late 60 s it was proposed to alleviate the problem of spectrum congestion by restructuring the coverage area of mobile radio systems. IMTS required a single powerful base station and line of sight (LOS) for the mobile units to cover areas of 50km radius. The cellular concept does not use broadcasting over large areas. Instead smaller areas called cells are handled by less powerful base stations that use less power for transmission. Now the available spectrum can be re-used from one cell to another thereby increasing the capacity of the system. However this did give rise to a new problem, as a mobile unit moved it could potentially leave the coverage area (cell) of a base station in which it established the call. This required complex controls that enabled the handing over of a connection (called handoff) to the new cell that the mobile unit moved into. In summary, the essential elements of a cellular system are: 1. Low power transmitter and small coverage areas called cells 2. Spectrum (frequency) re-use 3. Handoff 1

2 2.1.1 A Cellular Network PENN Other MSCs (IS 41) F1,F2,..,F6 PSTN MSC F7,F8,..,F12 F7,F8,..,F12 Base Station F1,F2,..,F6 Handoff Cell MSC: Mobile Switching Center (Theoretical) PSTN: Public Switched Telephone Network Practical Cell - coverage depends on antenna location and height, transmitter power, terrain, foliage, buildings, etc. 2

3 2.1.2 Some Definitions Forward path or down link - from base station down to the mobile Reverse path or up link - from the mobile up to the base station The mobile unit - a portable voice and/or data comm. transceiver. It has a 10 digit telephone number that is represented by a 34 bit mobile identification number -> (215) is divided into two parts: MIN1: 215 translated into 10bits and MIN2: translated into 24bits. In addition each mobile unit is also permanently programmed at the factory with a 32 bit electronic serial number (ESN) which guards against tampering. The cell - a geographical area covered by Radio Frequency (RF) signals. It is essentially a radio communication center comprising radios, antennas and supporting equipment to enable mobile to land and land to mobile communication. Its shape and size depend on the location, height, gain and directivity of the antenna, the power of the transmitter, the terrain, obstacles such as foliage, buildings, propagation paths, etc. It is a highly irregular shape, its boundaries defined by received signal strength! But for traffic engineering purposes and system planning and design a hexagonal shape is used. 3

4 The base station (BS) - a transmitter and receiver that relays signals (control and information (voice or data)) from the mobile unit to the MSC and vice versa. The mobile switching center (MSC) - a switching center that controls a cluster of cells. Base stations are connected to the MSC via wireline links. The MSCis directly connected to the PSTN and is responsible for all calls related to mobiles located within its domain. MSCs intercommunicate using a link protocol specified by IS (International Standard) 41. This enables roaming of mobile units (i.e. obtaining service outside of the home base). The MSC is also responsible for billing, it keeps track of air time, errors, delays, blocking, call dropping (due to handoff failure), etc. It is also responsible for the handoff process, it keeps track of signal strengths and will initiate a handoff when deemed necessary (note to handoff or not to handoff is not a trivial issue!) 4

5 2.1.3 The Basic Cellular Communication Protocol (based on using FDMA) Every mobile unit whether at home or roaming, has to register with the MSC controlling the area it is in. If it does not register then the MSC does not know of its existence and will not be able to process any of its calls. The home location register (HLR) is used to keep information regarding a mobile unit/user, it is a database for storing and managing subscriber information. When roaming, a mobile unit registers with a foreign MSC and data from its HRL is relayed to the visitor location register (VLR). The VLR is a dynamic database used to store roaming mobile subscriber information. The HLR and VLR communicate via the MSCs using IS 41. The cellular system uses out of band signalling. Most of the control information is sent over different channels from the user information (voice or data) channels. Inband signalling is used for control during the connection (disconnect, handoff, etc.) A mobile unit when enabled (power on) scans the control channels and tunes to the one with the strongest signal. The control channels are known and carry signals pertaining to the cell sites, e.g. transmission power to be used by the mobile unit in a particular cell. This process is called initialization. 5

6 If the mobile wants to initiate a call, it sends in a service request on the reverse path control link. The service request contains the destination phone number and identification information (MIN1, MIN2, and ESN) of the source mobile unit to verify the originator. When the base station receives the request, it relays it to the MSC. The MSC then checks to see it is it a number of another mobile or of a fixed user. If the latter the call is forwarded to the PSTN. If the former, it checks to see if the destination mobile unit is a subscriber (local or visitor/roamer). If not it relays the call to the PSTN to forward to the appropriate MSC. If the destination is within its cluster it sends out a paging message to all the base stations. Every base station then relays this message by broadcasting it on its control channel. If the destination mobile unit is enabled (power on) it will detect this message and respond to the base station. The base station relays this response to the MSC. The MSC then allocates channels to both the source mobile unit and the destination mobile unit. The corresponding base stations pass this information on to the respective mobile units. The mobile units then tune to the correct channels and the communication link is established. 6

7 2.1.4 Spectrum and Capacity issues: The available spectrum is limited. Allocated Spectrum F1 F2 F3 F4 F5 F6 F7 F8 F9 FDM F1,F2,...F9: frequency channels To be able to increase the capacity of the system, frequencies must be re-used in the cellular layout (unless we are using spread spectrum techniques). Frequencies cannot be re-used in adjacent cells because of co-channel interference. The cells using the same frequencies must be dispersed across the cellular layout. The closer the spacing the more efficient the scheme! 7

8 Fx:subset of frequencies used in a cell Cochannel Interference F1 F1 F2 F2 Minimum Re-use distance 8

9 2.1.5 Frequency Re-Use For an omni-directional antenna, with constant signal power, each cell site coverage area would be circular (barring any terrain irregularities or obstacles). To achieve full coverage without dead spots, a series of regular polygons for cell sites are required. The hexagonal was chosen as it comes the closest to the shape of a circle, and a hexagonal layout requires fewer cells (when compared to triangles or rectangles, it has the largest surface area given the same radius R) -> less cells. Goal is to find the minimum distance between cells using same frequencies. i,j - integers -> intercell distance along cell centers 60% i,j: multiples of 3 1/2 R A i D j A D - min. dist. R R: cell radius 9

10 D=3 1/2 R[i 2 +j 2 +ij] 1/2 i,j are integers v R R = radius of hexagonal u (u,v) D 2 3 1/2 R 3 1/2 R R 1 (0,0) u 2 -u 1 =3 1/2 Ri v 2 -v 1 =3 1/2 Rj For two adjacent cells: D=3 1/2 R The closest we can place the same frequencies is called the first tier around the center cell (minimal re-use distance -> lower -> more capacity!). For simplicity we only take the first tier of cells into account for co-channel interference (i.e., we ignore 2nd, 3rd, etc. tiers, cause much less interference, negligible!). 10

11 Original cell First tier of interferers Second tier of interferers They are all equidistant away from each other (D) Cluster of N cells with different frequencies Each cell has exactly six equidistant interfering cells Radius = D D Radius First Tier (all use same frequencies as center cell) R Cluster of N cells with frequencies different from center cell (large hexagon) 11

12 Radius = dist. between two co-channel cells = (3R 2 [i 2 +j 2 +ij]) 1/2 = D! Since the area of a hexagon is proportional to the square of the distance between its center and a vertex (i.e., its radius), the area of the large hexagon is: A large = k[radius] 2 = k[3r 2 [i 2 +j 2 +ij]] where k is a constant. Similarly the area of each cell (i.e., small hexagon) is: A small = k[r 2 ] Comparing these expressions we find that: A large /A small = 3[i 2 +j 2 +ij] = D 2 /R 2 From symmetry we can see that the large hexagon encloses the center cluster of N cells plus 1/3 the number of the cells associated with 6 other peripheral hexagons. Thus the total number of cells enclosed by the first tier is: N+6(1/3N) = 3N Since the area of a hexagon is proportional to the number of cells contained within it: A large /A small = 3N/1 = 3N 12

13 Substituting we get: Or: q is referred to as the reuse ratio! 3N = 3[i 2 +j 2 +ij] = D 2 /R 2 D/R = q =(3N) 1/2 The co-channel interference ratio S/I is given as: S -- I = S N i I k k = 1 S = desired signal power in a cell, I k = interference signal power from the k th cell, N i = number of interfering cells. If we only assume the first tier of interfering cells, then N i =6,and all cells interfere equally (they are all equidistant!). The signal power at any point is inversely proportional to the inverse of the distance from the source raised to the γ power. (2< γ <5) 13

14 I k is proportional to D γ, and S is proportional to R γ, where γ is the propagation path loss and is dependent upon terrain environment. A value of 4 is commonly used for cellular communication environemts. Therefore: S R γ 1 -- = I 6 D γ = q γ = Recall that: D/R = q =(3N) 1/2 S/I = 18db (decibels=10logs/i) = 63.1, gives an acceptable voice quality. Therefore q = [6x63.2] 1/4 = 4.41 when Substituting for N we get N = (4.41) 2 /3 = γ = This means that if we have 49 frequency channels available, each cell gets 49/7 = 7 frequency channels. If we have 82 available then 82/7 = > which means that 5 cells will have 12 and 2 cells will have 11! How does that translate to i and j for a cell layout? N = [i 2 +j 2 +ij], find i,j that satisfy the equation! q γ

15 7 1 N=7 -> i=2, j= i 2 D j D = 4.41R We can see that by reducing the area of a cell we can increase capacity as we will have more cells each with its own set of frequencies. What is drawback of shrinking the size of the cells (cell splitting)? Increase in the number of handoffs -> increased load on the system! Also need more infrastrucutre -> base stations (each cell needs a BS). An easier solution exists, sectorization. It does not reduce handoffs, its advantage: it does not require more infrastructure. 15

16 2.1.6 Sectorization We can also increase the capacity by using sectors in cells. Directional antennas instead of being omnidirectional, will only beam over a certain angle. F1+F2+F3=Fa 120% F1+F2+F3+F4+F5+F6=Fa 60% F1 F1 F2 F3 F3 F2 F6 F4 F5 3 sectors Fa: A cell s set of frequencies 6 sectors What does that mean? We can now assign frequency sets to sectors and decrease the re-use distance or improve S/I ratio (i.e. signal quality). Question: By how much? Depends on number of sectors (i.e., 60% or 120%). 16

17 A : set of frequencies in a sector PENN A:Do not interfere with A sector of center cell A A A A A A First Tier (all use same frequencies in sectors as center cell) A :Cause Cell site to mobile interference A A A :Cause Mobile to cell site interference For 120% sectors we get interference from two cells (whether interference for mobile or interference for BS). For 60% sectors we get interference from one cell only (whether interference for mobile or interference for BS). 17

18 For the worst case scenario (i.e., mobile is at the edge of its own cell) the distances from the interferers are: (D, and D+.7R for 120%, and D+.7R for 60%). Therefore the S/I ratio for 120% sectors can be given as: R 4 S -- = I D 4 + ( D R = ) ( q + 0.7) 4 q 4 And for 60% sectors S/I is given as: S -- I R 4 = = ( q + 0.7) 4 ( D R) 4 We see now that for the same q we get a better S/I ratio! Or for the same S/I we get a smaller q -> smaller N! Sectorization is generally used to improve the S/I ratio for improved signal quality once the N has been decided upon using a smaller S/I value (i.e., minimum required value!) 18

19 Other capacity or signal improvement techniques Dynamic channel allocation (DCA): allows cells to borrow frequencies from other cells within the cluster if not used by them. Can be used to alleviate hotspots. Another implementation basically has all channels available to all cells, they get allocated based upon demand. Power control: by reducing the transmitted power, the battery life of a mobile can be extended. It also helps in reducing -channel and adjacent channel interference. 19