Cellular Concept PDF

Cellular Concept  The limited capacity of the first mobile radio-telephone services was related to the spectrum used…not much sharing and a lot of bandwidth dedicated to a single call. – good coverage – interference: impossible to reuse the same frequency  The cellular concep...

Cellular Concept  The limited capacity of the first mobile radio-telephone services was related to the spectrum used…not much sharing and a lot of bandwidth dedicated to a single call. – good coverage – interference: impossible to reuse the same frequency  The cellular concept addressed many of the shortcomings of the first mobile telephones – Frequency reuse – Wasted spectrum allocated to a single user  In 1968, Bell Labs proposed the cellular telephony concept to the FCC  It was approved and then the work began! – FCC allocated spectrum (took away TV UHF channels 70-83) in the 825- 845 MHz and 870-890 MHz bands – AT&T put up a developmental system in Chicago Cellular Network Architecture Mobile Public Switching Telephone Center network and Internet Mobile Switching Center Wired network  Cellular Concept  developed by Bell Labs 1960’s-70’s  areas divided into cells  a system approach, no major technological changes  a few hundred meters in some cities, 10s km at country side  each served by base station with lower power transmitter  each gets portion of total number of channels  neighboring cells assigned different groups of channels, interference minimized  hexagon geometry cell shape Frequency reuse concept  Frequency Reuse  Adjacent cells assigned different frequencies to avoid interference or crosstalk  Objective is to reuse frequency in nearby cells – 10 to 50 frequencies assigned to each cell – transmission power controlled to limit power at that frequency escaping to adjacent cells – the issue is to determine how many cells must intervene between two cells using the same frequency EE 542/452 Spring 2007  Frequency Reuse  each cell allocated a group k channels – a cluster has N cells with unique and disjoint channel  groups, N typically 4, 7, 12  total number of duplex channels S = kN  Cluster repeated M times in a system  Total number of channels that can be used (capacity) – C = MkN = MS  Smaller cells  higher M  higher C + Channel reuse  higher capacity + Lower power requirements for mobiles – Additional base stations required – More frequent handoffs – Greater chance of ‘hot spots’ EE 542/452 Spring 2007  Effect of cluster size N  channels unique in same cluster, repeated over clusters  keep cell size same – large N : weaker interference, but lower capacity – small N: higher capacity, more interference need to maintain certain S/I level  frequency reuse factor: 1/N – each cell within a cluster assigned 1/N of the total available channels  In most of the current networks, frequency reuse factor is 1.  Design of cluster size  In order to connect without gaps between adjacent cells (to Tessellate)  N = i2 + ij + j2 where i and j are non-negative integers  Example i = 2, j = 1 – N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7  Next page example – move i cells along any chain or hexagon. – then turn 60 degrees counterclockwise and move j cells.  Example 3.1 in page 61  Example  N=19  (i=3, j=2)  Channel Assignment Strategies  Fixed Channel Assignments – Each cell is allocated a predetermined set of voice channels. – If all the channels in that 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 Mobile Switch Center: Connects cells to wide area network; Manages call setup; Handles mobility  Channel Assignment Strategies  Dynamic Channel Assignments – Voice channels are not allocated to different cells permanently. – Each time a call request is made, the serving base station requests a channel from the MSC. – The switch then allocates a channel to the requested call based on a decision algorithm taking into account different factors: frequency re-use of candidate channel and cost factors. – Dynamic channel assignment is more complex (real time), but reduces likelihood of blocking EE 542/452 Spring 2007  Handover/handoff  Reasons for handover – Moving out of range – Load balancing  Cell, BSC (base station controller), MSC (mobile switching center)  Handover scenarios – Intra-cell handover (e.g., change frequency due to narrowband interference) – Inter-cell, intra-BSC handover (e.g., movement across cells) – Inter-BSC, intra-MSC handover (e.g., movement across BSC) – Inter MSC handover (e.g., movement across MSC) EE 542/452 Spring 2007  Four Types of Handoff 1 2 3 4 MS MS MS MS BTS BTS BTS BTS BSC BSC BSC MSC MSC EE 542/452 Spring 2007  Handoffs  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  high priority than initiation request (block new calls rather than drop existing calls)  Handoff  =handoff threshold - Minimum acceptable signal to maintain the call   too small: – Insufficient time to complete handoff before call is lost – More call losses   too large: – Too many handoffs – Burden for MSC  Styles of Handoff  Network Controlled Handoff (NCHO) – in first generation cellular system, each base station constantly monitors signal strength from mobiles in its cell – based on the measures, MSC decides if handoff necessary – mobile plays passive role in process – burden on MSC  Mobile Assisted Handoff (MAHO) – present in second generation systems – mobile measures received power from surrounding base stations and report to serving base station – handoff initiated when power received from a neighboring cell exceeds current value by a certain level or for a certain period of time – faster since measurements made by mobiles, MSC don’t need monitor signal strength  Mobile Controlled Handoff EE 542/452 Spring 2007  Types of Handoff  Hard handoff - (break before make) – FDMA, TDMA – mobile has radio link with only one BS at anytime – old BS connection is terminated before new BS connection is made. EE 542/452 Spring 2007  Types of Handoff  Soft handoff (make before break) – CDMA systems – mobile has simultaneous radio link with more than one BS at any time – new BS connection is made before old BS connection is broken – mobile unit remains in this state until one base station clearly predominates  Types of Handoff  Vertical handoff  Prioritizing handoff  Dropping a call is more annoying than line busy  Guard channel concept – Reserve some channels for handoffs – Waste of bandwidth – But can be dynamically predicted  Queuing of handoff requests – There is a gap between time for handoff and time to drop. – Better tradeoff between dropping call probability and network traffic.  Reduce the burden for handoff – Cell dragging – Umbrella cell EE 542/452 Spring 2007 Umbrella Cell  Interference and System Capacity  major limiting factor in performance of cellular radio systems  sources of interference: – other mobiles in same cell – a call in progress in a neighboring cell – other base stations operating in the same frequency band – Non-cellular system leaking energy into the cellular frequency band  effect of interference: – voice channel: cross talk – control channel: missed or blocked calls  two main types: – co-channel interference – adjacent channel interference  Co-Channel Interference  cells that use the same set of frequencies are called co-channel cells.  Interference between the cells is called co-channel interference.  Co-channel reuse ratio: Q = D/R=sqrt(3N) – R: radius of cell – D: distance between nearest co-channel cells  Small Q  small cluster size N  large capacity  large Q  good transmission quality  tradeoff must be made in actual cellular design EE 542/452 Spring 2007  Co-channel Reuse Ratio   SINR Power  d  Pr Pt   S S  d0  I  i0 propagation  d     Ii 2 Pr (dBm) Pt (dBm)  10 log  i 1 factor 2-6  d0   Cochannel Interference S S  d    SINR  i0 Pr Pt   I    Ii 2  d0   Power: propagation factor 2-6 i 1  d  Pr (dBm) Pt (dBm)  10 log    d0  – Sun, nuclear bomb  Approximation S R  ( D / R ) ( 3 N )  i0   I i0 i0  i ( D )   AMPS example i 1  =4, S/I=18dB, N needs to be larger than 6.49. – Reuse factor 1/N small  Relations: cochannel interference, link quality, reuse factor  Example 3.2  Worst Case Interference  S/I ~ R-4 /[2(D-R)-4 + 2(D+R)-4 + 2D-4] EE 542/452 Spring 2007  Adjacent Channel Interference  Interference resulting from signals where are adjacent in frequency to the desired signal.  Due to imperfect receiver filters that allow nearby frequencies to leak into pass band.  Can be minimized by careful filtering and assignments, and by keeping frequency separation between channel in a given cell as large as possible, the adjacent channel interference may be reduced considerably.  Example 3.3  Channel Planning and Power Control  Cell planning – Control channel 5% – Voice/data channel – f1/f2 cell planning – Breathing cell  Power Control – Open loop – Close loop – 800Hz in CDMA2000, 1500Hz in WCDMA