wireless-communication-architecture
- 2. The cellular concept • Limited number of frequencies => limited channels • Single high power antenna => limited number of users • Smaller cells => frequency reuse possible => more number of users • Base stations (BS): implement space division multiplex – Each BS covers a certain transmission area (cell) – Each BS is allocated a portion of the total number of channels available – Cluster: group of nearby BSs that together use all available channels • Mobile stations communicate only via the base station – FDMA, TDMA, CDMA may be used within a cell • As demand increases (more channels are needed) – Number of base stations is increased – Transmitter power is decreased correspondingly to avoid interference
- 3. The cellular concept • Cell size: – 100 m in cities to 35 km on the country side (GSM) – even less for higher frequencies – Umbrella cell: large cell that includes several smaller cells • Avoid frequent handoffs for fast moving traffic • Cell shape: – Hexagonal is useful for theoretical analysis – Practical footprint (radio coverage area) is amorphous • BS placement: – Center-excited cell: BS near center of cell • omni-directional antenna – Edge-excited cell: BSs on three of the six cell vertices • sectored directional antennas
- 4. The cellular concept • Advantages: – higher capacity, higher number of users – less transmission power needed – more robust, decentralized – base station deals with interference, transmission area etc. locally • Problems: – fixed network needed for the base stations – handover (changing from one cell to another) necessary – interference with other cells: co-channel, adjacent-channel • Important Issues: – Cell sizing – Frequency reuse planning – Channel allocation strategies Bottom line: Attempt to maximize availability of channels in an area
- 5. Cellular Geometries Hexagonal pattern is preferred coz it supports equidistant to all adjacent cells.
- 6. Factors: •Equal area •No overlap between the cells For a given S, A3>A1 and A3>A2. Here A3 covers maximum area for a given value of S. By using hexagon geometry, the fewest number of cells covers a given geographic region.
- 7. Ideal Cell Hexagonal cell The hexagon is closely approximates a circular radiation pattern which would occur for an omni directional base station antenna and free space propagation
- 8. Channel Capacity Let a cellular system have total of S duplex channels for use. If S channels are divided into N cells (in a cluster) into unique and disjoint channel groups which each has the same number of channels, total number of available radio channels is: S = KN Where, K is the number of channels / cell. If a cluster is replicated M times within the system, the total number of duplex channels, C, or the capacity is C = MKN = MS Cluster size N = 4, 7 or 12
- 9. Design of cluster size N In order to connect without gaps between adjacent cells N= i2 + ij + j2 Where i and j are non non-negative integers Example i = 2, j = 1, then N=7 i, j = 0,1,2,3,…. Then the possible values of N are 1,3,4,7,9,….
- 10. Cellular interference issues The signal to interference ratio (SIR or S/I) gives an indication of the quality of the received signal. Smaller the cluster sizes, closer the reuse distance and therefore larger the system capacity or total number of possible users. But increases cost, complexity of the network, lower SIR and hence decrease in radio link quality.Co channel cells for cluster N=7
- 11. S= Signal power from designated base station Ii= Interference power caused by the ith interfering co- channel
- 12. Problems If a particular FDD cellular telephone system has a total bandwidth of 33 MHz, and if the phone system uses two 25 KHz simplex channels to provide full duplex voice and control channels. Compute the number of channels per cell if N = 4, 7, 12. Solution Total bandwidth = 33 MHz Channel bandwidth = 25 KHz x 2 = 50 KHz Total avail. channels = 33 MHz / 50 KHz = 660 N = 4, Channel per cell = 660 / 4 =165 channels N = 7, Channel per cell = 660 / 7 =95 channels N = 12, Channel per cell = 660 / 12 =55 channels
- 15. 1 11 1 2 2 22 3 3 3 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 7 8 8 88 9 99 9 10 1010 10 1111 1111 1212 12 12 Cell Structure for N = 12
- 16. Reuse number • Maximize frequency reuse • Determine minimum size cluster – To calculate interference levels • D = R(3N)1/2 R = cell radius N = reuse pattern= i2 + ij + j2 D = reuse distance • If N is small, closer is reuse distance (D), larger is system capacity • If D is reduced, increases subscribers that can be handled, increases cost of network hardware
- 17. N = i2 + ij + j2 N = 22 + 2*1 + 12 N = 4 + 2 + 1 N = 7 D = R(3N)1/2 D = 4.58R The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster
- 18. N = i2 + ij + j2 N = 22 + 2*0 + 02 N = 4 + 0 + 0 N = 4 D = R(3N)1/2 D = 3.46R i D R The Frequency Re-Use for N = 4
- 19. D L * i L * j r In this case: j=2, i=1 Compute D based on “law of cosine” NijjirD ijjiLD jiLjLiLD jLiLjLiLD 3)(3/ )( )5.0(2 )3/2cos())((2)()( 22 2222 222222 222 =++= ++= −⋅⋅⋅−⋅+⋅= ⋅⋅−⋅+⋅= π rL 3=
- 21. Capacity expansion techniques • Due to more users • Techniques – Additional frequency spectrum • Expensive – Change in architecture • Sectoring • Cell splitting • Overlaid cell schemes – Channel allocation scheme • Traffic parameters – Next generation technology adaptation
- 22. Cell splitting
- 23. Microcell Zone • The Problems of sectoring can be addressed by Microcell Zone Concept • A cell is conceptually divided into microcells or zones • Each microcell (zone) is connected to the same base station (fiber/microwave link) – Doing something in middle of cell splitting and sectoring by extracting good points of both • Each zone uses a directional antenna • Each zone radiates power into the cell. • MS is served by strongest zone • As mobile travels from one zone to another, it retains the same channel, i.e no hand off • The BS simply switches the channel to the next zone site
- 26. Cell splitting • Divides a large cell into smaller regions called microcells • Large-power antenna is replaced with multiple low-power antennas • It rescales system by reducing cell size • Advantages: – Allows orderly growth of the system – The number of cells in an area increases – Number of clusters increases – Capacity increases – Maintain the co-channel reuse ratio • More frequent handoffs between cells specially for high mobility users • More equipment: Cost issues with buying the equipment. A cell site costs in the range of $650,000-$800,000 • Power Remote generation equipment required that is a cost and a security problem
- 27. Large cellsLarge cells Medium cellsMedium cells Small cellsSmall cells R/4R/4 R/2R/2 •Suppose original congested area is originally covered by 5 cells each with 80 channels •Capacity=5x80=400 users •After cell splitting, Rnew= R/2 We now have 24 cells •New capacity = 24x80 = 19200 users For n = 4,Transmit Power of New BS is 12 dB lower than original
- 28. Cell sectoring • By subdividing a cell into sectors, we can increase the capacity – 3-sector cell (each sector is 1200 ) – 6-sector cell (each sector is 600 )
- 29. Cell sectoring basic idea • Replace an omni-directional antenna with directional antennas • Point them to reduce co-channel interference • Sectors are based on either 1200 or 600 sectoring • Divide cell channel pool among the sectors in the cell
- 30. Tri-Sector antenna for a cell
- 31. Cell sectoring basic idea • Antennas are placed at the outer edges of the cell • Any channel may be assigned to any zone by the base station • Mobile is served by the zone with the strongest signal.
- 32. Cell sectoring advantage and disadvantage • Advantages: • Improves the SIR of the system • A gain of ~ 7 dB is achieved over omnidirectional systems when 1200 sectoring is used. • Problems: • Increased handoff requirements • Multiple antennas are required in a BS • Decreased trunking efficiency (less number of channels per sector)
- 33. Overlaid cells • 2 methods to increase capacity – For split band analog systems • Using macrocell, microcells • Requires dual mode mobile systems – For GSM or TDM • Helps migrating to other systems – Use same base stations • Tiering – Subcell in large cell D3 D2 D1 B3 A3 C3 A2 B2 C2 A1 B1 C1 B2 A2 C2 B3 A3 C2 B1 A1 C1 B1 A2 C2 1 group of overlaid 3/9 plan under overlaid 4/12 plan Part of another Group of An overlaid 3/9 plan
- 34. Overlaid cells
- 35. Channel allocation • Traffic in each cell is dynamic – Ex - sporting events, rock concerts, natural disasters – Change with time • Portable cellular sites – COW (cell on wheels) • Channel allocation techniques • To avoid non-availability of service – Blocking – Configure entire network capacity – Should be less than 2% • Stabilizes temporal fluctuations of blockage – Minimize call blocking probability – Serve subscribers effectively
- 36. • 3 methods to achieve efficient channel allocation – Fixed channel scheme • Fine tune the system where needed • Instead of equally dividing up channels over cells, some cells will receive larger channel allocations. • Periodically update – Channel borrowing • High traffic cells borrow channels from low traffic cells • Other cells in the cell lose that frequency • Channel returned after traffic is cleared – Dynamic channel allocation (DCA) • Available channel are placed in channel pool • Each channel assigned new call based on Signal to interference statistics • Channel used until SIR is met • Complex • Every cell site must be capable of transmitting every one of system’s assigned channels
- 37. Other capacity expansion schemes • Lee’s Microcell technology – Sectoring increases handoffs increasing loads on switching elements – Use zones instead of sectors – Reduce number of hand offs – Uses 3 antennas in a cell connected to same RBS – Antenna with best reception used for uplink and downlink
- 38. RBS Antenna patterns High speed communication links Zone antenna site
- 39. Cellular backhaul networks • Backhaul – “Getting data to the network backbone“ or transmitting from a remote site or network to a central or main site • 1G – Voice voice + data – Change in requirements for PSTN and PDN – Separate facilities for voice and data networks • 2G – Voice band signals are transcoded (compressed and reformatted) at BSC – Fiber optic cables between MSC and PSTN – Minimized costs – CDMA had IWF for data but same connection maintained for voice • 2.5G, 2.5+G – Own private wideband networks to backhaul both voice and data between MSC and BS • GSM – Packet switched networks – GPRS, PLMN added – Access web sites through private servers • 3G – High speed data services – All IP network, ATM – SONET/SDH
- 40. RBS RBS RBS BSC MSC IWF PSTNPSTN PDNPDN CDMA Cellular system data network connections
- 41. GPR S PLM N GPR S PLM N GSM PLMN GSM PLMN BSC To PDN To PSTN GSM cellular system data network connection GSM and GPRS coverage area
- 42. Mobility management • Characteristics/features – Provide mobility to user – Contrasting wireless and PSTN network – Location management – Location update – Paging messages – Handoff management
- 43. Location management • Keeping track of user’s present location – Eg. Voice call through PSTN • Working When a call is made that passes through PSTN, – Dedicated traffic channel set up from BS to MS – PSTN sets up circuit over fixed part of network – Wireless network allocates radio channels for air interface – For this MS location must be known • Objectives – Provide continuous radio link – Direct the packet in a network – Determine MS status in network – Check availability of the MS • Basic functions – Location updating – paging messages – transmission of location information between network elements
- 44. Location updating • Performed by MS • MS attached to a base station and is located initially • Periodically checked for changes • MS sends update message every time it changes point of access in a network • Exchange information for handoff • If a connection fails, systems page group of surrounding stations to track a MS
- 45. BSC A MSC/VLR A Update info To VLR Cell A1 in LAI 15 MS in instructed to Activate an SDCCH MS requests radio resource BTS A1 MS performs authentication And updating proceduresMS LAI 15 Radio resource request SDCCH – stand alone dedicated control channel LAI – Location area ID Cellular location updating
- 46. Location updating • Balance required between number of update messages and number of cells to be paged • Greater degree of certainty in locating the MS • Call blocking due to frequent paging • 2 updating schemes – Static • Geographic layout determines updating requirements – Dynamic • User’s mobility determines updating requirements
- 47. Location update in BSS
- 48. Paging messages • Incoming call/message to MS initiates paging of mobile • Consists of – Broadcasting message • To bring response from a single particular mobile • Starts communication processing • Required if exact cell of mobile not known • This information not available always – Blanket paging • Broadcast to all cells in a location area • Initiates MS to respond – Sequential paging • Paged to the cell where it was last registered • Parameters measured – RSS (received signal strength) – BER (bit error ratio) – Symbol – Block error rate – Parameters can undergo fluctuations due to signal fading
- 49. Paging messages • Handoff initiated when power from current RBS drops • Reduce ping-pong effect – Handover to and fro between a cell pair frequently • Solution is to define threshold • Fine tuning algorithm to improve system performance – Provide required QOS continuity during handoff
- 50. Radio resources and power management • Transmission powers represent a key degree of freedom in the design of wireless networks – Interference management: Due to the broadcast nature of wireless communication, signals interfere with each other. Power control helps ensure efficient spectral reuse and desirable user experience. – Energy management: Due to limited battery power in mobile stations, handheld devices, or any “nodes”. Power control helps minimize a key component of the overall energy expenditure. – Connectivity management: Due to uncertainty and time variation of wireless channels, the receiver needs to be able to maintain a minimum level of received signal so that it can stay connected with the transmitter and estimate the channel state. Power control helps maintain logical connectivity for a given signal processing scheme.
- 51. Power control • Design issues making it desirable to include dynamic power control in a cellular system – Received power must be sufficiently above the background noise for effective communication – Desirable to minimize power in the transmitted signal from the mobile – Reduce co-channel interference, lessen health concerns, save battery power – Energy efficient hardware and software
- 52. Types of power control • Open-loop power control – Depends solely on mobile unit – No feedback from BS – Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength • Closed-loop power control – Adjusts signal strength in reverse channel based on metric of performance – BS makes power adjustment decision and communicates to mobile on control channel
- 53. Power control • Achieve SIR tolerance with good quality communications • Must constantly adjust to change in signal strength caused by fading or mobility of MS • Usual Power control algorithm has 2 phases – Phase I: • MS registers with BSS • Determine minimum output power • Avoid possibility of a call drop – Phase II: • Additional measurements to reduce power • Output power of RBS is adjusted • Use complex algorithms achieve maximum SIR for all radio links
- 54. Power saving schemes • Discontinuous transmission (DTX) – Transmit during speech only – Extra over head – Compensate low-power background during silence – Adopted by MS, TRC, BSC also • Sleep mode – No activity – RF circuitry is powered off – Periodical awakening • Energy efficient designs – Semiconductor technologies – Power efficient modulation schemes – Software/hardware design – DSP technology
- 55. Radio resources and power management • Radio resource management (RRM) is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems • Types – Static RRM: Involves manual as well as computer aided fixed cell planning or radio network planning. – Dynamic RRM: Adaptively adjust the radio network parameters to the traffic load, user positions, quality of service requirements, etc.
- 56. Radio resource management • Provide functional improvements for RF operation – Implement system power control to reduce interference – Maximize capacity from above concept – Best available radio channel selection – Use wireless radio resource management scheme to enable handoff operations
- 57. Wireless network security • Wireless medium has certain limitations over the wired medium – Open access – Limited bandwidth – Systems complexity • 3G networks have a packet switched core – Connected to external networks like Internet – vulnerable to new types of attack
- 58. Security Issues In Cellular Networks • Authentication • Integrity • Confidentiality • Access Control • Operating Systems • Web Services • Location Detection • Viruses And Malware • Downloaded Contents • Device Security
- 59. Wireless network security requirements • Limitations • Security issues • GSM security – Global control equipment identity register (CEIR) • Database in Dublin, Ireland • List of handsets approved for GSM • White/Black listed – GSM cellular operators employ an EIR • Keep track of handsets to be blocked • Registered user of CEIR share database – CEIR creates master black list for operator
- 60. Network security requirements • Identification • Authentication • Billing • Maintenance • All-IP network – Increased management issues – Prevent hacking of systems – Software virus prevention
- 61. Network security • Techniques – Encryption • Scrambling using key • Secret key algorithms • Prevent threat from global terrorism