Data and Computer Communications Chapter 10 Cellular Wireless Networks
Cellular Wireless Networks 5 PSTN Switch Mobile Telecomm Switching Office (MTSO) 3 4 2 1 Base Station 0 2016-08-30 2
Cellular Wireless Networks Developed to increase mobile phone capacity Area divided into cells Served by base station Each with own range of frequencies Adjacent cells use different frequencies to avoid crosstalk Cells sufficiently distant from each other can use the same frequency band
Increasing Capacity Frequency reuse Share nearby cell frequencies without interfering with each other Allow multiple simultaneous conversations 10 to 50 frequencies per cell Allow communications within cell on given frequency Limit escaping power to adjacent cells Frequency borrowing Taken from adjacent cells by congested cells Or assign frequencies dynamically
Increasing Capacity Cell splitting Microcells - Use smaller cells in high use areas Use reduced power to cover a much smaller area Good for city streets, roads, inside large buildings Cell Sectoring Cell is divided into wedge shaped sectors (3 6 per cell) Each sector is assigned a separate subset of the cell s channels Directional antennas at base station are used to focus on each sector
Operation of Cellular System Two types of channels are available between mobile unit and base station (BS) Traffic (Data) channels Carry voice and data Control channels Set up and maintain calls Establish relationship between mobile unit and nearest BS
Steps for Call Mobile unit initialization Cells with different frequency bands repetitively broadcast on different setup channels The mobile unit selects the strongest setup channel Handshake takes place between the mobile unit and the MTSO through the BS to identify the user and register its location Scanning procedure is repeated periodically to account for the motion of the unit If the unit enters a new cell, then a new BS is selected Mobile-originated call Mobile unit originates a call by sending the number of the called unit on the preselected setup channel BS sends the request to the MTSO
Steps for Call (continued) Paging MTSO sends a paging message to certain BSs depending on the called mobile number Each BS transmits the paging signal on its own assigned setup channel Call accepted The called mobile unit recognizes its number on the setup channel and responds to that BS, which sends the response to the MTSO MTSO sets up a circuit between the calling and called BSs At the same time, MTSO selects an available traffic channel within each BS s cell and notifies each BS, which in turn notifies its mobile unit Two mobile units tune to their respective assigned channels
Steps for Call (continued) Ongoing call While the connection is maintained, two mobile units exchange voice or data signals, going through their respective BSs and the MTSO Handoff If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell
Location Management Location Register HLR (Home Location Register) VLR (Visitor Location Register) HLR VLR HLR VLR MTSO MTSO BS BS BS BS Mobile Terminal 2016-08-30 10
Location Register Home Location Register (HLR) Database for subscriber Customer ID, Authentication, Billing detail The data it contains is remotely accessed by all the MSCs and the VLRs Visitor Location Register (VLR) A copy of most of the data stored at the HLR Temporary data which exists for only as long as the subscriber is active in the particular area covered by the VLR Eliminates the need for excessive and time-consuming references to the home HLR database
Handoff Intra-cell handoff A new channel is allocated when signal strength is degraded Inter-cell handoff MTSO A 2 1 3 When mobile terminal moves to other cells Frequent handoffs occur when cell size is too small in urban area MTSO B 2016-08-30 12
Mobile Radio Propagation Effects Signal strength Strength of signal between BS and mobile unit needs to be strong enough to maintain signal quality Not too strong so as to create co-channel interference Fading Time variation of received signal Caused by changes in transmission path Even if signal strength is in effective range, signal propagation effects may disrupt the signal
Types of Fading Fast fading Rapid variations in signal strength occur over distances of about one-half a wavelength Slow fading Change in the average received power level due to user passing different height buildings, vacant lots, intersections, etc. Flat fading All frequency components of the received signal fluctuate in the same proportions simultaneously Selective fading Attenuation occurring over a portion of the bandwidth of the signal
Wireless Network Generations Technology 1G 2G 2.5G 3G 4G Design began 1970 1980 1985 1990 2000 Implementation 1984 1991 1999 2002 2012 Services Analog voice Digital voice Higher capacity packetized data Higher capacity, broadband Completely IP based Data rate 1.9. kbps 14.4 kbps 384 kbps 2 Mbps 200 Mbps Multiplexing FDMA TDMA, CDMA TDMA, CDMA CDMA OFDMA, SC-FDMA Core network PSTN PSTN PSTN, packet network Packet network IP backbone
Summary Principles of cellular networks Cellular network organization Operation of cellular systems Mobile radio propagation effects Cellular network generations