W.wan.1-2 Wireless WAN: cellular basics Wireless network topologies Wireless WAN:Cellular Basics Cellular topology Module W.wan.1 Cellular hierarchy & geometry Cellular expansion Dr.M.Y.Wu@CSE Dr.W.Shu@ECE End of module W.wan.1 Shanghai Jiaotong University University of New Mexico Shanghai, China Albuquerque, NM, USA © by Dr.Wu@SJTU & Dr.Shu@UNM 1 W.wan.1-3 W.wan.1-4 Wireless network topologies Cellular topology How a MT (Mobile Terminal) communicate with A multi-BS infrastructure network configuration another? Æ cellular topology Infrastructure Frequency reuse: spatially reuse the available spectrum Centralized, with connected ¾ The same spectrum can support multiple users separated by ¾ BS (Base Station) or a distance ¾ AP (Access Point) Based on the attenuation of the signal strength of EM Any communication has to go through BS/AP waves with distance Ad hoc 9 The distance is sufficiently large Distributed, without the need for a fixed infrastructure 9 Transmit powers are reasonable controlled Rapid deployment, reconfigurable Cellular topology makes frequency reuse possible Single-hop, or multi-hop ad hoc networks © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM W.wan.1-5 W.wan.1-6 Cellular topology Cellular topology Basic unit: cell Channels Cells are grouped into a cluster, each cluster utilizes MT Æ BS, uplink the entire available radio spectrum MT Å BS, downlink No two cells within a cluster use same frequency N, # of cells in the cluster, named as Interference in a cellular architecture ¾ cluster size, or Cochannel interference, due to using the same frequency in ¾ frequency reuse factor cells of different clusters W, the total available spectrum ¾ The same frequency are separated by a minimum distance: B, the bandwidth needed per user reuse distance D m, # of cells required to cover an area ¾ Js: the number of cochannel cells with interference n, # of simultaneous users n = m/N × W/B, also defined as Adjacent channel interference, due to usage of adjacent capacity frequency within a cluster © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM W.wan.1-7 W.wan.1-8 Cellular hierarchy Cellular geometry Support heterogonous cell size To extend the coverage ¾ Megacells, hundreds of km, with satellites ¾ Macrocells, a few of km, outdoor ¾ Picocells, a few tens of m, inside building ¾ Microcells, a few hundreds of m, along streets or highways To increase the capacity where user density is high To adapt certain coverage needed by applications Equilateral Regular Square hexagon ¾ Femtocells, x m, connecting devices in a PAN area triangle © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM W.wan.1-9 W.wan.1-10 Cellular geometry Cellular geometry The hexagon cell shape is by default Signal-to-interference ratio Sr N = i2 + ij + j2, Pt, transmission power from a BS ¾ i=1, j=1, N = 3 P = P × d-α, signal strength from the desired BS ¾ i=2, j=1, N = 7, used in AMPS (advanced mobile phone service) d t × -α th R, cell radius Pf,i = Pt di , signal strength from the i interfering BS d, distance between R Sr = Pd / Σ Pf,i two adjacent cells The higher of Sr, the performance better d = 3½ × R D ¾ Closer to the desired BS D, distance between ¾ Further away from other interfering BS cochannel cells An example, N=7 ½ D/R = (3N) , reuse ratio d Consider only 6 cochannel cells that makes up the first tier of since: interference, di=D, d=R, α=4, 2 2 2 α cos (120°) = (d + (2d) –D )/4d 2 Sr = Pd / Σ Pf,I = (D/R) /6 = 3/2×N = (in dB) 1.76 + 20 log N ≅ 18 dB © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM W.wan.1-11 W.wan.1-12 Cellular geometry Cellular expansion Example: AMPS Capacity expansion M = 25 MHz, FDMA Obtain additional spectrum ¾ 12.5 MHz uplink R ¾ Simple but expensive ¾ 12.5 MHz downlink Change the cellular architecture K = 30 KHz D M/K = 416 channels ¾ Cell splitting ¾ 21 control ¾ Cell sectoring with directional antenna ¾ 395 voice d ¾ Lee’s microcell zone architecture N=7, i=2,j=1 ¾ Multiple reuse factors ¾ D = 4.58R Dynamical frequency allocation & nonuniform distribution ¾ Sr = 18 dB Enhancement from analog FM into digital TDMA, CDMA N=12, i=j=2 © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM W.wan.1-13 W.wan.1-14 Cellular expansion Cellular expansion Capacity expansion: cell splitting Capacity expansion: cell sectoring Splitting cells into smaller cells Use directional antenna to restrict ¾ Macrocells: A and partition the cell into ¾ Microcells: a A sectors For smaller cell a, R’ = R/2, Increase Sr to a level that P remains the same, enables us to use a lower t a A D’ = D/2 frequency reuse factor Example, N=7 Sr remains the same A For larger cell A, Originally, Sr = 18 dB, Js = 6 ’ With 120° sectoring, Js = 2 D has changed 4 2 Sr = Pd / Σ Pf,I = (D/R) /Js = 9N /Js how to retain Sr ??? ’ 2 ’ Î Sr = 9N /Js = 3Sr © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM W.wan.1-15 W.wan.1-16 Cellular expansion Wireless WAN: cellular basics Capacity expansion: cell sectoring Wireless network topologies Example, N=4 With omni-direction Cellular topology Js = 6 Cellular hierarchy & geometry Originally, Sr = 13.8 dB, With 120° sectoring, Cellular expansion ’ Js = 2 End of module W.wan.1 4 Sr = Pd / Σ Pf,i = (D/R) /Js 2 From 1G to 4G: module W.wan.2 = 9N /Js ’ 2 ’ GSM & CDMAone: W.wan.3 ÎSr = 9N /Js = 3Sr Î= 19.9 dB > 18 dB UMST/W-CDMA: W.wan.4 WLL & 802.16: W.wan.5 WiMAX/802.16: W.wan.6 © by Dr.Wu@SJTU & Dr.Shu@UNM © by Dr.Wu@SJTU & Dr.Shu@UNM.