CSC 290e Mobile Computing Wireless Medium Access 26 September 2001 Reference: Mobile Communications by Schiller Chapter 3 ------------------------ Last time we talked about dimensions along which to multiplex 1. space 2. frequency 3. time code * analogy to crowded party, can tune to different languages, languages can use same frequency ------------------------ Cellular Systems * cell radios can vary from tens of meters in buildings, and hundreds of meters in cities, and tens of kilometers in countryside * GSM: max 35km. Practical max: 10km * airplanes fly at 30000ft ~ 9.1 km * Why use small cells and not fewer big cells? 1. higher capacity - frequency reuse 2. less transmission power, not problem for base station, but problem for clients 3. local interference only - shorter distances to cause problems 4. robustness - system becomes decentralized * Some negatives 1. Infrastructure complex to connect all base stations, lots of switches 2. handovers - can happen frequently with fast movement and small cells 3. frequency planning - to avoid interference * can increase the number of cells within a cell by using sectorized antennas ------------------------ Medium access control - managing shared media wired strategies won't always work. CSMA/CD - carrier sense multiple access with collision detection (for wired) sender senses meduim if medium is busy, sender waits until free if medium is free, sender starts transmitting and keeps listening if sender detects a collision while sending, it stops immediately and sends a jamming signal why won't work: cannot accurately sense wireless medium (hidden terminal) cannot detect wireless collision (hidden terminal) hidden terminal: a reaches b but not c c reaches b but not a b reaches a and c a sarts sending to b, c doesn't see it and starts sending to b collision at b, neither a nor c can detect a is hidden from c exposed terminal b sends something to a. c wants to taransmit data to some to other phone outside the interefeerence ranges of a and b. c senses busy and waits. c postpones transmission. c is exposed to b ------------------------ Space Division Multiple Access (SDMA) * assign an optimal base station based on which frequencies, time slots, or code are available in each cell. * used in combination with other schemes Frequency division multiple access (FDMA) * can be fixed (e.g., radio stations) or dynamic (demand driven) * can use same frequency all the time, * or (FDMA+TDMA) do frequency hopping - s/r both need to know the hopping pattern. Keep pattern for "long" time. Keep each freq for short time. * important for uplink/downlink pairs. allocate part of band to uplink, part to downlink Time Division multiple access (TDMA) * receiver tunes to only one frequency (simpler technology) * synchronize sender/receiver in time * combine with FDMA to increase flexibility * Fixed TDM fixed allocation of the timeslots requires synchronization, but this is straightforward works well for fixed bandwidth apps: used in many digital phone systems including GSM uplink and downlink are separated in time * classical Aloha. time division, but no access control! University of Hawaii, ALOHANET, wirelessly connect buildings each station can accesses the medium at ANY time if collision occurs, data is lost resolving data loss is left to higher layers (e.g., retransmission) works fine for light loads simple, no complicated access mechanisms, no central coordinator adapts to varying number of stations theoretical maximum throughput = 18$ * slotted Aloha. time division, synchronized senders transmission can only start at beginning of time slot access still not coordinated doubles the throughput over classical Aloha because potential overlap is reduced theoretial maximum throughput = 36% why: when packets collide, they will collide completely, not partially CDMA use a code which can be used to separate signal from noise No initial allocation of frequencies among base stations. adjacent cells use same frequencies, different codes