More Wireless Networks

802.16

• Fixed broadband wireless  ("wireless Metropolitan Area Network")
• WiMAX - "worldwide interoperability for mobile access" - commercialized version of 802.16, promoted by Intel and others
• Provide DSL-like service without need for connection to phone or cable company
• Use a set of base stations (similar to cell phone towers) in an area
• A single base station can have a high enough data rate to simultaneously support more than 60 businesses with T1 level connectivity and hundreds of homes with DSL level connectivity
• "WiMax will be to DSL and cable modems what cellular was to land-line phones."

Physical Layer

• Original standard uses 10-66 MHz band - requires "line of sight" towers
• 802.16a revision (January 2003) uses 2-5 GHz band.  Does not require "line of sight" access.  Closer to 802.11, but with much longer range (3-10 miles)
• 802.16e extension intended to support roaming; i.e., mobile access, handoffs between base stations, handoffs between 802.16 nets and 802.11 nets.
• Modulation technique may be QPSK (50Mbps), QAM-16 (100 Mbps), or QAM-64 (150 Mbps) - which one is used depends on distance from base station.
• Basic multiplexing technique is OFDM (Orthogonal Frequency Division Multiplexing)

MAC Sublayer

• Centrally controlled by base station
• Base station sends frames containing data downstream; frame contains map of frame slots, indicating who is granted upstream slots. Some slots are available for contention
• All connection-oriented, with four classes of service (listed from best to worst):
• Constant bit rate
• Real time variable bit rate
• Non real time variable bit rate
• Best effort service

3G technologies

WCDMA - Wideband Code Division Multiple Access DSSS up to 384 Kbps (wide area) 2 Mbps (hot-spot areas); use QPSK and QAM modulation
HSDPA - High Speed Downlink Packet Access -- Transport channel for shared data (cell networks, CDMA optimized for voice ("always on")

3G vs 802.16

• WCDMA more susceptible to multipath fading, because of very fast chipping rate
  
• Frequency selective fading.   CDMA is more susceptible because of wideband signalling.
• Frequency offset.   Doppler shift due to mobility.  OFDM is more susceptible, subcarriers may no longer be orthogonal.
• AMC - Adaptive Modulation and Coding
• Change bit rate and modulation scheme to maximize throughput.  For example closer to base station, use higher level modulation scheme.
• With OFDM, different frequencies can use different data rates.
• Both use forward error correction; i.e., error correcting codes. In OFDM, strong channels can help weak ones. (error correction bits can be spread over subchannels.)
• SDMA - Space Division Multiple Access Allocate channels to users geographically to avoid interference, multipath fading, etc. OFDM can do this, CDMA cannot.
• Frequency reuse.  OFDM - Adjacent cells don't use the same frequencies (to avoid interference).  This reduces the available bandwidth in any given cell.
• CDMA is voice-oriented ("always on"),  OFDM is data oriented.  For example, each voice user needs a unique chipping code.  Each data user needs a unique code for control.  HSDPA uses up to 15 shared codes (using TDM) for data, but only for downstream.