Outline Outline • What is wireless? – Overview – Wireless parameters – Cellular architecture Wireless Networks Wireless Networks – Media multiple access • Wireless LAN 802.11 • MANET – Mobile Ad-hoc NETworks Amnon Jonas Amnon Jonas April 2003 April 2003 • Broadband fixed wireless access 802.16 May 18, 2003 April 2003 - 2 - Wireless Networks Wireless Networks • Architecture – Point to multi-point: many stations connected to the network through a single base station (Fixed W.L., 802.11, Cell. Phones, satellite) – Point to point: A wireless link between 2 points (IrDA, B.T.) – Multi-point to multi-point: Ad-hoc network (802.11) What is wireless? What is wireless? • Services – Internet/Network access – Voice – Video streaming • Target market – Residential (802.16-MMDS, cell. Phones) – Business (802.16-LMDS, satellite) May 18, 2003 April 2003 - 4 - 1
Wireless standards Path Loss Phenomena Wireless standards Path Loss Phenomena • Open space attenuation WAN Mobility – Signal strength ~ 1/d 2 • Flat fading WLAN Vehicle – Lower signal strength due to interference of multiple signal phases S Outdoor • Frequency selective fading High performance WLAN P M Walk – Fading is depend on the frequency within the signal bandwidth – results in signal A distortion - UMTS D • Distance related attenuation , 802.11a/g 5 802.11b Fixed HomeRF Wideband 9 - – Signal strength ~ 1/d γ , 2< γ <5 S Cellular Wired LAN H2 I , M Walk S Indoor p a t h h t G s i g e o f L i n Fixed wireless Fixed/ Bluetooth 802.16 Desktop Reflected path 0,1 1 10 100 Mbps (PHY layer) May 18, 2003 May 18, 2003 - 5 - - 6 - PHY parameters PHY parameters Symbol Rate & Modulation Symbol Rate & Modulation • Bandwidth • Modulation – The width of the range of frequencies that an electronic – Modulation is the addition of information to an signal occupies on a given transmission medium electronic or optical signal carrier – Examples: voice – 3 KHz; TV – 6 MHz – Common modulations: AM, FM, – There is a relation between the width of the band to its – More complex modulations: QAM, QPSK robustness (~QAM4) • Roll off factor - a lpha – QAM N provides log2N bits/signal – defines the sharpness of the filter in the frequency domain – TV: QAM 64 – 256, wireless: QAM 4 - 64 – The lower the alpha, the sharper the filter in the frequency QPSK/QAM4 • Example for channel rate calculation (6 MHz, domain and the higher the overshoot in the time domain α =13%, QAM64): – TV – 13% • Symbol rate 6 Msym Mbits log 64 ~ 5 . 3 ⋅ 6 ~ 32 Bandwidth = Symbol Rate (1 + α ) sec sec 2 1 . 13 – TV – 6,000,000/1.13 = 5,309,735 Symbols/sec QAM16 May 18, 2003 May 18, 2003 - 7 - - 8 - 2
Error correction Interleaving Error correction Interleaving • Errors caused by interference, fading and random • Fading tends to result errors in blocks noise • Interleaver randomizes the errors • It works by adding redundancy to the transmitted • Interleavers introduce delays to the system signal • Mainly two types: – Block coding (e.g. Reed Solomon) rows rows – Convolutional coding (e.g. trellis code with Viterbi decoder) Transmitter Transmitter 1 2 3 4 5 6 • Example: RS (188,204) 7 8 9 10 11 12 – Corrects 8 bytes from 188 bytes 1 2 rows rows 13 14 15 16 17 18 – Can improve 10 -4 to 10 -14 19 20 21 22 23 24 7 8 Receiver Receiver 13 25 26 27 28 29 30 • FEC works best when the errors are randomly 19 … … 8 2 31 25 19 13 7 1 8 2 31 25 19 13 7 1 31 32 33 34 35 36 distributed columns columns 25 31 columns columns May 18, 2003 May 18, 2003 - 9 - - 10 - Super cell Supercell Architecture Supercell Architecture • Single cell configuration: • Very large cell – Large service area - a typical radius of up to 30 mile is covered. – Large service area with a radius of up to 30 mile is covered. – High antenna at base transceiver station (BTS) ~ 80m • Advantages – A high-gain rooftop directional CPE antenna – Fast and low-cost initial deployment services a large area – Line-of-sight (LOS) connection between transmitter and receiver – All the equipment is located in a single location – simplify maintenance • Advantages – Frequency reuse in angle and polarization may be possible with – Due to LOS propagation, carrier-to-noise ratio (C/N) values of sectorization. around 30 dB can be sustained • Disadvantages: – Use of high-order modulation possible – Not scalable. • Disadvantages: – Limited bandwidth – Not scalable – Limited coverage – dead areas – Limited coverage due to the strict need for LOS links – High power – battery lifespan, high radiation, interference to other • Attractive for: systems, regulation limits – Initial network roll out – Expansive antennas • Toll towers already available in many place – Rural, low density population areas May 18, 2003 May 18, 2003 - 11 - - 12 - 3
Supercell Architecture Multicell Architecture Architecture Supercell Architecture Multicell • SECTOR COVERAGE • OMNIDIRECTIONAL • • • Cellular architecture: SECTOR COVERAGE OMNIDIRECTIONAL – Multiple cells, each cover a smaller area • Advantages – Frequency reuse => unlimited capacity – The architecture is scalable in both capacity and coverage • Disadvantages: – Infrastructure is distributed in many locations – Higher deployment and maintenance costs May 18, 2003 May 18, 2003 - 13 - - 14 - Adaptive PHY parameters Adaptive PHY parameters • Burst profile – Modulation and FEC and FEC • Dynamically assigned according to link conditions – Burst by burst, per subscriber station – Trade-off capacity vs. robustness in Multiple Access Multiple Access real time real time • Roughly doubled capacity for the same cell area • Burst profile for downlink broadcast channel is well - k n own – All other burst profiles could be configured “on the fly” – Subscriber station capabilities recognized at registration May 18, 2003 April 2003 - 15 - 4
Frequency Division Multiple Access Frequency Division Multiple Access Duplexing Duplexing FDMA FDMA • Separation of uplink and downlink channels • System bandwidth divided into channels assigned to different users • Frequency Division Duplexing – FDD: – No synchronization issues. • Advantages – Uplink and downlink channels may fade independently – Narrowband channels – Duplexor required to separate signals 1 1 – Low complexity • Time Division Duplexing – TDD: 2 2 – Allows channel estimation 3 3 – Simple duplex equipment • Disadvantages 4 4 – Transmission in one direction can be used to measure 5 5 – Multiple radios at base station channel in other direction. 6 6 – Dedicated channels (idle ones wasted) – Flexible bandwidth allocation between uplink and downlink – Interference between adjacent channels – Requires synchronization – Need guard bands to prevent overlap – Difficult frequency reuse May 18, 2003 May 18, 2003 - 17 - - 18 - Time Division Multiple Access Time Division Multiple Access Code Division Multiple Access Code Division Multiple Access TDMA TDMA CDMA CDMA • CDMA • Time divided into slots assigned to different users – Each station can transmit over the entire spectrum all the time • Advantages – Multiple simultaneous transmissions are separated using orthogonal codes – Common radio equipment for all users at the base station • Each station is assigned a unique m-bit code – Overbooking can be easily supported • The signal is multiplied with the code • The signal is recovered by multiplying with the negation code • Disadvantages • If the codes are orthogonal, other stations transmissions are canceled – Synchronization required • Otherwise, other stations transmissions are filtered out as white noise 1 2 3 1 2 3 1 1 2 3 1 2 3 1 – Wider channel exacerbate ISI • Advantages – Guard band between transmissions – The bandwidth is shared over several users is needed • Disadvantages 4 5 6 4 5 6 4 4 5 6 4 5 6 4 – Preamble and equalization training – Requires strict time and power synchronization sequence is needed – Maximum rate is limited – High complexity May 18, 2003 May 18, 2003 - 19 - - 20 - 5
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