Overview of Wireless Communications ECE 480 Wireless Systems Lecture 6 Overview of Wireless Communications 22 Feb 2006

History of Wireless Communications Most radio systems transmit data through the use of a “data packet”, a burst of digital data First established in 1971, “ALOHANET” Connected 7 campuses on 4 islands Used “star” topology Initial LANs using this technology were not successful (only 20 kbps speed and low coverage) Wired LANs had 10 Mbps speed Today, wireless LANs have a speed in the tens of Mbps Wired LANs still faster, 1Gbps

Cellular Telephone System Not successful early due to lack of capacity AT&T developed the cellular concept Power falls off at a distance Two users separated by a distance can use the same frequency Analog systems introduced in 1983 – saturated by 1984 FCC increased the bandwidth Digital systems introduced in early 1990’s Higher capacity, lower cost, higher efficiency, and faster speed Standards still an issue

Satellite Systems LEO (Low Earth Orbit) – 2000 km altitude MEO (Medium Earth Orbit) – 9000 km altitude Geosynchronous (GEO) – 40,000 km altitude The higher the satellite, the more coverage, BUT the more power required Suppose that we have a cellular phone with a dipole antenna that delivers 1 mw in a symmetric pattern S = 2 x 10 – 11 W/m2 for LEO S = 5 x 10 – 14 W/m2 for LEO t d = 1.3 S for LEO t d = 0.27 mS for GEO (Significant)

What is Wireless Communications? Applications Voice Internet access Sensing and controls Data transfer LANs Text messaging Entertainment

What is Wireless Communications? Systems Cellular telephones Wireless LANs Wide area wireless data systems Satellite Systems Problem: These applications/systems all have different requirements Result: Fragmentation of standards, services, and products

Voice systems Low data rate requirements (20 kbps) High Bit Error Rate (10 – 3) Low total delay (100 ms) Data systems High data rates (1 – 100 Mbps) Low BER (10 – 8) No absolute delay requirement Real time video systems High data rate requirement High bit error rate Low total delay Paging, Text messaging Low data rate requirements Low BER

The most stringent of these requirements can readily be met by wired systems Data rates of several GHz BER – 10 – 12 Wireless systems must be tailored to the application More fragmentation Different protocols

Technical Issues Wireless channels are a difficult and capacity-limited broadcast communications medium Traffic patterns, user locations, and network conditions are constantly changing Applications are heterogeneous with hard constraints that must be met by the network Energy and delay constraints change design principles across all layers of the protocol stack

Wireless channels are a difficult and capacity-limited broadcast communications medium Spectrum is very crowded and expensive Bandwidth is usually auctioned to the highest bidder Spectrum must be reused in the same geographical area Need breakthroughs to enable systems to operate at higher frequencies or to use bandwidth more efficiently

Traffic patterns, user locations, and network conditions are constantly changing Mobility is both an advantage and a curse Signal experiences random fluctuations in time due to movement, obstacles, or reflection Channel characteristics appear to change randomly with time Security

Applications are heterogeneous with hard constraints that must be met by the network Must locate a user among billions traveling at 100 km/sec Must interface with wired networks Energy and delay constraints change design principles across all layers of the protocol stack Most wired systems are designed in layers Layers are designed in isolation with standards to interface between layers Wireless systems do not have the same baseline conditions Transmission is spotty and may change with time

Energy and delay constraints change design principles across all layers of the protocol stack Most wired systems are designed in layers Layers are designed in isolation with standards to interface between layers Wireless systems do not have the same baseline conditions Transmission is spotty and may change with time Energy use is also critical – batteries are large, heavy, and expensive

Evolution of Current Systems Wireless systems today 2G Cellular: ~30-70 Kbps. WLANs: ~10 Mbps. Next Generation 3G Cellular: ~300 Kbps. WLANs: ~70 Mbps. Technology Enhancements Hardware: Better batteries. Better circuits/processors. Link: Antennas, modulation, coding, adaptivity, DSP, BW. Network: Dynamic resource allocation. Mobility support. Application: Soft and adaptive QoS. (Quality of Service)

Consensus among experts: Other Tradeoffs: Rate vs. Coverage Rate vs. Delay Rate vs. Cost Rate vs. Energy Rate 4G 802.11b WLAN 3G 2G 2G Cellular Mobility Consensus among experts: Design breakthroughs are needed – not just improvements on present designs

Multimedia Design Requirements Voice Data Video Delay <100ms - <100ms Packet Loss <1% <1% BER 10-3 10-6 10-6 Data Rate 8-32 Kbps 1-100 Mbps 1-20 Mbps Traffic Continuous Bursty Continuous One-size-fits-all protocols and design do not work well Wired networks use this approach with poor results

Wireless Performance Gap WIDE AREA CIRCUIT SWITCHING User Bit-Rate (kbps) 14.4 digital cellular 28.8 modem ISDN ATM 9.6 modem 2.4 modem 2.4 cellular 32 kbps PCS 9.6 cellular wired- wireless bit-rate "gap" 1970 2000 1990 1980 YEAR LOCAL AREA PACKET SWITCHING Ethernet FDDI 100 M Polling Packet Radio 1st gen WLAN 2nd gen wired- wireless bit-rate "gap" .01 .1 1 10 100 1000 10,000 100,000

Quality of Service (QoS) QoS refers to the requirements associated with a given application, typically rate and delay requirements It is hard to make a one-size-fits all network that supports requirements of different applications Wired networks often use this approach with poor results, and they have much higher data rates and better reliability than wireless QoS for all applications requires a cross-layer design approach

Crosslayer Techniques Adaptive techniques Link, MAC, network, and application adaptation Resource management and allocation (power control) Diversity techniques Link diversity (antennas, channels, etc.) Access diversity Route diversity Application diversity Content location/server diversity Scheduling Application scheduling/data prioritization Resource reservation Access scheduling

Current Wireless Systems Cellular Systems Wireless LANs Satellite Systems Paging Systems Bluetooth Ultrawideband radios Zigbee radios

Cellular Telephone Systems Geographic region divided into cells Frequencies/timeslots/codes reused at spatially-separated locations. Co-channel interference between same color cells. Base stations/MTSOs coordinate handoff and control functions Shrinking cell size increases capacity, as well as networking burden BASE STATION MTSO Mobile Telephone Switching Office

Intercell Interference: Interference caused by users in different cells operating on the same channel set Reuse distance: the spatial separation of cells that reuse the same channel set BASE STATION

Reuse distance cannot be reduced below a minimum value depending on the characteristics of signal propagation within the aggregate of cells Early base stations were few and far between, usually on a high spot to cover as much area as possible (macrocells) Approximately uniform signal Circular cells (approximated by hexagon) Present – day base stations are smaller and closer to street level (microcells or picocells) Increases capacity Lower cost More complicated network design

More complicated network design Mobile phones change cells more frequently Handoffs must be processed more quickly Hexagonal shape may no longer be a good approximation Location management is more complicated

Cellular Phone Networks All base stations are connected to a mobile telephone switching office (MTSO) by a high speed link MTSO is a central controller Allocates channels within cell Coordinates handoffs Routing calls to and from mobile users San Francisco BS BS Internet New York MTSO MTSO PSTN BS Public Switched Telephone Network

MTSO routes voice calls through PSTN or to the internet User request a channel through a separate control channel Call handoff occurs when the base station or mobile detects that a signal has fallen below a minimum threshold MTSO queries whether a surrounding station can detect the signal Call is dropped if the signal strength drops below the minimum threshold San Francisco BS BS Internet New York MTSO MTSO PSTN BS Public Switched Telephone Network

Cellular systems are primarily digital Cheaper, faster, smaller, and use less power Higher capacity More efficient modulation techniques Compression techniques Encryption techniques Data services

Spectral Sharing (Multiple Access) Divides the signal dimensions along time, frequency, and/or code space axes Frequency Division Multiple Access (FDMA) Total System bandwidth is divided into orthogonal frequency channels The subcarrier pulse used for transmission is chosen to be rectangular. This has the advantage that the task of pulse forming and modulation can be performed by a simple Inverse Discrete Fourier Transform (IDFT) which can be implemented very efficiently as a I Fast Fourier Transform (IFFT). Receiver design is simplified

Time Division Multiple Access (TDMA) Time is divided orthogonally and each channel occupies the entire frequency band over its assigned timeslot More difficult to implement than FDMA – must be time synchronized Easier to accommodate multiple data rates

Code – Division Multiple Access (CDMA) Implemented using direct – sequence or frequency – hopping Direct sequence, each user modulates its data sequence by a different data sequence that is much faster In frequency hopping the carrier frequency used to modulate the narrowband data signal is varied by a chip (binary) sequence that may be faster or slower than the data sequence Results in a modulated signal that hops over different carrier frequencies

Efficient cellular system designs are "interference limited" Interference is higher than random noise Methods for improvement Cell sectorization Directional and smart antennas Multiuser detection Dynamic resource allocation

Second Generation (2G) Standards Europe uses GSM (Global Systems for Mobile Communications) standards Combination of TDMA and slow frequency hopping with frequency – shift keying for voice modulation US has several incompatible standards 900 MHz band has 2 standards IS-136 uses a combination of TDMA and FDMA and phase - shift keyed modulation IS-95 uses direct – sequence CDMA with phase - shift keyed modulation and coding 2 GHz PCS (personal communication system) has 3 standards, IS-36, IS-95, and GSM

All 2G standards support high - rate packet data services GSM supports data rates up to 140 kbps (GPRS) Enhanced Data Rates for GSM Evolution (EDGE) increases data rates up to 384 kbps Defines 9 different coding and modulation combinations, each optimized to a different value of S/N ratio (SNR) IS - 136 systems use GPRS and EDGE as well with rates up to 384 kbps IS - 95 supports data rates up to 115 kbps

3G Cellular : Voice and Data Data is bursty, whereas voice is continuous Typically require different access and routing strategies 3G “widens the data pipe”: 384 Kbps. Standard based on wideband CDMA Packet-based switching for both voice and data 3G cellular struggling in Europe and Asia Evolution of existing systems (2.5G,2.6798G): GSM + EDGE IS-95 (CDMA)+HDR 100 Kbps may be enough What is beyond 3G? The trillion dollar question

Wireless Local Area Networks (WLANS) 1011 01011011 0101 Internet Access Point WLANs connect “local” computers (100m range) Breaks data into packets Channel access is shared (random access) Backbone Internet provides best-effort service Poor performance in some applications (e.g. video)

All wireless LAN standards operate in unlicensed frequency bands 900 MHz, 2.4 GHz, 5.8 GHz, and the Unlicensed National Information Infrastructure (U - NII) band at 5 GHz No FCC license required – can cause interference with other users 1G systems were unsuccessful due to the large number of protocols

Wireless LAN Standards 802.11b (Current Generation) Standard for 2.4GHz ISM band (80 MHz) Frequency hopped spread spectrum 1.6-10 Mbps, 500 ft range 802.11a (Emerging Generation) Standard for 5GHz N-II band (300 MHz) OFDM with time division 20-70 Mbps, variable range Similar to HiperLAN in Europe 802.11g (New Standard) Standard in 2.4 GHz and 5 GHz bands OFDM Speeds up to 54 Mbps In 200?, all WLAN cards will have all 3 standards

Satellite Systems Cover very large areas Different orbit heights GEOs (39000 Km) versus LEOs (2000 Km) Optimized for one-way transmission Radio (XM, DAB) and movie (SatTV) broadcasting Most two-way systems struggling or bankrupt Expensive alternative to terrestrial system A few ambitious systems on the horizon

Paging Systems Broad coverage for short messaging Message broadcast from all base stations Simple terminals Optimized for 1-way transmission Answer-back is hard Overtaken by cellular

Bluetooth Cable replacement RF technology (low cost) Short range (10m, extendable to 100m) 2.4 GHz band (crowded) 1 Data (700 Kbps) and 3 voice channels Widely supported by telecommunications, PC, and consumer electronics companies Few applications beyond cable replacement Transmitter is imbedded into an IC Uses frequency hopping Applications – connection to a printer, etc..

IEEE 802.15.4 / ZigBee Radios Low-Rate WPAN Data rates of 20, 40, 250 kbps (slower than Bluetooth) Star clusters or peer-to-peer operation Support for low latency devices CSMA-CA channel access Very low power consumption (months to years) Frequency of operation in ISM bands

Ultrawideband (UWB) Radios UWB is an impulse radio: sends pulses of tens of picoseconds(10-12) to nanoseconds (10-9) Duty cycle of only a fraction of a percent A carrier is not necessarily needed Uses a lot of bandwidth (GHz) Low probability of detection Excellent ranging capability Multipath highly resolvable: good and bad Can use OFDM to get around multipath problem

Why is UWB Interesting? Unique Location and Positioning propertie 1 cm accuracy possible Low Power CMOS transmitters 100 times lower than Bluetooth for same range/data rate Very high data rates possible 500 Mbps at ~10 feet under current regulations 7.5 Ghz of “free spectrum” in the U.S. FCC recently legalized UWB for commercial use Spectrum allocation overlays existing users, but its allowed power level is very low to minimize interference “Moore’s Law Radio” # of transistors on a chip will double every 18 months Data rate scales with the shorter pulse widths made possible with ever faster CMOS circuits

Power Dissipation 10 W 802.11a 3G 802.11bg 1 W 100 mW Bluetooth UWB ZigBee 10 mW ZigBee UWB 1 mW 0 GHz 1GHz 2 GHz 3 GHz 4 GHz 5 GHz 6 GHz

Data Rate UWB 802.11g 802.11a 802.11b 10 Mbit/sec 1 Mbit/sec 3G Bluetooth ZigBee ZigBee 10 kbits/sec UWB 0 GHz 1GHz 2 GHz 3 GHz 4 GHz 5 GHz 6 GHz

Range 10 km 3G 1 km 100 m 802.11b,g 802.11a Bluetooth 10 m ZigBee ZigBee UWB 1 m UWB 0 GHz 1GHz 2 GHz 3 GHz 4 GHz 5 GHz 6 GHz

The Wireless Spectrum Licensed Government allocates specific frequency bands for specific purposes Usually auctioned to highest bidder FCC license required Unlicensed Created to encourage innovation Become very crowded very fast No FCC license required

Problem 1 - 10 This problem demonstrates the capacity increase associated with a decrease in cell size. Consider a square city of 100 square kilometers. Suppose you design a cellular system for this city with square cells, where every cell (regardless of cell size) has 100 channels and so can support 100 active users. (In practice, the number of users that can be supported per cell is mostly independent of cell size as long as the propagation model and power scale appropriately.) What is the total number of active users that your system can support for a cell size of 1 km 2? What cell size would you use if your system had to support 250,000 active users?

Solution What is the total number of active users that your system can support for a cell size of 1 km 2? # of cells = Area of city/Area per cell # of users = # of cells x users/cell = 100 x 100 = 10,000 b. What cell size would you use if your system had to support 250,000 active users? # cells = # users/users/cell

Now we consider some financial implications based on the fact that users do not talk continuously. Assume that Friday from 5 – 6 pm is the busiest hour for cell-phone users. During this time, the average user places a single call, and this call lasts two minutes. Your system should be designed so that subscribers need tolerate no greater than a 2% blocking probability during this peak hour. Blocking probability is computed using the Erlang B model: C = number of Channels A = U  H U = number of users  = average # of call requests per unit time per user H = average duration of a call

c. How many total subscribers can be supported in the macrocell system (1 km 2 cells) and in the microcell system (with cell size from part (b))? Macrocell C = 100 x 100 = 10,000 The unknown is U, the number of users Iterate U until P b = 0.02 U = 2670 subscribers Microcell: C = 267,000 U = 6,675,000 subscribers

d. If a base station costs $500,000, what are the base station costs for each system Macrocell: $ 50,000,000 Microcell: $ 1,250,000,000 e. If the monthly user fee in each system is $ 50, what will be the monthly revenue in each case? How long will it take to recoup the infrastructure (base station) cost for each system? Macrocell: $ 13,350,000/month 3.75 months Microcell: $ 333,750,000/month 3.75 months