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  1. 1. ECE 4331, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 26 Nov. 19 th , 2009                                                             
  2. 2. Outline <ul><li>Term paper, only journal. For those who did not give me the title on time, I think you would not work for the term project, right?  </li></ul><ul><li>General Wireless System Architecture </li></ul><ul><li>Media Access Control </li></ul><ul><ul><li>Classes of MAC protocols </li></ul></ul><ul><ul><li>Simplex and Duplex Channels </li></ul></ul><ul><li>Coordinated MAC Schemes </li></ul><ul><ul><li>FDMA </li></ul></ul><ul><ul><li>TDMA </li></ul></ul><ul><ul><ul><li>Capacity of TDMA systems and which factors affect the capacity. </li></ul></ul></ul><ul><ul><li>Spread Spectrum Access Methods </li></ul></ul><ul><ul><ul><li>FHMA </li></ul></ul></ul><ul><ul><ul><ul><li>Case study: Bluetooth </li></ul></ul></ul></ul><ul><ul><ul><li>CDMA </li></ul></ul></ul><ul><ul><ul><li>Hybrid Spread Spectrum Schemes. </li></ul></ul></ul><ul><li>Case Study </li></ul>
  3. 3. Multiple Access <ul><li>How can we share a wireless channel: </li></ul><ul><ul><li>Results in Wireless Medi a Access Control Protocols </li></ul></ul><ul><li>How we can change base stations: Results in Handoff algorithms and protocols </li></ul><ul><li>How can we seamlessly support mobile applications over wireless links: </li></ul><ul><ul><li>Results in mobility protocols like Mobile IP, Cellular IP, etc. </li></ul></ul><ul><li>How can we design efficient transport protocols over wireless links: </li></ul><ul><ul><li>Results in solutions like SNOOP, I-TCP, M-TCP, etc. </li></ul></ul><ul><li>How different wireless networks/systems are designed? </li></ul><ul><ul><li>Bluetooth, IEEE 802.11, GSM, etc. </li></ul></ul>
  4. 4. Wireless System Architecture and Functions Physical Radio Transceiver Frame Controller Link Controller Wireless Subnet Controller TCP/IP Carrier frequency, channel bandwidth, carrier detect, Captude detect, channel data rate, modulation, Received signal strength (RSSI), transmit power, Power control, … Framing and frame synchronization, error control, CRC, bit scrambling, widening, …. Medium Access Control, MAC level Scheduling, Link Layer Queueing, Link Layer Reliability – ACKs, NACKs, …. Neighbor Discovery and Registration, Multicasting, Power Saving Modes, Address Translation (IP-MAC), Routing, Quality of Services, Subnet Security Wireless Link Layer (Layers 1 and 2 in ISO/OSI Network Reference Model) Applications
  5. 5. Medium Access Control <ul><li>Wireless spectrum (frequency band) is a very precious and limited resource. </li></ul><ul><ul><li>We need to use this resource very efficiently </li></ul></ul><ul><li>We also want our wireless system to have high user capacit y </li></ul><ul><ul><li>A lot of (multiple) users should be able to use the system at the same time. </li></ul></ul><ul><li>For th ese reason s most of the time , multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system. </li></ul><ul><ul><li>The algorithms and protocols that enables this sharing by multiple users and controls /coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…) </li></ul></ul>
  6. 6. Wireless Media Access Control <ul><li>Random Schemes (Less-Coordinated) </li></ul><ul><ul><li>Examples: MACA, MACAW, Aloha, 802.11 MAC,… </li></ul></ul><ul><ul><li>More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs </li></ul></ul><ul><li>Coordinated Schemes </li></ul><ul><ul><li>Examples: TDMA, FDMA, CDMA </li></ul></ul><ul><ul><li>More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,… </li></ul></ul><ul><li>Polling based Schemes </li></ul><ul><ul><li>Examples: Bluetooth, BlueSky,… </li></ul></ul><ul><ul><li>Access is coordinated by a central node </li></ul></ul><ul><ul><li>Suitable for Systems that wants low-power, aims to carry voice and data at the same time. </li></ul></ul>
  7. 7. Duplexing <ul><li>It is sharing the media between two parties . </li></ul><ul><li>If the communication between two parties is one way, the it is called simplex communication . </li></ul><ul><li>If the communication between two parties is two- way, then it is called duplex communication . </li></ul><ul><li>Simplex commu nication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver. </li></ul><ul><li>Duplex communication achieved by: </li></ul><ul><ul><li>Time Division (TDD) </li></ul></ul><ul><ul><li>Frequency Division (FDD) </li></ul></ul><ul><ul><li>Some other method like a random access method </li></ul></ul>
  8. 8. Duplexing <ul><li>Usually the two parties that want to communication in a duplex manner (both send and receive) are: </li></ul><ul><ul><li>A mobile station </li></ul></ul><ul><ul><li>A base station </li></ul></ul><ul><li>Two famous methods for duplexing in cellular systems are: </li></ul><ul><ul><li>TDD: Time Division Duplex </li></ul></ul><ul><ul><li>FDD: Frequency Division Duplex </li></ul></ul>
  9. 9. Duplexing - FDD <ul><li>A duplex channel consists of two simplex channel with different carrier frequencies </li></ul><ul><ul><li>Forward band : carries traffic from base to mobile </li></ul></ul><ul><ul><li>Reverse band : carries traffic from mobile to base </li></ul></ul>Reverse Channel Forward Channel frequency f c,R f c,,F Frequency separation Frequency separation should be carefully decided Frequency separation is constant B M F R Base Station Mobile Station
  10. 10. Duplexing - TDD <ul><li>A single radio channel (carrier frequency) is shared in time in a deterministic manner. </li></ul><ul><ul><li>The time is slotted with fixed slot length (sec) </li></ul></ul><ul><ul><li>Some slots are used for forward channel (traffic from base to mobile) </li></ul></ul><ul><ul><li>Some slots are used for reverse channel (traffic from mobile to base) </li></ul></ul>B M Base Station Mobile Station F R R R R 0 1 2 3 4 5 6 7 … … . Reverse Channel Forward Channel time T i Time separation T i+1 channel Slot number F F F
  11. 11. Duplexing – TDD versus FDD <ul><li>FDD </li></ul><ul><ul><li>FDD is used in radio systems that can allocate individual radio frequencies for each user. </li></ul></ul><ul><ul><ul><li>For example analog systems: AMPS </li></ul></ul></ul><ul><ul><li>In FDD channels are allocate d by a base station. </li></ul></ul><ul><ul><li>A channel for a mobile is allocated dynamically </li></ul></ul><ul><ul><li>All channel s that a base station will use are allocated usually statically. </li></ul></ul><ul><ul><li>More suitable for wide-area cellular networks: GSM, AMPS all use FDD </li></ul></ul><ul><li>TDD </li></ul><ul><ul><li>Can only be used in digital wireless systems (digital modulation). </li></ul></ul><ul><ul><li>Requires rigid timing and synchronization </li></ul></ul><ul><ul><li>Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile. </li></ul></ul><ul><ul><ul><li>Such as cordless phones… </li></ul></ul></ul>
  12. 12. Multiple Access - Coordinated <ul><li>We will look now sharing the media by more than two users. </li></ul><ul><li>Three major multiple access schemes </li></ul><ul><ul><li>Time Division Multiple Access (TDMA) </li></ul></ul><ul><ul><ul><li>Could be used in narrowband or wideband systems </li></ul></ul></ul><ul><ul><li>Frequency Division Multiple Access (FDMA) </li></ul></ul><ul><ul><ul><li>Usually used narrowband systems </li></ul></ul></ul><ul><ul><li>Code Division Multiple Access </li></ul></ul><ul><ul><ul><li>Used in wideband systems. </li></ul></ul></ul>
  13. 13. Narrow- and Wideband Systems <ul><li>Narrowband System </li></ul><ul><ul><li>The channel bandwidth (frequency band allocated for the channel is small) </li></ul></ul><ul><ul><ul><li>More precisely, the channel bandwidth is large compared to the coherence bandwi d th of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel) </li></ul></ul></ul><ul><ul><ul><li>AMPS is a narrowband system (channel bandwidth is 30kHz in one-way) </li></ul></ul></ul><ul><li>Wideband Systems </li></ul><ul><ul><li>The channel bandwidth is large </li></ul></ul><ul><ul><ul><li>More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel. </li></ul></ul></ul><ul><ul><ul><li>A large number of users can access the same channel (frequency band) at the same time. </li></ul></ul></ul>
  14. 14. Narrow- and Wideband Systems <ul><li>Narrowband Systems </li></ul><ul><ul><li>Could be employing one of the following multiple access and duplexing schems </li></ul></ul><ul><ul><ul><li>FDMA/FDD </li></ul></ul></ul><ul><ul><ul><li>TDMA/FDD </li></ul></ul></ul><ul><ul><ul><li>TDMA/TDD </li></ul></ul></ul><ul><li>Wideband systems </li></ul><ul><ul><li>Could be employing of the followin g multiple access and duplexing schemes </li></ul></ul><ul><ul><ul><li>TDMA/FDD </li></ul></ul></ul><ul><ul><ul><li>TDMA/TDD </li></ul></ul></ul><ul><ul><ul><li>CDMA/FDD CDMA/TDD </li></ul></ul></ul>
  15. 15. Cellular Systems and MAC Cellular System Multiple Access Technique AMPS FDMA/FDD GSM TDMA/FDD USDC (IS-54 and IS-136) TDMA/FDD PDC TDMA/FDD CT2 Cordless Phone FDMA/TDD DECT Cordless Phone FDMA/TDD US IS-95 CDMA/FDD W-CDMA CDMA/FDD CDMA/TDD cdma2000 CDMA/FDD CDMA/TDD
  16. 16. Frequency Division Multiple Access <ul><li>Individual radio c hannels are assigned to individual users </li></ul><ul><li>Each user is allocated a frequency band (channel) </li></ul><ul><ul><li>During this time, no other user can share the channel </li></ul></ul><ul><li>Base station allocates channels to the users </li></ul>B M M M … f 1,F f 2,F f N,F f N,R f 2,R f 1,R
  17. 17. Features of FDMA <ul><li>An FDMA channel carries one phone circuit at a time </li></ul><ul><li>If channel allocated to a user is idle, then it is not used by someone else: waste of resource. </li></ul><ul><li>Mobile and base can transmit and receive simultaneously </li></ul><ul><li>Bandwidth of FDMA channels are relatively low. </li></ul><ul><li>Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low) </li></ul><ul><li>Lower complexity systems that TDMA systems. </li></ul>
  18. 18. Capacity of FDMA Systems … Frequency spectrum allocated for FDMA system Guard Band Guard Band channel B t : Total spectrum allocation B guard : Guard band allocated at the edge of the spectrum band B c : Bandwidth of a channel AMPS has 12.M H z simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416.
  19. 19. Time Division Multiple Access <ul><li>The allocated radio spectrum for the system is divided into time slots </li></ul><ul><ul><li>In each slot a user can transmit or receive </li></ul></ul><ul><ul><li>A user occupies s a cyclically repeating slots. </li></ul></ul><ul><ul><li>A channel is logically defined as a particular time slot that repeats with some period. </li></ul></ul><ul><li>TDMA systems buffer the data, until its turn (time slot) comes to transmit. </li></ul><ul><ul><li>This is called buffer-and-burst method. </li></ul></ul><ul><ul><li>Leaky bucket </li></ul></ul><ul><li>Requires digital modulation </li></ul>
  20. 20. TDMA Concept 1 2 3 … N 1 2 3 … . N … 1 2 3 … N 1 2 3 … . N … Downstream Traffic: Forward Channels: (from base to mobiles) Upstream Traffic: Reverse Channels: (from mobile to base) Logical forward channel to a mobile Base station broadcasts to mobiles on each slot A mobile transmits to the base station in its allocated slot Logical reverse channel from a mobile Upstream and downstream traffic uses of the two different carrier frequencies .
  21. 21. TDMA Frames <ul><li>Multiple, fixed number of slots are put together into a frame . </li></ul><ul><li>A frame repeats. </li></ul><ul><li>In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels. </li></ul><ul><li>In TDMA/FDD: a different carrier frequency is used for a reverse or forward </li></ul><ul><ul><li>Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa). </li></ul></ul><ul><ul><li>Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame. </li></ul></ul>
  22. 22. General Frame and Time Slot Structure in TDMA Systems Preamble Information Trail Bits Slot 1 Slot 2 … Slot 3 Slot N Guard Bits Sync Bits CRC Information One TDMA Frame One TDMA Slot A Frame repeats in time Control Bits
  23. 23. A TDMA Frame <ul><li>Preamble contains address and synchronization info to identify base station and mobile s to each other </li></ul><ul><li>Guard time s are used to allow synchronization of the receivers between different slots and frames </li></ul><ul><ul><li>Different mobiles may have different propagation delays to a base station because of different distances. </li></ul></ul>
  24. 24. Efficiency of a Frame/TDMA-System <ul><li>Each frame contains overhead bits and data bits. </li></ul><ul><ul><li>Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits. </li></ul></ul>b T : total number of bits in a frame T f : frame duration (seconds) b OH : number of overhead bits Number of channels in a TDMA cell: m: maximum number of TDMA users supported in a radio channel
  25. 25. TDMA <ul><li>TDMA Efficiency </li></ul><ul><ul><li>GSM: 30% overhead </li></ul></ul><ul><ul><li>DECT: 30% overhead </li></ul></ul><ul><ul><li>IS-54: 20% overhead. </li></ul></ul><ul><li>TDMA is usually combined with FDMA </li></ul><ul><ul><li>Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other. </li></ul></ul><ul><ul><li>There may be more than one carrier frequency (radio channel) allocated and used inside each cell . Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels) </li></ul></ul><ul><ul><ul><li>For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA. </li></ul></ul></ul>
  26. 26. Contemporary TDMA Systems GSM (Europa) IS-54 (USA) PDC (Japan) DECT (European Cordless) Bit Rate 270.8 Kbps 48.6 Kbps 42 Kbps 1.152 Mbps Bandwidth 200 KHz 30 KHz 25 KHz 1.728 MHz Time Slot 0.577 ms 6.7 ms 6.7 ms 0.417 ms Upstream slots per frame 8 3 3 12 Duplexing FDD FDD FDD TDD Efficiency of Time Slots 73 % 80 % 80 % 67 % Modulation GMSK  /4 DQPSK  /4 DQPSK GMSK Adaptive Equalized Mandatory Mandatory Optional None
  27. 27. Features of TDMA <ul><li>Enables the sharing of a single radio channel among N users </li></ul><ul><li>Requires high data-rate per radio channel to support N users simultaneously. </li></ul><ul><ul><li>High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread  parameter) </li></ul></ul><ul><li>Transmission occurs in bursts (not continues) </li></ul><ul><ul><li>Enables power saving by going to sleep modes in unrelated slots </li></ul></ul><ul><ul><li>Discontinues transmission also enables mobile assisted handoff </li></ul></ul><ul><li>Requires synchronization of the receivers. </li></ul><ul><ul><li>Need guard bits, sync bits.  large overhead per slot . </li></ul></ul><ul><li>Allocation of slots to mobile users should not be uniform. </li></ul><ul><ul><li>It may depend on the traffic requirement of mobiles. </li></ul></ul><ul><ul><li>This brings extra flexibility and efficiency compared to FDMA systems. </li></ul></ul>
  28. 28. Capacity of TDMA Systems <ul><li>Capacity can be expressed as </li></ul><ul><ul><li>System Capacity (the capacity of the overall system covering a region) </li></ul></ul><ul><ul><ul><li>Depends on: </li></ul></ul></ul><ul><ul><ul><ul><li>Range of cells </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Whether the system can support macro-cells, micro-cells or pico-cells. </li></ul></ul></ul></ul><ul><ul><li>Cell Capacity </li></ul></ul><ul><ul><ul><li>Depends on the radio link performance between a base-station and mobiles: </li></ul></ul></ul><ul><ul><ul><ul><li>The lowest C/I (carrier-to-interence) ratio the system can operate for example quality of transmission. This in turn depends on the speech coding technique, desired speech quality, etc. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Data-rate over the channel which depends modulation efficiency (bits_per_second/Hz) and channel bandwidth . </li></ul></ul></ul></ul><ul><ul><ul><ul><li>The frequency re-use factor </li></ul></ul></ul></ul>
  29. 29. Spread Spectrum Access <ul><li>SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth. </li></ul><ul><li>Provides </li></ul><ul><ul><li>Immunity to multipath interference </li></ul></ul><ul><ul><li>Robust multiple access. </li></ul></ul><ul><li>Two techniques </li></ul><ul><ul><li>Frequency Hopped Multiple Access (FHMA) </li></ul></ul><ul><ul><li>Direct Sequence Multiple Access (DSMA) </li></ul></ul><ul><ul><ul><li>Also called Code Division Multiple Access – CDMA </li></ul></ul></ul>
  30. 30. Frequency Hopping (FHMA) <ul><li>Digital muliple access technique </li></ul><ul><li>A wideband radio channel is used. </li></ul><ul><ul><li>Same wideband spectrum is used </li></ul></ul><ul><li>The carrier frequency of users are varied in a pseudo-random fashion. </li></ul><ul><ul><li>Each user is using a narrowband channel (spectrum) at a specific instance of time. </li></ul></ul><ul><ul><li>The random change in frequency make the change of using the same narrowband channel very low. </li></ul></ul><ul><li>The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized . </li></ul><ul><li>Rate of hopping versus Symbol rate </li></ul><ul><ul><li>If hopping rate is greather: Called Fast Frequency Hopping </li></ul></ul><ul><ul><ul><li>One bit transmitted in multiple hops. </li></ul></ul></ul><ul><ul><li>If symbol rate is greater: Called Slow Frequency Hopping </li></ul></ul><ul><ul><ul><li>Multiple bits are transmitted in a hopping period </li></ul></ul></ul><ul><ul><ul><li>GSM and Bluetooth are example systems </li></ul></ul></ul>
  31. 31. Capacity of CDMA Systems <ul><li>Uplink Single-cell System Model </li></ul>... . . . ... . . . User 1 User 2 User k User K u User n <ul><li>Assumptions </li></ul><ul><li>Total active users K u </li></ul><ul><li>The intra-cell MAI can be </li></ul><ul><li>modeled as AWGN </li></ul><ul><li>Perfect power control is assumed </li></ul><ul><li>Random sequences </li></ul>
  32. 32. Capacity of CDMA Systems <ul><li>Coarse estimate of the reverse link (uplink) capacity </li></ul><ul><ul><li>Assumptions: </li></ul></ul><ul><ul><ul><li>Single Cell. </li></ul></ul></ul><ul><ul><ul><li>The interference caused by other users in the cell can be modeled as AWGN. </li></ul></ul></ul><ul><ul><ul><li>Perfect power control is used, i.e. the received power of each user at the base station is the same. </li></ul></ul></ul><ul><ul><li>If the received power of each user is Ps watts, and the background noise can be ignored (ex: microcells), then the total interference power (MAI) at the output of the desired user’s detector is </li></ul></ul><ul><ul><li>where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as: </li></ul></ul>
  33. 33. Case Study - Bluetooth <ul><li>Uses Frequency Hopping in cell (piconet) over a 79 MHz wideband radio channel. </li></ul><ul><li>Uses 79 narrowband channels (carrier frequencies) to hop through. </li></ul><ul><ul><li>Freq (f) = 2402+k MHz, k = 0,...,78 </li></ul></ul><ul><ul><li>Channel spacing is 1 MHz (narrowband channel bandwidth) </li></ul></ul><ul><ul><li>Wideband spectrum width = 79 MHz. </li></ul></ul><ul><ul><li>Hopping Rate = 1600 Hops/Second </li></ul></ul><ul><ul><li>Hopping sequence is determined by Bluetooth Hardware address and Clocks that are syncrozied between sender and receiver </li></ul></ul>0 1 2 3 77 78 ..... 79-Hop System A hop sequence could be: 7,1,78,67,0, 56,39,....... 79 MHZ 1 MHZ
  34. 34. Case Study: Bluetooth – P iconet and FHSS S S S M Each node is classified as master or slave. Master defines a piconet (a cell). Maximum 7 slaves can be connected to a master. Master coordinates access to the the media. All traffic has to go over master. Slaves can not talk to each-other directly. Range = 10m Raw Data-rate: 1 Mbps/piconet Radio channel used by devices in a piconet is 79MHz channel, which Is frequency hopped: hopping though 789 channels. Hoprate = 1600 hops/sec FHSS Picocell All slaves and the master hops according to the same hopping sequence . The hopping sequence is determined by the clock and BT_address of the master.
  35. 35. Case Study: Bluetooth – Scatternet and FHSS S S S M1 FHSS Piconet S S S M2 Piconet Piconet can be combined into scatternets. Red slave acts as a bridge between two piconets. FHSS Each piconet uses FHSS with different hopping sequences (masters are different). This prevents interference between piconets.
  36. 36. Case Study: Bluetooth - Media access in a piconet S2 S3 S1 M FHSS Piconet Inside a piconet, access to the frequency hopped radio channel is coordinated using time division multiple access : TDMA/TDD. Slot duration = 1/1600 sec = 625  s M-S1 S1-M M-S2 S2-M M-S3 S3-M M-S1 S1-M 0 1 2 3 4 5 6 7 ….. …… In an even slot , master transmits to a slave. In an odd slot , the slave that is addressed in the previous master-to-slave slot transmits. slot time=625  s
  37. 37. 802.11 <ul><li>2.4G-2.4835G , 5.725-5.825G </li></ul><ul><li>802.11a/g, OFDM, 802.11b: CDMA </li></ul>
  38. 38. Channel <ul><li>11, 5.5, 2, 1Mbps </li></ul>
  39. 39. Channelization scheme <ul><li>channels </li></ul>
  40. 40. 802.11 <ul><li>802.11a/g: 54, 48, 36, 24, 18, 6Mbps </li></ul><ul><li>802.11e - MAC Enhancements-Security/QoS </li></ul><ul><li>802.11f- Inter-Access Point Protocol </li></ul><ul><li>802.11h- Spectrum Managed 5Ghz </li></ul><ul><li>802.11i- Enhanced Security (TKIP and 802.1x) </li></ul><ul><li>802. 11p- vehicular </li></ul><ul><li>802. 11n- MIMO </li></ul>Application Presentation Session Transport Network Data Link Physical ISO OSI 7-layer model Logical Link Control Medium Access (MAC) Physical (PHY) IEEE 802 standards
  41. 41. Wireless hotpot planner <ul><li>Wireless valley </li></ul>
  42. 42. Design Procedure
  43. 43. Future WIFI
  44. 44. Signal to Noise Ratio at home
  45. 45. Fail of Iridium Satellite System <ul><li>The system was originally to have 77 active satellites, and as such was named for the element iridium , which has atomic number 77. </li></ul><ul><li>Too few users per square miles, cost too much. </li></ul><ul><li>Chapter 11 bankruptcy on August 13 , 1999 </li></ul><ul><li>In 70s, 12 calls per NYC. </li></ul>
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