ECE442: Wireless Communications
Lecture 10: Existing Cellular Systems - GSM and IS-95


          Prof. Sudharman K. Jayaw...
Existing Wireless Technologies


      Cellular telephony
      Cordless phones
      Wireless Local Loop (WLL) systems
  ...
Emerging Wireless Technologies


     Mobile satellite services (MSS)
     Wireless geo-location systems (E-911)
     Thir...
GSM: Global System for Mobile Communications

     1982: The Conference of European Posts and Telegraphs
     (CEPT) forme...
GSM Network Architecture
     A GSM network consists of three main parts:




         Mobile Station (MS): carried by the...
Mobile Station (MS)


  The MS consists of two main parts:
   1 Mobile equipment (the terminal)

           The mobile equ...
Architecture of a GSM Network




            Prof. Sudharman K. Jayaweera   ECE442: Wireless Communications
Base Station Subsystem (BSS)
  The BSS is also called the radio subsystem. It consists of two main
  parts:
    1 Base Tra...
Network and Switching Subsystem (NSS)




                                                    I


                        ...
Databases and Registers in NSS

     The NSS contains several databases that connect to the MSC.
     Home Location Regist...
GSM Call Routing and Roaming: HLR and VLR

     Call routing and roaming capability of GSM is managed by
     the Home Loc...
GSM Interfaces




     Um interface: MS and BSS (specifically the BTS)
     communicate across the Um interface
         a...
GSM Radio Subsystem




     In Europe, GSM systems operate in the ITU allocated bands
     of 890-915 MHz (uplink) and 93...
Physical Channels in GSM

     Multiple-access capability is a hybrid of time and frequency
     division multiple-access
...
Logical Channels in GSM

  Two basic types of logical channels in GSM are:
    1   Traffic channels (TCH)
    2   Control Ch...
GSM Traffic Channels (TCH)
                          156.25 bits
                          576.92/.ls
                      ...
GSM Frame Structure


              Superframe                                                                         51 ...
Control Channels in GSM: Broadcast channels (BCH)

     Operates only in the forward link of a specific ARFCN within
     e...
Control Channels in GSM: Common Control channels
(CCCH)

     On the same ARFCN used for BCH, the Common Control
     Chan...
Control Channels in GSM: Dedicated Control Channels
(DCCH)

     Dedicated Control Channels (DCCHs) are bidirectional and
...
GSM Speech Coding

     The 64 kbps PCM voice output stream in toll-quality voice is
     too high for wireless communicat...
Channel Coding in GSM

     To protect against man-made as well as natural
     electromagnetic interference, GSM uses con...
Channel Coding for Speech Signals

     First a 3-bit cyclic Redundancy Check (CRC) code is added to
     the Class Ia bit...
Modulation in GSM

     The resulting digital signal is modulated onto the analog
     carrier signal using so-called Gaus...
Power Control in GSM

     Based on the peak transmitter power, the GSM system
     defines 5 classes of mobile stations:
 ...
Global Market Comparisons (Figures from 1998 onward are
projections




  Source: Ojanpera and Prasad, Wideband CDMA for T...
GSM Standard Parameter Summary
                          Parameter               Specifications

             Reverse Chan...
CDMA IS-95 Systems

     Interim Standard 95 (IS-95) defines 10 CDMA bands (each of
     1.25 MHz) in the already allocated...
IS-95 Systems



     Modulation: QPSK
     Nominal data rate: 9600 bps
     Spreading factor: 128
     Chip rate: 1.2288 ...
Logical Channels in IS-95


                                                Pilot channels


                             ...
IS-95 Forward Link


     Forward link (downlink) includes:
         one pilot channel, one sync. channel, up to seven pag...
IS-95 Reverse Link


      Reverse link (uplink) channels are only access channels or
      traffic channels
          there...
IS-95 Speech Coding


     In IS-95 speech is encoded using a variable-rate vocoder,
     named the Code Excited Linear Pr...
IS-95 Froward Link Modulation Process


                                                                                  ...
IS-95 Reverse Link Modulation Process


                                                                                  ...
Power Control in IS-95

      Power control is an important aspect in IS-95 CDMA systems
          Goal is to ensure that ...
References



     D. P. Agrawal and Q. Zeng, Introduction to Wireless and
     Mobile Systems, Second Edition, Thomson, 2...
Next time...




     Existing Wireless Technologies: WLLs, WLANs, and Paging
                              Systems

     ...
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ECE442: Wireless Communications - Lecture 10: Existing ...

  1. 1. ECE442: Wireless Communications Lecture 10: Existing Cellular Systems - GSM and IS-95 Prof. Sudharman K. Jayaweera Department of Electrical and Computer Engineering University of New Mexico. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  2. 2. Existing Wireless Technologies Cellular telephony Cordless phones Wireless Local Loop (WLL) systems Wireless Local Area Networks (WLAN’s) Wireless Personal Digital Assistants (PDA’s) Pagers Bluetooth Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  3. 3. Emerging Wireless Technologies Mobile satellite services (MSS) Wireless geo-location systems (E-911) Third Generation (3G) and beyond (4G etc.) cellular WiMax Wireless Personal Area networks (WPAN’s) Wireless Internet Mobile ad-hoc networks (MANET’s) and Wireless Sensor Networks (WSN’s) Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  4. 4. GSM: Global System for Mobile Communications 1982: The Conference of European Posts and Telegraphs (CEPT) formed the study group named Groupe Special Mobile (GSM) to develop a pan-European public and land mobile system 1989: GSM responsibility was transferred to European Telecommunications Standards Institute (ETSI) 1990: GSM Phase I specifications were published 1991: Commercial GSM service was started in mid-1991 Although GSM was standardized in Europe it was quickly adapted worldwide becoming a global standard As a result now GSM is used to mean Global System for Mobile communications In US, PCS1900 (also called the GSM1900) system is based on a derivative of the GSM standard In other parts of the world DCS1800 is a GSM-based system Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  5. 5. GSM Network Architecture A GSM network consists of three main parts: Mobile Station (MS): carried by the subscriber Base Station Subsystem (BSS): controls the radio link with the MS Network and Switching Subsystem (NSS): main component is the Mobile Switching Center (MSC) that controls the switching of calls between mobile users, between mobile and fixed network users, and handles the mobility management An Operations Support Subsystem (OSS) provided solely for the system engineers of the operating company to monitor, diagnose and troubleshoot the GSM system Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  6. 6. Mobile Station (MS) The MS consists of two main parts: 1 Mobile equipment (the terminal) The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). 2 Subscriber Identity Module (SIM) The SIM is a smart card that provides personal mobility such that the user can have access to subscriber services from any terminal. The SIM contains a unique identifier called International Mobile Subscriber Identity (IMSI) that identifies the subscriber to the system It also contains a secret authentication and other information The IMEI and IMSI are independent of each other (for the flexibility in mobility) Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  7. 7. Architecture of a GSM Network Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  8. 8. Base Station Subsystem (BSS) The BSS is also called the radio subsystem. It consists of two main parts: 1 Base Transceiver Station (BTS) The BTS contains the radio transceivers that define a cell It also handles the radio-link protocols between the MS In any given area there might be a large number of BTS’s 2 Base Station Controller (BSC) A BSS consists of many BSC’s that connect to a single MSC. The BSC manages the radio resources for one or more BTS’s (usually a BSC controls up to several hundred BTS’s). BSC handles radio-channel set-up and frequency hopping. When a handoff is between two BTS’s under the control of the same BSC, the BSC will also handle the handoff without going through the MSC. The BSC is the connection between the MS and the MSC 3 The BTS and BSC communicate across a standardized interface called Abis interface allowing operation between components made by different manufacturers Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  9. 9. Network and Switching Subsystem (NSS) I ,;/ I ~Ji_ MS I~ I ~~r~tion I Subsystem Support Base Station Subsystem Network Switching Subsystem I Public Networks NSS handles the switching of GSM calls between external networks and the BSC’s of the GSM systems The main component of the network subsystem is the Mobile Switching Center (MSC) MSC is very much like a normal switching node in a PSTN, but also provides the additional functionalities needed to support subscriber mobility such as registration, authentication, location updating, handoffs and call routing for roaming The MSC connects the GSM network to the fixed networks such as PSTN or ISDN Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  10. 10. Databases and Registers in NSS The NSS contains several databases that connect to the MSC. Home Location Register (HLR) and the Visitor Location Register (VLR) supports call routing The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network and the current location of each of those mobiles There is only one logical HLR in a GSM network (although it may be implemented as a distributed data base) The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment in the network each mobile station is identified by its unique IMEI A protected database called the Authentication Center (AuC) stores a copy of the secret key stored in each subscriber’s SIM card Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  11. 11. GSM Call Routing and Roaming: HLR and VLR Call routing and roaming capability of GSM is managed by the Home Location Register (HLR) and the Visitor Location Register (VLR) in conjunction with the MSC The VLR contains selected administrative information from the HLR that are necessary for call control and provision of the subscribed services for each of the mobiles currently located in the geographical area controlled by VLR Usually the VLR is implemented together with the MSC (though not necessary) so that the area controlled by an MSC corresponds to the area controlled by the VLR The MSC contains no information about particular mobile stations. This information is stored in location registers Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  12. 12. GSM Interfaces Um interface: MS and BSS (specifically the BTS) communicate across the Um interface also called the air interface or the radio link A interface: The BSS (specifically BSC) communicates with the MSC across the A interface Abis interface: The BTS communicates with the BSC across the Abis interface Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  13. 13. GSM Radio Subsystem In Europe, GSM systems operate in the ITU allocated bands of 890-915 MHz (uplink) and 935-960 MHz (downlink) As GSM was adopted worldwide the operating frequency bands vary in in different markets Each 25MHz bandwidth (uplink or downlink) is divided first into 124 channels of 200KHz each Since a guard band of 100kHz is left at each end of the spectrum band Each of these carriers, called the Absolute Radio Frequency Channel Numbers (ARFCN), are then divided in time via TDMA Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  14. 14. Physical Channels in GSM Multiple-access capability is a hybrid of time and frequency division multiple-access Each TDMA frame is made of 8 so-called burst periods or time slots A time slot (TS) is the fundamental unit of time in GSM TDMA scheme and is equal to 15/26 ms (≈ 0.577 ms or 576.92 µ s) Hence a TDMA frame is equal to 120/26 ms (≈ 4.615 ms) One physical channel corresponds to one time slot per TDMA frame i.e. a physical channel constitutes the combinations of one ARFCN and one TS number Each physical channel can be mapped into various logical channels at any given time Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  15. 15. Logical Channels in GSM Two basic types of logical channels in GSM are: 1 Traffic channels (TCH) 2 Control Channels (CCH) There are three different types of control channels also: Broadcast channels (BCH) Common Control Channels (CCCH) Dedicated Control Channels (CCCH) Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  16. 16. GSM Traffic Channels (TCH) 156.25 bits 576.92/.ls .....--. TS2 TS3 TS1 TS7 TS6 TS5 TS4 4.615 ms • TSo TSn: nth Time Slot (Normal) Speech Multiframe = 26 TDMA frames T22 TlO IIS Tn ........ T12 ......... S T24 .T13 T2 T23 T1 T14 T15 120 ms Tn: nth TCH frame S: Slow Associated Control Channel frame I: Idle frame GSM traffic channels can carry either digitized speech or data at full-rate or half-rate In full rate, data is transmitted in one TS per each frame In half-rate, user data is transmitted in one TS per every two frame (i.e. in alternating frames) After every 13 consecutive frames of TCH data, GSM inserts either a Slow Associated Control Channel (SACCH) data or an idle frame Each group of 26 consecutive TDMA frames is called a multiframe Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  17. 17. GSM Frame Structure Superframe 51 Multiframes ~ / "- / "- Multiframe 26 Frames ~ / " / " / "" Frame 8 Time slots ~ /' -- /' 1 8.25 57 3 1 5726 -- -- 576.92 Jls Time slot 156.25 bits Tail Coded Stealing Midamble Stealing Coded Tail Guard bit Data flag flag data bit Period Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  18. 18. Control Channels in GSM: Broadcast channels (BCH) Operates only in the forward link of a specific ARFCN within each cell, and only in the first time slot (TS0) of certain GSM frames Serves as a TDMA beacon and provides synchronization to all mobiles within the cell There are three different types of BCH’s: Broadcast Control Channels (BCCH), Frequency Correction Channels (FCCH), and Synchronization Channels (SCH) These 3 types of BCH’s are given access to TS0 during various frames of a 51-length frame sequence Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  19. 19. Control Channels in GSM: Common Control channels (CCCH) On the same ARFCN used for BCH, the Common Control Channels (CCCHs) occupy the TS0 of every GSM frame that is not used by the BCH or an Idle frame There are three different types of CCCH’s as well: 1 Paging Channels (PCH): Only in forward link. Used by the BS to notifies a MS of an incoming call 2 Random Access Channels (RACH): Only in reverse link. Used by the MS to either originate a call or to acknowledge a page from the BS on a PCH. Uses slotted-ALOHA protocol. 3 Access Grant Channels (AGCH): Only in Forward link. Used by the BS to provide the MS with information on which physical channel to be used (i.e. the ARFCN and the TS number), in conjunction with which dedicated control channel. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  20. 20. Control Channels in GSM: Dedicated Control Channels (DCCH) Dedicated Control Channels (DCCHs) are bidirectional and their functionality is the same in both forward and reverse links DCCH can be operated in any time slot (except for the TS0) of any ARFCN There are three different types of DCCH channels: 1 Stand-alone Dedicated Control Channels (SDCCH): Provide signalling services required by the MS 2 Slow Associated Control Channels (SACCH): Always associated with a traffic channel (TCH) or a SDCCH and maps onto the same physical channel as them. On the forward link SAACH is used to provide slowly changing control information such as transmit power level to the MS. 3 Fast Associated Control Channels (FACCH): Carries urgent messages whenever a SDCCH is not yet dedicated. An example is a handoff request. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  21. 21. GSM Speech Coding The 64 kbps PCM voice output stream in toll-quality voice is too high for wireless communications GSM implements a speech coding algorithm called Regular Pulse Excited Linear Predictive Coding (RPE-LPC) to reduce the required bit rate The RPE-LPC algorithm uses the past samples to linearly predict the current sample Then the coefficients of the linear predictor and an encoded version of the prediction error is send to the receiver Speech is divided into 20 ms samples and is encoded using 260 bits Thus the total bit rate of GSM voice signals is 13 kbps (this is called the full-rate speech coding) Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  22. 22. Channel Coding in GSM To protect against man-made as well as natural electromagnetic interference, GSM uses convolutional encoding and block interleaving The exact methods used for speech and data signals are different For speech signals, the 260 bits in each 20 ms block are divided first into three classes based on their perceived importance in determining the speech quality: Class Ia: most sensitive to bit error (50 bits) Class Ib: moderately sensitive to bit error (132 bits) Class II: least sensitive to bit error (78 bits) Each class is encoded differently to provide different levels of protection Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  23. 23. Channel Coding for Speech Signals First a 3-bit cyclic Redundancy Check (CRC) code is added to the Class Ia bits for error detection (this gives 53 bits) Next, these 53 bits together with 132 Class Ib bits are encoded using a rate 1/2 convolutional encoder with constraint length 4. The 78 Class II bits are then added (unprotected) to the 378 bits from the convolutional encoder output resulting in 456 bits (in a 20 ms block) This gives a bit rate of 22.8 kbps To further protect against burst errors, each sample is interleaved before transmission Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  24. 24. Modulation in GSM The resulting digital signal is modulated onto the analog carrier signal using so-called Gaussian Minimum Shift Keying (GMSK) GMSK provides a compromise among spectral efficiency, transmitter complexity and limited spurious emissions Note that, Reduced transmitter complexity is important to reduce the power consumption at the mobile terminals In order to reduce the co-channel interference it is important that the spurious emissions outside the allocated bandwidth is minimized Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  25. 25. Power Control in GSM Based on the peak transmitter power, the GSM system defines 5 classes of mobile stations: - rated at 20, 8, 5, 2 and 0.8 Watts In order to minimize the co-channel interference, both the MS and the BTS operate at the minimum power level required to maintain an acceptable signal quality Power levels can be stepped up or down in steps of 2dB from the peak power for the class down to a minimum of 13dBm (i.e. down to 20mW) The MS measures the signal strength and signal quality (based on the BER) and passes that information to the BSC The BSC then decides if and when the power level needed to be changed based on that information Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  26. 26. Global Market Comparisons (Figures from 1998 onward are projections Source: Ojanpera and Prasad, Wideband CDMA for Third Generation Mobile Communications. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  27. 27. GSM Standard Parameter Summary Parameter Specifications Reverse Channel Frequency 890-915 MHz Forward Channel Frequency 935-960 MHz ARFCN Number o to 124 and 975 to 1023 TxlRx Frequency Spacing 45 MHz TxIRx Time Slot Spacing 3 Time slots Modulation Data Rate 270.833333 kbps Frame Period 4.615 ms Users per Frame (Full Rate) 8 Time Slot Period 576.91ls Bit Period 3.6921ls Modulation 0.3 GMSK ARFCN Channel Spacing 200 kHz Interleaving (max. delay) 40ms Voice Coder Bit Rate 13.4 kbps Source: Rapppaport. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  28. 28. CDMA IS-95 Systems Interim Standard 95 (IS-95) defines 10 CDMA bands (each of 1.25 MHz) in the already allocated 25 MHz of AMPS frequency bands uplink: 869-894 MHz downlink: 824-849 MHz The multiple-access capability is provided using DS-CDMA: In each frequency band 64 orthogonal Walsh codes (W0 through W63 ) are used to identify downlink channels In each frequency band 64 long PN codes are used to identify uplink channels Each physical IS-95 channel is identified by specifying a carrier frequency as well as a code sequence A PCS version of IS-95 is also available for international use in the 1.8GHz-2GHz band Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  29. 29. IS-95 Systems Modulation: QPSK Nominal data rate: 9600 bps Spreading factor: 128 Chip rate: 1.2288 Mcps Filtered bandwidth: 1.23 MHz Coding: convolutional coding with a Viterbi decoder Interleaving: in 20 ms blocks IS-95 uses a RAKE receiver to combine multipath energy Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  30. 30. Logical Channels in IS-95 Pilot channels Paging channels Variable-bit-rate user information Forward Sync channels channels Power control Traffic channels Logical channels I Signaling messages Logical channels Variable-bit-rate Logical channels < user information [-S-l-' g-n-a-li-n-g-m-es-s-a-g-e- Source: Textbook. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  31. 31. IS-95 Forward Link Forward link (downlink) includes: one pilot channel, one sync. channel, up to seven paging channels, and a set of forward traffic channels Information on each channel is modulated by an appropriate Walsh code at a fixed chip rate of 1.2288 Mcps Each user in a cell is assigned a different Walsh code To reduce co-channel interference among cells, all signals in one cell are also scrambled by a length 215 PN sequence The pilot and sync. channels are always assigned to code channel number 0 and 32, respectively (i.e. W0 and W32 ) Forward traffic channels are grouped into rate-sets: RS1: contains rates 9.6, 4.8, 2.4 and 1.2 kbps RS2: contains rates 14.4, 7.2, 3.6 and 1.8 kpbs Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  32. 32. IS-95 Reverse Link Reverse link (uplink) channels are only access channels or traffic channels there are 62 traffic channels there can be up to 32 access channels The access channel operates at the fixed rate of 4.8 kbps The spreading method used in the reverse link is different from that in forward link: The reverse link traffic channel can operate at any RS1 rate (i.e. 9.6, 4.8, 2.4 or 1.2 kbps) Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  33. 33. IS-95 Speech Coding In IS-95 speech is encoded using a variable-rate vocoder, named the Code Excited Linear Predictive (CELP) coder, that depends on the voice activity Since frame duration is fixed at 20 ms, this results in a variable number of bits per frame Rate can be changed from 2400 to 19,200 symbols per second downlink: The resulting bits are channel encoded using a rate 1/2 convolutional encoder and then interleaved uplink: The resulting bits are channel encoded using a rate 1/3 convolutional encoder and then interleaved Both convolutional encoders have a constraint length of 9 Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  34. 34. IS-95 Froward Link Modulation Process I-Channel Pilot PN Sequence Walsh code User data kbpsInterleaver Convolutional Block Encoder Code for nth and Long user Generator Long Code 19.2 Mcps 4800 bps 2400 + 41.2288 Symbol Cover Q-Channel Pilot PN Sequence Source: Rapppaport. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  35. 35. IS-95 Reverse Link Modulation Process Zero-offset Pilot PN Long Code 1.2288 Mcps Sequence I-channel Mask for user n PN chip Baseband let) Code Code Filter Convolutional Symbol 64-ary Symbol Block Data Burst Encoder and Orthogonal Repetition In terleaver Modulator Walsh Randomizer 9600 bps r=1I3K=9 chip Q(t) 4800 bps 28.8 ksps 307.2 kcps 2400 bps 1200 bps Zero-offset Pilot PN Sequence Q-channel Source: Rapppaport. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  36. 36. Power Control in IS-95 Power control is an important aspect in IS-95 CDMA systems Goal is to ensure that the received power at the BS due to all the MS’s are equal Power control is applied at both the MS as well as the BS Open loop power control at the MS: The mobile senses the strength of the pilot channel and adjusts its power according to that Closed-loop power control at the MS: Mobiles received the power control information from the BS (at a rate of 800b/sec). Open-loop power control at the BS: the BS decreases its power level gradually and waits to hear the frame error rate (FER) from the MS. If the FER is high the BS will again increase its power Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  37. 37. References D. P. Agrawal and Q. Zeng, Introduction to Wireless and Mobile Systems, Second Edition, Thomson, 2006. Chapter 10. T. S. Rappaport, Wireless Communications: Principles and Practices, Prentice-Hall, Second edition, 2006. Chapter 11. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications
  38. 38. Next time... Existing Wireless Technologies: WLLs, WLANs, and Paging Systems References: D. P. Agrawal and Q. Zeng, Introduction to Wireless and Mobile Systems, Second Edition, Thomson, 2006. Chapter 10. Prof. Sudharman K. Jayaweera ECE442: Wireless Communications

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