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Telecommunications Engineering Topic 5: Wireless Architectures

Telecommunications Engineering Topic 5: Wireless Architectures. James K Beard, Ph.D. jkbeard@temple.edu http://astro.temple.edu/~jkbeard/. Essentials. Text: Simon Haykin and Michael Moher, Modern Wireless Communications SystemView Use the full version in E&A 603A for your term project

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Telecommunications Engineering Topic 5: Wireless Architectures

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  1. Telecommunications EngineeringTopic 5: Wireless Architectures James K Beard, Ph.D. jkbeard@temple.edu http://astro.temple.edu/~jkbeard/ Topic 5

  2. Essentials • Text: Simon Haykin and Michael Moher, Modern Wireless Communications • SystemView • Use the full version in E&A 603A for your term project • Web Site • URL http://astro.temple.edu/~jkbeard/ • Content includes slides for EE320 and EE521 • SystemView page • A few links • Office Hours • E&A 349 • Hours Tuesday afternoons 3:00 PM to 4:30 PM • MWF 10:30 AM to 11:30 AM • Others by appointment; ask by email Topic 5

  3. Topics • Architecture topics • Open System Interconnection (OSI) model • Power control • Handover • The Network Layer • Other areas from earlier chapters are reviewed Topic 5

  4. Open System Interconnection (OSI) Model • Seven-layer model • Physical layer (modem) • Data link layer • Network layer • Transport layer (packetizing, ACK/NAK) • Session layer (Service selection and access) • Presentation layer (encryption, compression) • Application layer (HMI) • Layers designed together as a system Topic 5

  5. Example 7.1: E-mail and the Seven-Layer Model • Application layer – e-mail client software • Presentation layer – compression and encryption (SSH) • Session layer – interface with host • Transport layer – TCP interface, IP addressing • Network layer – routing, adds header • Data link layer – adds header and addresses of host, adds CRD bits; medium access layer (MAC) selects free channel and passes to… • Physical layer – FEC and modulation, yet another header Topic 5

  6. Power Control Architectures • Open Loop • Mobile terminals measure strength of pilot channel • Transmit power decreased for strong pilot channels • Fast and simple, but must be approximate • Closed Loop • Base station measures mobile terminal signal strength • Mobile station receives signal strength by downlink • Accurate but delay and averaging must be smaller than channel coherence time • Outer Loop Control • Base station uses expected signal strength in control algorithm • Complexity can result in a slow loop Topic 5

  7. Power Control: Summary • Power control minimizes SINR in busy cells • Handset power control minimizes SINR in the base station but not at the mobile terminal • Methods still evolving • Next generation standards will implement • Newer techniques such as outer-loop control • Base station power control for SINR control at the mobile station Topic 5

  8. Example 7.3: The Near-Far Problem • Mobile terminal distance to base station varies from 100 m to 10 km • Power differences • Given a path loss exponent of 4 • Difference in received power at base station is 80 dB • Spreading rate of 128 million required to prevent jamming of weaker user • Solution is power control Topic 5

  9. Handover Issues • Purpose • Address operational transition of mobile terminals between cells • Maintain continuity of calls • Calls are dropped in handover because • Mobile station signal strength drops too low before handover is completed • The new cell doesn’t have a free channel Topic 5

  10. Handover Techniques • Start handover when signal strength is decreasing but a margin still exists • Common technique with first-generation systems • Margin can be small with second-generation systems that switch cells quickly • Mobile assisted • Use base station signal strength in handover logic • Avoids cell dragging in which mobile station operates well into another cell, and causes interference with other mobile stations Topic 5

  11. Handover Multiple-Access Issues • FDMA and TDMA • Mobile station must change signaling channels and traffic channels in handover • Called hard handover • CDMA • Signaling channels are the same during handover • Called soft handover • SDMA • Switch stations when mobile station transitions between beam boundaries • Can become complex when base station tracks users with steerable beams Topic 5

  12. The Network Layer Components • Base station • RF links to mobile terminals • RF, wire, fiber or other links to mobile switching center • Switching Center • Handles billing and authorization • Executes interconnects between base stations, other networks, or land line telecommunications Topic 5

  13. Mobile Switching Center Functions • Billing and authorization • Counts the minutes • Determines roaming status and finds home station/account • Rings the cash register • Modifies routing where appropriate • Interface between cellular and public land line telephone networks • Overall supervision of mobile access control (MAC) wireless communications network • Power control functions • Handover • Provide data capability to mobile terminals Topic 5

  14. Indoor LANs • Terminology • Cells are service sets • User terminals are stations • Base stations are stations • Peculiarities • Often design and growth is ad hoc without planning • Dissimilar packet sizes through network • Wired and 802.11 terminals on same station Topic 5

  15. Physical Layer for Various MAC Standards (Table 7.2 p. 470) Topic 5

  16. Physical Layer for Various Data Network Standards (Table 7.3) Topic 5

  17. Theme Example 5: 802.11(Wi-Fi) Pages 328-331 • Timeline • User station (STA) logs onto local base station (AP), AP authenticates STA and provides ID • STA listens • Inactive channel – STA sends RTS, AP sends CTS • Active channel – listens for gap and sends packet • STA fragments and sends packet • AP reassembles packet and sends to network layer • AP disassembles packet from network layer and sends to STA • Random time access (like Ethernet) Topic 5

  18. (5)(7) Convolutional Code with Hard Decoding Topic 5

  19. (5)(7) Convolutional Code with Soft Decoding Topic 5

  20. Problem 2.59 Page 102 When G. Marconi made the first radio transmission in 1899 across the Atlantic Ocean, he used all of the spectrum available worldwide to transmit a few bits per second. It has been suggested that, in the period since then, spectrum usage (bits/s/Hz worldwide) has increased by a factor of a million. List the factors that have resulted in this substantial increase. Which factor will likely result in the largest increase in the future? Topic 5

  21. Factors That Increase Spectral Usage Topic 5

  22. Problem 3.2 Page 110 • Consider the sinusoidal modulating signal • Show that the use of double sideband, suppressed carrier (DSB-SC) modulation produces a pair of side frequencies, one at fc+fm and the other at fc-fm, where fc is the carrier frequency. What is the condition that the modulator has to satisfy in order to make sure that the two side-frequencies do not overlap? Topic 5

  23. Solution for Problem 3.2 Topic 5

  24. Polynomial Arithmetic Modulo 2 • Integer arithmetic modulo 2 • Add, subtract, multiply integers • Take this result modulo 2 • Solution is always 0 or 1 • Division? Reciprocal of odd numbers is 1 • Polynomial arithmetic modulo 2 • Integers are coefficients of polynomials • Perform polynomial arithmetic as usual • Take coefficients of result modulo 2 Topic 5

  25. Examples • Two polynomials • Multiplying them • Taking the result modulo 2 Topic 5

  26. Finite Fields • Example • Integer arithmetic modulo 7 • Elements are {0,1,2,3,4,5,6} • Reciprocals pairs are (1,1), (2,4), (3,5), (6,6) • Division is defined as multiplication by reciprocal • All integer arithmetic modulo a prime defines a finite field Topic 5

  27. Vector Extensions of Finite Fields • Sometimes called polynomial fields or Galois fields • The exist for orders N equal to any power k of a prime p: N=pk • Arithmetic • Elements are characterized as the coefficients of a polynomial of order k-1 • Addition and subtraction is done modulo p • Multiplication is defined as modulo a generating polynomial of order k Topic 5

  28. Defining Characteristics of Galois Fields • Successive multiplication by x • Begin with 1 • Steps through all N elements except zero • A sequence of length N-1 • A reciprocal • Defined as producing 1 as a product • Always exists • Division is defined as multiplication by reciprocal Topic 5

  29. Special Case for Signal Processing • Galois fields of order 2k • The series of coefficients is a sequence of k zeros and ones • Addition and subtraction • Are identical operations in this field • Result is a bit-by-bit XOR • Multiplication • Modulo a generating polynomial of order k • Generating polynomial can be added or subtracted • Table 5.1 page 272 lists some generating polynomials Topic 5

  30. Generation with Shift Registers • Basis is a shift register with k latches • Shifting is equivalent to multiplication by x • A 1 shifted out • Fed back in according to the 1s in the generating polynomial • Addition is done with an XOR Topic 5

  31. Example Generating Polynomial: Topic 5

  32. Definitions of Orthogonality • Vectors with arithmetic modulo 2 • Addition of two orthogonal vectors gives the zero vector • A set of vectors that is closed on addition has the properties • Sum of any two is another in the set • The zero vector is always included • A basis set has the property • Sum of any two is never another in the basis set or zero • The opposite of closed on addition • Orthogonal signals Topic 5

  33. Next Time • Assignment: • Look at the study guide • Go over the slides to date • Look particularly at Chapters 4 and 7 • Make up a list of questions • Send them to me by email: jkbeard@temple.edu Topic 5

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