ECE 6332, Spring, 2014 Wireless Communication

1. ECE 6332, Spring, 2014Wireless Communication Zhu Han Department of Electrical and Computer Engineering Class 25 April. 23rd, 2014

2. Outline • Adaptive Modulation and Coding • Diversity • Network Basics • OSI Model; TCP/IP Model

3. Adaptive Modulation • Change modulation relative to fading • Parameters to adapt: • Constellation size • Transmit power • Instantaneous BER • Symbol time • Coding rate/scheme • Optimization criterion: • Maximize throughput • Minimize average power • Minimize average BER Only 1-2 degrees of freedom needed for good performance

4. One of the M(g) Points log2 M(g) Bits To Channel M(g)-QAM Modulator Power: P(g) Point Selector Uncoded Data Bits Delay g(t) g(t) 16-QAM 4-QAM BSPK Variable-Rate Variable-Power MQAM Goal: Optimize P(g) and M(g) to maximize R=Elog[M(g)]

5. Optimization Formulation • Adaptive MQAM: Rate for fixed BER • Rate and Power Optimization Same maximization as for capacity, except for K=-1.5/ln(5BER).

6. gk g Optimal Adaptive Scheme • Power Adaptation • Spectral Efficiency g Equals capacity with effective power loss K=-1.5/ln(5BER).

7. K2 K1 K=-1.5/ln(5BER) Spectral Efficiency Can reduce gap by superimposing a trellis code

8. Constellation Restriction • Restrict MD(g) to {M0=0,…,MN}. • Let M(g)=g/gK*, where gK* is later optimized. • Set MD(g) to maxj Mj: Mj M(g). • Region boundaries are gj=MjgK*, j=0,…,N • Power control maintains target BER M3 M(g)=g/gK* MD(g) M3 M2 M2 M1 M1 Outage 0 g0 g1=M1gK* g2 g3 g

9. Power Adaptation and Average Rate • Power adaptation: • Fixed BER within each region • Es/N0=(Mj-1)/K • Channel inversion within a region • Requires power increase when increasing M(g) • Average Rate

10. Efficiency in Rayleigh Fading

11. Practical Constraints • Constellation updates: fade region duration • Error floor from estimation error • Estimation error at RX can cause error in absence of noise (e.g. for MQAM) • Estimation error at TX causes mismatch of adaptive power and rate to actual channel • Error floor from delay: let r(t,t)=g(t-t)/g(t). • Feedback delay causes mismatch of adaptive power and rate to actual channel

12. Main Points • Adaptive modulation leverages fast fading to improve performance (throughput, BER, etc.) • Adaptive MQAM uses capacity-achieving power and rate adaptation, with power penalty K. • Comes within 5-6 dB of capacity • Discretizing the constellation size results in negligible performance loss. • Constellations cannot be updated faster than 10s to 100s of symbol times: OK for most dopplers. • Estimation error/delay causes error floor

13. Diversity • Send bits over independent fading paths • Combine paths to mitigate fading effects. • Independent fading paths • Space, time, frequency, polarization diversity. • Combining techniques • Selection combining (SC) • Equal gain combining (EGC) • Maximal ratio combining (MRC) • Can have diversity at TX or RX • In TX diversity, weights constrained by TX power

14. Multiple Input Multiple Output (MIMO)Systems • MIMO systems have multiple (M) transmit and receiver antennas • With perfect channel estimates at TX and RX, decomposes to M indep. channels • M-fold capacity increase over SISO system • Demodulation complexity reduction • Beamforming alternative: • Send same symbol on each antenna (diversity gain)

15. Beamforming • Scalar codes with transmit precoding y=uHHvx+uHn • Transforms system into a SISO system with diversity. • Array and diversity gain • Greatly simplifies encoding and decoding. • Channel indicates the best direction to beamform • Need “sufficient” knowledge for optimality of beamforming • Precoding transmits more than 1 and less than RH streams • Transmits along some number of dominant singular values

16. Error Prone Low Pe Diversity vs. Multiplexing • Use antennas for multiplexing or diversity • Diversity/Multiplexing tradeoffs (Zheng/Tse)

17. ST Code High Rate High-Rate Quantizer Decoder Error Prone ST Code High Diversity Low-Rate Quantizer Decoder Low Pe How should antennas be used? • Use antennas for multiplexing: • Use antennas for diversity Depends on end-to-end metric: Solve by optimizing app. metric

18. Multiaccess vs. Point-to-point • Multiaccess means shared medium. • many end-systems share the same physical communication resources (wire, frequency, ...) • There must be some arbitration mechanism. • Point-to-point • only 2 systems involved • no doubt about where data came from !

19. Internetwork • Connection of 2 or more distinct (possibly dissimilar) networks. • Requires some kind of network device to facilitate the connection. Net A Net B

20. Comparison • Speed and Range

21. ISO/OSI Reference Model • To address the growing tangle of incompatible proprietary network protocols, in 1984 the ISO formed a committee to devise a unified protocol standard. • The result of this effort is the ISO Open Systems Interconnect Reference Model (ISO/OSI RM). • The ISO’s work is called a reference model because virtually no commercial system uses all of the features precisely as specified in the model. • The ISO/OSI model does, however, lend itself to understanding the concept of a unified communications architecture.

22. ISO/OSI Reference Model • The OSI RM contains seven protocol layers, starting with physical media interconnections at Layer 1, through applications at Layer 7. • OSI model defines only the functions of each of the seven layers and the interfaces between them. • Implementation details are not part of the model.

23. ISO/OSI Reference Model: Physical Layer • The Physical layer receives a stream of bits from the Data Link layer above it, encodes them and places them on the communications medium. • The Physical layer conveys transmission frames, called Physical Protocol Data Units, or Physical PDUs. Each physical PDU carries an address and has delimiter signal patterns that surround the payload, or contents, of the PDU. • Issues: • mechanical and electrical interfaces • time per bit • distances

24. Modulation • Process of varying a carrier signal in order to use that signal to convey information • Carrier signal can transmit far away, but information cannot • Modem: amplitude, phase, and frequency • Analog: AM, amplitude, FM, frequency, Vestigial sideband modulation, TV • Digital: mapping digital information to different constellation: Frequency-shift key (FSK)

25. ISO/OSI Reference Model: Data Link • The Data Link layer negotiates frame sizes and the speed at which they are sent with the Data Link layer at the other end. • The timing of frame transmission is called flow control. • Data Link layers at both ends acknowledge packets as they are exchanged. The sender retransmits the packet if no acknowledgement is received within a given time interval. ARQ • Medium Access Control - needed by mutiaccess networks. • Issues: • framing (dividing data into chunks) • header & trailer bits • addressing 01100010011 10110000001

26. Automatic Repeat-reQuest (ARQ) • Alice and Bob on their cell phones • Both Alice and Bob are talking • What if Alice couldn’t understand Bob? • Bob asks Alice to repeat what she said • What if Bob hasn’t heard Alice for a while? • Is Alice just being quiet? • Or, have Bob and Alice lost reception? • How long should Bob just keep on talking? • Maybe Alice should periodically say “uh huh” • … or Bob should ask “Can you hear me now?” 

27. Time-Division Multiplexing Figure Block diagram of TDM system.

28. ISO/OSI Reference Model: Network • At the originating computers, the Network layer adds addressing information to the Transport layer PDUs. • The Network layer establishes the route and ensures that the PDU size is compatible with all of the equipment between the source and the destination. • Its most important job is in moving PDUs across intermediate nodes. • Issues: • packet headers • virtual circuits

29. London Metro Map

30. Dijkstra's algorithm Dijkstra's algorithm-is a solution to the single-source shortest path problem in graph theory.  Works on both directed and undirected graphs. However, all edges must have nonnegative weights. Approach: Greedy Input: Weighted graph G={E,V} and source vertex v∈V, such that all edge weights are nonnegative Output: Lengths of shortest paths (or the shortest paths themselves) from a given source vertex v∈V to all other vertices

31. Dijkstra's algorithm - Pseudocode dist[s] ←0 (distance to source vertex is zero)for  all v ∈ V–{s}        do  dist[v] ←∞ (set all other distances to infinity) S←∅ (S, the set of visited vertices is initially empty) Q←V  (Q, the queue initially contains all vertices) while Q ≠∅ (while the queue is not empty) do   u ← mindistance(Q,dist) (select the element of Q with the min. distance)   S←S∪{u} (add u to list of visited vertices)        for all v ∈ neighbors[u]               do  if   dist[v] > dist[u] + w(u, v) (if new shortest path found)                         then      d[v] ←d[u] + w(u, v) (set new value of shortest path) (if desired, add traceback code) return dist

32. Dijkstra Animated Example

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41. Dijkstra Animated Example

42. ISO/OSI Reference Model: Transport • the OSI Transport layer provides end-to-end acknowledgement and error correction through its handshaking with the Transport layer at the other end of the conversation. • The Transport layer is the lowest layer of the OSI model at which there is any awareness of the network or its protocols. • Transport layer assures the Session layer that there are no network-induced errors in the PDU. • Issues: • headers • error detection: CRC • reliable communication

43. Parity Check • Add one bit so that xor of all bit is zero • Send, correction, miss • Add vertically or horizontally • Applications: ASCII, Serial port transmission

44. ISO/OSI Reference Model: Session • The Session layer arbitrates the dialogue between two communicating nodes, opening and closing that dialogue as necessary. • It controls the direction and mode (half -duplex or full-duplex). • It also supplies recovery checkpoints during file transfers. • Checkpoints are issued each time a block of data is acknowledged as being received in good condition. • Responsibilities: • establishes, manages, and terminates sessions between applications. • service location lookup

45. ISO/OSI Reference Model: Presetation • The Presentation layer provides high-level data interpretation services for the Application layer above it, such as EBCDIC-to-ASCII translation. • Presentation layer services are also called into play if we use encryption or certain types of data compression. • Responsibilities: • data encryption • data compression • data conversion

46. Substitution Method • Shift Cipher (Caesar’s Cipher) I CAME I SAW I CONQUERED H BZLD H TZV H BNMPTDSDC Julius Caesar to communicate with his army Language, wind talker

47. Public-Key Cryptography

48. RSA • by Rivest, Shamir & Adleman of MIT in 1977 • best known & widely used public-key scheme • based on exponentiation in a finite (Galois) field over integers modulo a prime • nb. exponentiation takes O((log n)3) operations (easy) • uses large integers (eg. 1024 bits) • security due to cost of factoring large numbers • nb. factorization takes O(e log n log log n) operations (hard)

49. ISO/OSI Reference Model • The Application layer supplies meaningful information and services to users at one end of the communication and interfaces with system resources (programs and data files) at the other end of the communication. • All that applications need to do is to send messages to the Presentation layer, and the lower layers take care of the hard part. • Issues: • application level protocols • appropriate selection of “type of service” • Responsibilities: • anything not provided by any of the other layers

50. TCP/IP Architecture • TCP/IP is the de facto global data communications standard. • It has a lean 3-layer protocol stack that can be mapped to five of the seven in the OSI model. • TCP/IP can be used with any type of network, even different types of networks within a single session.