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

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

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

Variable-Rate Variable-Power MQAM Uncoded Data Bits Delay Point Selector M(  )-QAM Modulator Power: P(  ) To Channel  (t) log 2 M(  ) Bits One of the M(  ) Points BSPK 4-QAM 16-QAM Goal: Optimize P(  ) and M(  ) to maximize R=Elog[M(  )]

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

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

Spectral Efficiency Can reduce gap by superimposing a trellis code

Constellation Restriction Restrict M D (  ) to {M 0 =0,…,M N }. Let M(  )=  /   *, where   * is later optimized. Set M D (  ) to max j M j : M j  M(  ). Region boundaries are  j =M j   *, j=0,…,N Power control maintains target BER M(  )=  /   *  00  1 =M 1  K * 22 33 0 M1M1 M2M2 Outage M1M1 M3M3 M2M2 M3M3 MD()MD()

Power Adaptation and Average Rate Power adaptation: –Fixed BER within each region u E s /N 0 =(M j -1)/K u Channel inversion within a region –Requires power increase when increasing M(  ) Average Rate

Efficiency in Rayleigh Fading

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  (t,  )=  (t-  )/  (t). –Feedback delay causes mismatch of adaptive power and rate to actual channel

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

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

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)

Beamforming Scalar codes with transmit precoding 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 R H streams Transmits along some number of dominant singular values y=u H Hvx+u H n

Diversity vs. Multiplexing Use antennas for multiplexing or diversity Diversity/Multiplexing tradeoffs (Zheng/Tse) Error Prone Low P e

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

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 !

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

Comparison Speed and Range

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.

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.

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

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)

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) u header & trailer bits –addressing

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?”

Time-Division Multiplexing Figure Block diagram of TDM system.

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

London Metro Map

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

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

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

Dijkstra Animated Example

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

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

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

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

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

Public-Key Cryptography

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 bits) security due to cost of factoring large numbers –nb. factorization takes O(e log n log log n ) operations (hard)

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

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.

TCP/IP Architecture The concept of the datagram was fundamental to the robustness of ARPAnet, and now, the Internet. Datagrams can take any route available to them without human intervention.

Layering & Headers Each layer needs to add some control information to the data to do it’s job. This information is typically pre-pended to the data before being given to the lower layer. Once the lower layers deliver the data and control information - the peer layer uses the control information. Process Transport Network Data Link Process Transport Network Data Link DATA H H H H HH

Protocols and networks in the TCP/IP model How a call is made?

IEEE 802 Standards The 802 working groups. The important ones are marked with *. The ones marked with  are hibernating. The one marked with † gave up.

Summary Physical: Language between two machines Data-Link: communication between machines on the same network. Network: communication between machines on possibly different networks. Transport: communication between processes (running on machines on possibly different networks). Connecting Networks –Repeater: physical layer –Bridge: data link layer –Router: network layer –Gateway: network layer and above.

Final Words The world has been changed, have you?