1. Mobile & Wireless Lecture # 1 By Engr. Sajjad karim

2. Communications The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point - Claude Shannon source destination Transmission Media

3. Why Digital Easy to regenerate the distorted signal Regenerative repeaters along the transmission path can detect a digital signal and retransmit a new, clean (noise free) signalThese repeaters prevent accumulation of noise along the pathThis is not possible with analog communication systems Finite-state signal representation The input to a digital system is in the form of a sequence of bits (binary or M-ary) Immunity to distortion and interference Digital communication is rugged in the sense that it is more immune to channel noise and distortion

4. Why Digital Hardware is more flexible Digital hardware implementation is flexible and permits the use of microprocessors, mini-processors, digital switching and VLSI Shorter design and production cycle Low cost The use of LSI and VLSI in the design of components and systems have resulted in lower cost Easier and more efficient to multiplex several digital signals Digital multiplexing techniques – Time & Code Division Multiple Access - are easier to implement than analog techniques such as Frequency Division Multiple

5. Why Digital Can combine different signal types – data, voice, text, etc. Data communication in computers is digital in nature whereas voice communication between people is analog in nature The two types of communication are difficult to combine over thesame medium in the analog domain. Using digital techniques, it is possible to combine both format for transmission through a common medium Can use packet switching Encryption and privacy techniques are easier to implement Better overall performance Digital communication is inherently more efficient than analog in realizing the exchange of SNR for bandwidth Digital signals can be coded to yield extremely low rates and high fidelity as well as privacy

6. Why Digital Good processing techniques are available for digital signals, such as Data compression (or source coding) Error Correction (or channel coding) Equalization Security Easy to mix signals and data using digital techniques Disadvantages Requires reliable “synchronization” Requires A/D conversions at high rate Requires larger bandwidth Performance Criteria Probability of error or Bit Error Rate

7. Basic Digital Comm. Transformations Formatting/Source Coding Transforms source info into digital symbols (digitization) Selects compatible waveforms (matching function) Removes redundancy so as to compress the required information Modulation/Demodulation Modulation is the process of modifying the info signal to facilitate transmission Demodulation reverses the process of modulation. It involves the detection and retrieval of the info signal Types Coherent: Requires a reference info for detection Non coherent: Does not require reference phase information

8. Coding/Decoding Translating info bits to transmitter data symbols Techniques used to enhance info signal so that they are less vulnerable to channel impairment (e.g. noise, fading, jamming, interference) Two Categories Waveform Coding Produces new waveforms with better performance Structured Sequences Involves the use of redundant bits to determine the occurrence of error (and sometimes correct it) Basic Digital Comm. Transformations

9. Multiplexing/Multiple Access Is synonymous with resource sharing with other users Frequency Division Multiplexing/Multiple Access (FDM/FDMA) Time Division Multiplexing/Multiple Access (TDM/TDMA) Code Division Multiplexing/Multiple Access (CDM/CDMA) Spread Spectrum (SS) Techniques It is usually used to protect privacy, protect against interference and allow flexible access to resources Common techniques include Direct Sequence (DS) Spread Spectrum –DSSS Frequency Hopping (FH) Spread Spectrum -FHSS Basic Digital Comm. Transformations

10. Wireless Wireless is a term used to describe telecommunications in which electromagnetic waves (rather than some form of wire) carry the piece of information over part or the entire communication path Wireless communication enables information transfer among a network of disconnected, and often mobile, devices.

11. Wireless Wireless communication has always lagged far behind wired communication. YearNetwork typeWireless rateWired Rate 1999Wide Area Network Cellular,10kb/sDSL 1MB/s 2004Wide Area Network Fixed Broadband,1 MB/s Cable, Modem VDSL 10MB/s 2004Local area Network 802.11, 5- 30Mb/s Ethernet 100- 1000 MB/s

12. Wireless Three Killers in wireless Rapidly varying SNR Worst-case received SNR dominates performance Multipath delay spread >> T Many “echoes” are received, ruining our perfect pulses. “Intersymbol Interference” biggest problem in achieving high data rates. Multiple access interference Must share the spectrum with others

13. Wireless Rapidly varying SNR Mobility causes the received SNR to change Quickly Solution Diversity Facilitate more than one independent channel Diversity Types: Spatial (multiple antennas) Time (Coding/Interleaving, adaptive modulation) Frequency (spread spectrum and multicarrier)

14. Wireless Solutions to Multipath There are three common solutions 1. Equalization: Receiver filters out the echoes A traditional approach Used in TDMA cellular and many applications 2. Multicarrier Modulation: Use many parallel channels, each with a low data rate, e.g. OFDM 3. Spread Spectrum: Send artificially fast, resolve the echoes individually using modulation structure Conveniently also allows potential of “multiple access” CDMA, 3G cellular, 802.11b, UWB

15. Wireless Multiuser interference Wireless frequency spectrum is limited All wireless users inherently interfere with each other (Power falls off rapidly with distance) How to divide the resources and be robust to interference?

16. Wireless Frequency Division Multiple Access (FDMA). -Simple and effective - high-Q RF filters Then, TDMA was popular - Better for digital than FDMA -Still with weaknesses, inc. synchronization, poor freq. reuse, equalization Code Division Multiple Access (CDMA) Each user uses a unique code, receiver that knows that code can extract out the desired user

17. Pre-cellular wireless One highly-elevated, high-powered antenna in a large service area Small number of channels (few users) Analog transmission, inefficient use of spectrum (no frequency reuse) Very low capacity, power-inefficient

18. Pre-cellular wireless First patent for wireless telephone? 1908 to Nathan B. Stubblefield of Murray, Kentucky zero generation (0G) of mobile telephones introduced in? In 1945, such as Mobile Telephone Service, were not cellular, and so did not feature "handover" from one base station to the next and reuse of radio frequency channels Cells for mobile phone base stations were invented in? 1947 by Bell Labs engineers at AT&T and further developed by Bell Labs during the 1960s The concepts of frequency reuse and handoff was first introduced in? May 1979 (Patent) to Charles A. Gladden and Martin H. Parelman,