EC MOBILE COMPUTING Dr. S. Sathiyan Assistant Professor (O.G)

EC1119 - MOBILE COMPUTING Dr. S. Sathiyan Assistant Professor (O.G)
Department of ECE SRM University

UNIT I-INTRODUCTION (9 hours)
Introduction to Mobile Computing – Wireless transmission: Signal Propagation – Multiplexing – Modulation – Spread Spectrum and Error Correction and Detection.

INTRODUCTION

What is mobile computing?
Mobile computing is to describe technologies that enable people to access network services anyplace, anytime, and anywhere, with portable and wireless computing and communication devices. Two aspects of mobility User mobility: users communicate (wireless) “anytime, anywhere with anyone” Device portability: devices can be connected anytime, anywhere to the network Wireless vs. mobile Examples   Desktop computer in a office   notebook in a hotel, on-board networks   wireless LANs in historic buildings   Smartphone

Why Mobile Communications?
Largest SW/HW/networked system Largest number of subscribers Mobile devices dominate the Internet Mobile applications dominate Internet usage New possibilities, new threats Technology fully integrated into everybody's life almost 24/7, almost anywhere

Overview of mobile devices
Pager receive only tiny displays simple text messages Smart phone voice, data simple graphical displays Laptop fully functional standard applications Wearable device human wearable non standard I/O Sensors, embedded controllers PDA graphical displays character recognition Performance

Effects of device portability
Power consumption limited computing power, low quality displays, small disks due to limited battery capacity CPU: power consumption ~ CV2f C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit f: clock frequency, can be reduced temporally Loss of data higher probability, has to be included in advance into the design (e.g., defects, theft) Limited user interfaces compromise between size of fingers and portability integration of character/voice recognition, abstract symbols

Wireless networks in comparison to fixed networks
Higher loss-rates due to interference emissions of, e.g., engines, lightning Restrictive regulations of frequencies frequencies have to be coordinated, useful frequencies are almost all occupied Lower transmission rates local some Mbit/s, regional sometimes only, e.g., 53kbit/s with GSM/GPRS or about 150 kbit/s using EDGE – some Mbit/s with LTE Higher delays, higher jitter connection setup time with GSM in the second range, several hundred milliseconds for other wireless systems – in ms range with LTE Lower security, simpler active attacking radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones Always shared medium secure access mechanisms important

Early history of wireless communication
Many people in history used light for communication heliographs, flags („semaphore“), ... 150 BC smoke signals for communication; (Polybius, Greece) 1794, optical telegraph, Claude Chappe Here electromagnetic waves are of special importance: 1831 Faraday demonstrates electromagnetic induction J. Maxwell ( ): theory of electromagnetic Fields, wave equations (1864) H. Hertz ( ): demonstrates with an experiment the wave character of electrical transmission through space (1886, in Karlsruhe, Germany, at the location of today’s University of Karlsruhe)

1896 Galileo Marconi, First demonstration of wireless telegraphy Based on long wave, requiring very large transmitters 1907 Commercial Trans-Atlantic Wireless Service Huge ground stations: 30 x 100m antenna masts 1920 Discovery of short waves by Marconi Cheaper, smaller, better quality transmitters by vacuum tube 1933 Frequency modulation (E. H. Armstrong) 1982 Start of GSM in Europe (1G analog) -goal: pan-European digital mobile phone system with roaming 1983 Start of AMPS in America (1G analog) 1992 Start of GSM (2G digital) in D as D1 and D2, fully digital, 900MHz, 124 channels automatic location, hand-over, cellular roaming in Europe - now worldwide in more than 200 countries services: data with 9.6kbit/s, FAX, voice, ...

1997 Wireless LAN - IEEE802.11 1998 Specification of GSM Successors Iridium satellite system (66 satellites) 1999 Standardization of additional wireless LANs IEEE standard b Bluetooth WAP (Wireless Application Protocol): access to many services via the mobile phone 2000 GSM with higher data rates (2.5G digital) HSCSD offers up to 57,6kbit/s First GPRS trials with up to 50 kbit/s 2001 Start of 3G systems IMT , several “members” of a “family”, CDMA2000 in Korea, UMTS tests in Europe 2002 WLAN hot-spots start to spread 2003 UMTS starts in Germany Start of DVB-T in Germany replacing analog TV

2005 Wi-MAX starts as DSL alternative (not mobile) first ZigBee products 2006 HSDPA starts in Germany as fast UMTS download version offering > 3 Mbit/s WLAN draft for 250 Mbit/s (802.11n) using MIMO WPA2 mandatory for Wi-Fi WLAN devices 2007 over 3.3 billion subscribers for mobile phones 2008 “real” Internet widely available on mobile phones (standard browsers, decent data rates) 7.2 Mbit/s HSDPA, 1.4 Mbit/s HSUPA available in Germany, more than 100 operators support HSPA worldwide, first LTE tests (>100 Mbit/s) 2009 – the story continues with netbooks, iphones, VoIPoWLAN… 2010 – LTE available in some cities, new frequencies allocated Reuse of old analog TV bands, LTE as DSL replacement for rural areas 2015 – VoLTE, LTE advanced 2020 – Start of 5G planned

Areas of research in mobile communication
Wireless Communication transmission quality (bandwidth, error rate, delay) modulation, coding, interference media access, regulations ... Mobility location dependent services location transparency quality of service support (delay, jitter, security) Portability power consumption limited computing power, sizes of display, ... usability and always: security (privacy, data integrity, tracking, encryption, law enforcement…)

Applications of Mobile Computing:
Vehicles Emergencies 3. Business 4. Credit Card Verification 5. Replacement of Wired Networks 6. Infotainment Limitations of Mobile Computing: 1. Resource Constraints : Battery 2. Interference 3. Bandwidth 4. Dynamic changes in communication environment 5. Network Issues 6. Interoperability issues 7. Security Constraints

Frequencies for communication
twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared UV visible light VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length:  = c/f wave length , speed of light c  3x108m/s, frequency f

Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections SHF and higher for directed radio links, satellite communication small antenna, focusing large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance frequencies) weather dependent fading, signal loss caused by heavy rainfall etc.

Wireless Transmission : Signal Propagation

Signals Physical representation of data Function of time and location
Signal parameters: parameters representing the value of data Classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values Signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t)

Fourier representation of periodic signals
1 1 t t ideal periodic signal real composition (based on harmonics)

Signals (Contd..,) Different representations of signals
amplitude (amplitude domain) frequency spectrum (frequency domain) constellation diagram (amplitude M and phase  in polar coordinates) Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!) A [V] Q = M sin  A [V] t[s]  I= M cos   f [Hz]

Signal propagation ranges
Transmission range receiver receives signal with an error rate low enough to be able to communicate Detection range transmitted power is high enough to detect the transmitter, but high error rate forbids communication Interference range sender interferes with other transmissions by adding to the noise distance sender transmission detection interference

Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² in vacuum – much more attenuation in real environments, e.g., d3.5…d4 (d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges shadowing reflection refraction scattering diffraction

Multipath propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts signal at sender signal at receiver LOS pulses multipath pulses LOS (line-of-sight)

Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts  quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away  slow changes in the average power received (long term fading) short term fading long term fading t power

Wireless Transmission : Multiplexing

Multiplexing A fundamental mechanism in communication system and networks Enables multiple users to share a medium For wireless communication, multiplexing can be carried out in four dimensions: space, time, frequency and code

Multiplexing Multiplexing in 4 dimensions
space (si) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed!

Space Division Multiplexing
Channels are assigned on the basis of “space” (but operate on same frequency) The assignment makes sure that the transmission do not interfere with each (with a guard band in between) s2 s3 s1 f t c k2 k3 k4 k5 k6 k1 channels ki

Frequency division multiplexing
Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages no dynamic coordination necessary works also for analog signals Disadvantages waste of bandwidth if the traffic is distributed unevenly inflexible k1 k2 k3 k4 k5 k6 c f t

Time division multiplexing
A channel gets the whole spectrum for a certain amount of time Advantages only one carrier in the medium at any time throughput high even for many users Disadvantages precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t

Time and frequency division multiplexing
Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM, Bluetooth Advantages better protection against tapping protection against frequency selective interference but: precise coordination required k1 k2 k3 k4 k5 k6 c f t

Code division multiplexing
k2 k3 k4 k5 k6 k1 f t c separation of channels achieved by assigning each channel its own code guard spaces are realized by having distance in code space (e.g. orthogonal codes) transmitter can transmit in the same frequency band at the same time, but have to use different code Provides good protection against interference and tapping but the receivers have relatively high complexity has to know the code and must separate the channel with user data from the noise composed of other transmission has to be synchronized with the transmitter

Wireless Transmission : Modulation

Modulation Process of combining input signal and a carrier frequency at the transmitter Digital to analog modulation necessary if the medium only carries analog signal Analog to analog modulation needed to have effective transmission (otherwise the antenna needed to transmit original signal could be large) permits frequency division multiplexing

Modulation Digital modulation
digital data is translated into an analog signal (baseband) ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), PSK (Phase Shift Keying) differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., /4) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Modulation and demodulation
analog baseband signal digital data digital modulation analog modulation radio transmitter radio carrier analog baseband signal digital data analog demodulation synchronization decision radio receiver radio carrier

Digital modulation Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK): ASK is the most simple digital modulation scheme Two binary values, 0 and 1, are represented by two different amplitude In wireless, a constant amplitude cannot be guaranteed, so ASK is typically not used Frequency Shift Keying (FSK): The simplest form of FSK is binary FSK assigns one frequency f1 to binary 1 and another frequency f2 binary 0 Simple way to implement is to switch between two oscillators one with f1 and the other with f2 The receiver can demodulate by having two band pass filter Phase Shift Keying (PSK): Uses shifts in the phase of a signal to represent data Shifting the phase by 1800 each time data changes: called binary PSK The receiver must synchronize in frequency and phase with the transmitter 1 t

Advanced Frequency Shift Keying
Bandwidth needed for FSK depends on the distance between the carrier frequencies Special pre-computation avoids sudden phase shifts  MSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK Even higher bandwidth efficiency using a Gaussian low-pass filter  GMSK (Gaussian MSK), used in GSM

Example of MSK 1 1 1 1 data even bits bit even 0 1 0 1 odd bits
1 1 1 data even bits bit even odd bits odd signal h n n h value low frequency h: high frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t No phase shifts!

Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex Higher bit rate can be achieved for the same bandwidth by coding two bits into one phase shift. Q I 1 Q I 11 01 10 00 A 450 for data 11 1350 for data 10 2250 for data 00 3150 for data 01 t 11 10 00 01

Wireless Transmission : Spread Spectrum

Spread spectrum technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping signal power interference spread signal power detection at receiver spread interference f f

Effects of spreading and interference
ii) iii) i) dP/df dP/df dP/df with interference spreading user signal f f f Sender iv) v) dP/df dP/df despread apply bandpass filter user signal f f Receiver broadband interference narrowband interference

Spreading and frequency selective fading
channel quality 2 1 5 6 narrowband channels 3 4 frequency narrow band signal guard space 2 frequency channel quality 1 spread spectrum spread spectrum channels

DSSS (Direct Sequence Spread Spectrum) I
Takes a user bit sequence and performs an XOR with, what is known as, chipping sequence Each user bit duration tb chipping sequence has smaller pulses tc If chipping sequence is generated properly it may appear as random noise sometimes called pseudo-noise (PN) tb/tc is known as the spreading factor determines the bandwidth of the resultant signal Used by b many chips per bit (e.g., 128) result in higher bandwidth of the signal tb user data 1 XOR tc chipping sequence 1 1 1 1 1 1 1 1 = resulting signal 1 1 1 1 1 1 1 tb: bit period tc: chip period

DSSS (Direct Sequence Spread Spectrum)
signal transmit signal user data X modulator chipping sequence radio carrier transmitter low pass filtered signal correlator sampled sums products received signal data demodulator X integrator decision radio carrier chipping sequence receiver

DSSS (Direct Sequence Spread Spectrum)
Advantages reduces frequency selective fading in cellular networks base stations can use the same frequency range several base stations can detect and recover the signal soft handover Disadvantages precise power control necessary

FHSS (Frequency Hopping Spread Spectrum)
Total available bandwidth is split into many channels of smaller bandwidth and guard spaces Transmitter and receiver stay on one of these channels for a certain time and then hop to another channel Implements FDM and TDM Pattern of channel usage : hopping sequence Time spent on a particular channel: dwell time

FHSS (Frequency Hopping Spread Spectrum)
Slow hopping Transmitter uses one frequency for several bit period systems are cheaper, but are prone to narrow band interference Fast hopping Transmitter changes frequency several times in one bit period Transmitter and receivers have to stay synchronized within smaller tolerances Better resistant to narrow band interference as they stick to one frequency for a very short period Receiver must know the hopping sequence and stay synchronized with the transmitter Used by Bluetooth

FHSS (Frequency Hopping Spread Spectrum) II
user data slow hopping (3 bits/hop) fast (3 hops/bit) 1 tb t f f1 f2 f3 td tb: bit period td: dwell time

FHSS (Frequency Hopping Spread Spectrum) III
modulator user data hopping sequence narrowband signal spread transmit Transmitter frequency synthesizer received signal Receiver demodulator data hopping sequence frequency synthesizer narrowband

FHSS (Frequency Hopping Spread Spectrum) IV
Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS simpler to detect

Error Detecting and Correcting Codes

Error Detecting and Correcting Codes
When the digital information in the binary form is transmitted from one circuit or system to another circuit or system an error may occur. This means a signal corresponding to 0 may change to 1 or vice versa due to presence of noise. To maintain the data integrity between the transmitter and receiver, extra bit or more than one-bit are added in the data. These extra bits allow the detection and sometimes correction of error in the data. The data along with the extra bit/bits forms the codes. Error Detecting Codes: Codes which allow only error detection are called error detecting codes. Error detecting and correcting codes: Codes which allow error detection and correction are called error detecting and correcting codes.

Parity Bits Parity Bit: It is used for the purpose of detecting errors during transmission of binary information. A parity bit is an extra bit included with a binary message to make the number of 1s either Odd or Even. The message including the parity bit is transmitted and then checked at the receiving end for errors. An error is detected if the checked parity does not corresponds with the transmitted ones. Parity Generator and Parity Checker: Parity Generator: The circuit that generates the parity bit in the transmitter is called a parity generator. Parity Checker: The circuit that checks the parity in the receiver is called parity checker. Even, Odd parity: In Even parity, the adder parity bit will make the total number of 1s Even amount In Odd parity, the added parity bit will make the total number of 1s an Odd amount

Hamming Codes Hamming code not only provides the detection of a bit error, but also identifies which bit is in error so that it can be corrected. Thus Hamming code is called error detecting and correcting code. Number of parity bits: Number of parity bits depends on the number of information bits. If the number of information bits is designed x, then the number of parity bits, p is determined by the following relationship: 2p ≥ x + p (1) For example, if we have four information bit, i.e. x=4, then p is found by trial and error using eqn. (1). Let p=2 then, 2p=22=4 and x+p+1 = = 7 Since 2p must be equal to or greater than x+p+1, the relationship in eqn (1) is not satisfied. Hence we have to try with the next value of p. Let p=3 Then, 2p=23=8 and x+p+1 = 4+3+1=8 This value of p satisfies the relationship given in eqn(1) and therefore we can say that three parity bits are required to provide single error correction for four information bits.

Locations of the parity bits in the code
Now we know that how to calculate the number of parity bits required to provide single error correction for given number of information bits. In our example, we have four information bits and three parity bits. Therefore, the code is of seven-bits. The rightmost bit is designated bit 1, the next bit is 2 and so on, Bit 7, Bit 6, Bit 5, Bit 4, Bit 3, Bit 2, Bit 1 The parity bits are located in the positions that are numbered corresponding to ascending powers of two (1,2,4,8…). Therefore, for 7-bit code, locations for parity bits and information bit are as shown below m7,m6, m5, c4, m3, c2, c1 cn designates a particular parity bit, mn designated a particular information bit and n is the location number Bit designation m7 m6 m5 c4 m3 c2 c1 Bit Location 7 6 5 4 3 2 1 Binary Locations 111 110 101 100 011 010 001 Information bit Parity Bit

Assignments of Parity Bits
Assignment of C1: C1 has a 1 for its right most digit. This parity bit checks all bit locations including itself, that have 1s in the same location in the binary location numbers Parity bit C1 checks bit location 1,3,5,7 and assign C1 according to even or odd parity Assignment of C2: The binary location number of parity bit C2 has a 1 for its middle bit. This parity bit checks all bit location, including itself that have 1s in the middle bit C2 2,3,6,7 and assign C2 according to even or odd parity Assignment of C4: C4 has 1 for its left most digit. This parity bit checks all bit location, including itself, that have 1s in the LSB C44,5,6,7 and assign the Odd or Even parity.

Seven Bit Hamming Code: 1010101
PROBLEMS RELATED TO ERROR DETECTION AND CORRECTING CODES Encode the binary code 1011 into seven bit even parity using Hamming Code Soln: We find the number of parity bits required. Let p=3 2p=23=8; x+3+1 = 4+3+1=8 Three parity bits are sufficient Total Code  4+3 =7 For C1 Bit location 3,5,7 have three 1s and therefore to have an even parity, C1 must be 1 For C2 Bit location 3,6,7 have two 1s and therefore to have an even parity, C2 must be 0 For C4 Bit location 5,6,7 have two 1s and therefore to have an even parity, C4 must be 0 Seven Bit Hamming Code: Bit designation m7 m6 m5 c4 m3 c2 c1 Bit Location 7 6 5 4 3 2 1 Binary Locations 111 110 101 100 011 010 001 Information bit Parity Bit

2. Determine the error correcting code for the information code 10111 for Odd parity
Soln: Let p=3, 2p=23=8 x+p+1 = 5+3+1=9  This will not work Let p=4, 2p=24=16 x+p+1 = = 10 Total bit 5+4 = 9 For C1: Bit Location, 3,5,7,9 Three 1s: Odd parity C1 must be 0 For C2: Bit Location 3,6,7 Two 1s: to have odd C2 must be 1 For C3: Bit Location, 5,6,7 Two 1s: to have Odd C4 must be 1 For C8: Bit Location, 8 and 9 must be 0 to have an Odd parity Nine Bit Hamming Code: Bit designation m9 c8 m7 m6 m5 c4 m3 c2 c1 Bit Location 9 8 7 6 5 4 3 2 1 Binary Locations 1001 1000 0111 0110 0101 0100 0011 0010 0001 Information bit Parity Bit

3. Assume that even parity Hamming code in example (0110011) is transmitted and (0100011) is
received. The receiver does not know what was transmitted. Determine bit location where error has occurred using received code Soln: C1 checks 1,3,5,7 There is one 1 in the group, parity checks for even parity is wrong 1 C2  checks 2,3,6,7 There are two 1 in the group, parity checks for even parity is correct 0 C4 checks 4,5,6,7 There are one 1 in the group, parity checks for even parity is wrong1 Resultant Word 101  5 location is in error It is 0 must be a 1 The correct code: Bit designation m7 m6 m5 c4 m3 c2 c1 Bit Location 7 6 5 4 3 2 1 Binary Locations 111 110 101 100 011 010 001 Received Code

Error bit position 2. Hamming code must be 1001011 and message is 1000
4. The message coded below in the even parity Hamming code and transmitted through a noisy channel. Decode the message assuming that almost a single error has occurred in each word code ii) iii) iv) Soln: Error bit position 2. Hamming code must be and message is 1000 Bit designation m7 m6 m5 c4 m3 c2 c1 Error Code Bit Location 7 6 5 4 3 2 1 Hamming coded message 1,3,5,7 Check C1 0 (LSB) 2,3,6,7 Check C2 4,5,6,7 Check C4 0 (MSB)

ii) Bit designation m7 m6 m5 c4 m3 c2 c1 Error Code Bit Location 7 6 5 4 3 2 1 Hamming coded message 1,3,5,7 Check C1 0 (LSB) 2,3,6,7 Check C2 4,5,6,7 Check C4 1 (MSB) Error bit position 6. Hamming code must be and message is 0010 iii) Bit designation m7 m6 m5 c4 m3 c2 c1 Error Code Bit Location 7 6 5 4 3 2 1 Hamming coded message 1,3,5,7 Check C1 1 (LSB) 2,3,6,7 Check C2 4,5,6,7 Check C4 1 (MSB) Error is in bit position 5. Hamming code must be and message is 1101

iv) Bit designation m7 m6 m5 c4 m3 c2 c1 Error Code Bit Location 7 6 5 4 3 2 1 Hamming coded message 1,3,5,7 Check C1 0 (LSB) 2,3,6,7 Check C2 4,5,6,7 Check C4 0 (MSB) Error is in bit position 2. Hamming code must be and message is 0010

EC MOBILE COMPUTING Dr. S. Sathiyan Assistant Professor (O.G)

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