Wireless Communication

Evolution of Mobile Phone

Electromagnetic Waves

Electromagnetic waves travel VERY FAST – around 300,000 kilometres per second (the speed of light).

At this speed they can go around the world 8 times in one second.

Radio transmission can take place using many different frequency bands, with each band having certain advantages and disadvantages.

Frequency and wave length are directly coupled

λ= c/f where wave length λ, speed of light c 3x10≅ 8m/s, and frequency f

• Electromagnetic Spectrum—name for the range of electromagnetic waves when placed in order of increasing frequency

RADIO WAVES

MICROWAVES

INFRARED RAYS

VISIBLE LIGHT

ULTRAVIOLET RAYS

X-RAYS

GAMMA RAYS

Electromagnetic Spectrum

Radio waves are basically frequencies that travel at the speed of light and deliver

information to transmitters and receivers. Radio waves can easily suffer from interference

due to natural causes such as stars and gases that emit radio waves. They can propagate

from millimeters to thousands of miles, also the frequency ranges from 3KHz to 300GHz.

All electromagnetic waves travel at the same speed. (300,000,000 meters/second) in a vacuum.

They all have different wavelengths and different frequencies.Long wavelength-lowest frequencyShort wavelength highest frequencyThe higher the frequency the higher the energy.

Frequencies and Regulations Radio frequencies are scarce resources. The International Telecommunications Union (ITU) located in Geneva is

responsible for worldwide coordination of telecommunication activities. {ITU-R handles standardization in wireless sector.

The ITU-R has split the world into three regions: Region-1 covers Europe, the Middle East, countries of soviet union and Africa Region-2 includes Greenland, North and South America Region-3 the Far East, Australia and New Zealand The ITU-R holds, the World Radio Conference (WRC), to periodically discuss and

decide frequency allocations for all three regions.

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 ϕsine wave as special periodic signal for a carrier. The general function of a sine wave is

Fourier representation of signals

Sine waves are of special interest because it is possible to construct every periodic

signal g by using only sine and cosine functions according to a fundamental equation of

Fourier

The c determines the Direct Current (DC) component of the signal, the

coefficient an and bn are the amplitudes of the nth sine and cosine function.

Signal RepresentationDifferent representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) constellation diagram (amplitude M and phase ϕ in polar coordinates)

A tool to display frequencies is a spectrum analyzer. Fourier transformations are a mathematical tool for translating from the time domain into the frequency domain and vice versa (using the inverse Fourier transformation).

The x-axis represents a phase of 0 and is also called In-Phase (I). A phase shift of 90° or π/2 would be a point on the y-axis, called

Quadrature (Q).

Ground Wave Propagation

Sky Wave Propagation

Line-of-Sight Propagation

Signal Propagation

• Propagation in free space always like light (straight line)• Receiving power proportional to 1/d²

(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

Multipath propagationSignal can take many different paths between sender and receiver due to reflection, scattering, diffraction.

Multipath propagationDue to the finite speed of light, signals travelling along different paths with different lengths arrive at the receiver at different times. This effect (caused by multi-path propagation) is called delay spread: the original signal is spread due to different delays of parts of the signal.

The energy intended for one symbol now spills over to the adjacent symbol, an effect which is called intersymbol interference (ISI). The higher the symbol rate to be transmitted, the worse the effects of ISI will be, as the original symbols are moved closer and closer to each other.

In case of mobile senders and receivers, the paths the signal can travel varies, the power of the received signal changes considerably over time. These quick changes in the received power are also called short-term fading. Depending on the different paths the signals take, these signals may have a different phase and cancel each other. An additional effect is the long-term fading of the received signal. This long-term fading is caused by varying distance to the sender or more remote obstacles.

Multiplexing

Multiplexing means combining multiple streams of information for transmission over a shared medium. Demultiplexing performs the reverse function, split a combined stream arriving from a shared medium into the original information streams. For wireless communication, multiplexing can be carried out in four dimensions: space, time, frequency, and code.

Multiplexing

Frequency division multiplexing (FDM) describes schemes to subdivide the frequency dimension into several non- overlapping frequency bands. Each channel is now allotted its own frequency band as indicated. Senders using a certain frequency band can use this band continuously.

Time division multiplexing (TDM) is a more flexible scheme where a channel is given the whole bandwidth for a certain amount of time, i.e., all senders use the same frequency but at different points in time.

Code division multiplexing (CDM) is a relatively new scheme in commercial communication systems. All channels use the same frequency at the same time for transmission. Separation is now achieved by assigning each channel its own ‘code’.

ModulationThe two types of signals used in a telecommunication network are- 1. Analog 2. DigitalAnalog signals take an infinite number of values in a range.

Digital signals take a limited number of values in a range.

(a) Analog signal (b) Digital signal

Modulation is the process of varying one or more properties of a high frequency periodic waveform, called the carrier signal, with respect to a modulating signal.

In modulation, a message signal, which contains the information is used to control the parameters of a carrier signal, so as to impress the information onto the carrier.

Types of Modulation

¨ Digital data, digital signals: simplest form of digital encoding of digital data

¨ Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog

¨ Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities

¨ Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system

There are two basic types:- Analog modulation and Digital modulationThe main difference between analog modulation and digital modulation is in the manner that they transmit data. With analog modulation, the input needs to be in the analog format, while digital modulation needs the data in a digital format.

Analog ModulationAnalog modulations techniques are as follows AM: The carrier amplitude is changed as per analog baseband signal.FM: The carrier frequency is changed as per analog baseband signal.PM: The carrier phase is changed as per analog baseband signal.Applications: Radio and television broadcast stations use AM and FM form of modulation.

Digital Modulation

In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion.

Digital modulation modulates three parameters of sinusoidal signal. The three parameters are Amplitude A, phase θ, and frequency f. s(t) = A·cos( 2π·fc·t + θk )

There are three type digital modulation. ASK: Amplitude Shift KeyingFSK: Frequency Shift KeyingPSK: Phase Shift Keying

Radio Transmitter & Receiver

Apart from the translation of digital data into analog signals, wireless transmission requires an additional modulation, an analog modulation that shifts the center frequency of the baseband signal generated by the digital modulation up to the radio carrier. The main reasons are length of the antennas, Frequency division multiplexing and the medium characteristics. The Digital Modulation schemes discussed vary in spectral efficiency, power efficiency and robustness.

Amplitude Shift Keying (ASK)Amplitude-shift keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave.

Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over optical fiber.

Frequency Shift Keying (ASK)The simplest form of FSK, also called binary FSK (BFSK), assigns one frequency f1 to the binary 1 and another frequency f2 to the binary 0.

To avoid sudden changes in phase, special frequency modulators with continuous phase modulation, (CPM) can be used. Sudden changes in phase cause high frequencies, which is an undesired side-effect.

FSK needs a larger bandwidth compared to ASK but is much less susceptible to errors.

Phase Shift Keying (ASK)Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). PSK uses a finite number of phases; each assigned a unique pattern of binary digits.

To receive the signal correctly, the receiver must synchronize in frequency and phase with the transmitter. This can be done using a phase lock loop (PLL). Compared to FSK, PSK is more resistant to interference, but receiver and transmitter are also more complex

An Example

Advanced FSK- Minimum 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)

MSK is basically BFSK without abrupt phase changes, i.e., it belongs

to CPM schemes.

Data bits are separated into even and odd bits, the duration of each bit

being doubled.

Depending on the bit values (even, odd) the higher or lower frequency,

original or inverted is chosen

The scheme also uses two frequencies: f1, the lower frequency, and f2,

the higher frequency, with f2 = 2f1, i.e. the frequency of one carrier is

twice the frequency of the other

if the even and the odd bit are both 0, then the higher frequency f2 is inverted (i.e., f2 is used with a phase shift of 180°)

if the even bit is 1, the odd bit 0, then the lower frequency f1 is inverted. This is the case, e.g., in the fifth to seventh columns of figure.

if the even bit is 0 and the odd bit is 1, as in columns 1 to 3, f1 is taken without changing the phase,

if both bits are 1 then the original f2 is taken.

A high frequency is always chosen if even and odd bits are equal. The signal is inverted if the odd bit equals 0.

Adding a so-called Gaussian low pass filter to the MSK scheme results in Gaussian MSK (GMSK), which is the digital modulation scheme used for many European wireless standards like GSM, DECT etc.

GMSK is a spectrally efficient modulation scheme and is particularly useful in mobile radio systems. It has a constant envelope, spectral efficiency, good BER performance.

Another Example of MSK

Advanced Phase Shift KeyingIn M-ary or multiple phase-shift keying (MPSK), there are more than two phases, usually four (0, +90, -90, and 180 degrees) or eight (0, +45, -45, +90, -90, +135, -135, and 180 degrees). If there are four phases (m = 4), the MPSK mode is called quadrature phase-shift keying or quaternary phase-shift keying (QPSK), and each phase shift represents two signal elements.

If there are eight phases (m = 8), the MPSK mode is known as octal phase-shift keying (OPSK), and each phase shift represents three signal elements. In MPSK, data can be transmitted at a faster rate, relative to the number of phase changes per unit time, than is the case in BPSK.

Quadrature Amplitude Modulation

Quadrature Amplitude Modulation (QAM): combines amplitude

and phase modulation it is possible to code n bits using one symbol 2n discrete levels, n=2 identical to QPSK Bit error rate increases with n, but less errors compared to

comparable PSK schemes

Quadrature Amplitude Modulation

Quadrature Amplitude Modulation

16-QAM 64-QAMQAM is used in many radio communications and data delivery applications. For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in digital cable television and cable modem applications. In the UK, 16 QAM and 64 QAM are currently used for digital terrestrial television using DVB - Digital Video Broadcasting. In the US, 64 QAM and 256 QAM are the mandated modulation schemes for digital cable as standardized by the SCTE in the standard ANSI/SCTE 07 2000. In addition to this, variants of QAM are also used for many wireless and cellular technology applications.

Multi-carrier Modulation• Multi-carrier modulation (MCM) is a method of

transmitting data by splitting it into several components, and sending each of these components over separate carrier signals. The individual carriers have narrow bandwidth , but the composite signal can have broad bandwidth.

• The advantages of MCM include relative immunity to fading caused by transmission over more than one path at a time (multipath fading), less susceptibility than single-carrier systems to interference caused by impulse noise, and enhanced immunity to inter-symbol interference. Limitations include difficulty in synchronizing the carriers under marginal conditions, and a relatively strict requirement that amplification be linear.

• MCM was first used in analog military communications in the 1950s. Recently, MCM has attracted attention as a means of enhancing the bandwidth of digital communications over media with physical limitations. The scheme is used in some audio broadcast services.

Spread SpectrumIn telecommunication and radio communication, spread-spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth.

• Spread spectrum is an technique used in radio transmission based on the concept that the narrowband signal is manipulated (scrambled) prior to transmission in such a way that the signal occupies a much larger part of the RF spectrum then strictly needed. This makes the signal more robust against interference and jamming.

• The manipulation requires a pseudo random noise code which, in the original concept, was only known to the parties at each end of the radio connection. Spread spectrum technology was invented in the 1940s, and has been used extensively since then for military and other applications that require robustness and resistance to jamming or eavesdropping.

Spread Spectrum Merits Because Spread Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals are harder to jam (interfere with) than

narrowband signals Spread Spectrum transmitters use similar transmit power levels to narrow

band transmitters. The Ability to Selectively Address. If we are clever about how we

spread the signal, and use the proper encoding method, then the signal can

only be decoded by a receiver which knows the transmitter's code. Bandwidth Sharing. If we are clever about selecting our modulation

codes, it is entirely feasible to have multiple pairs of receivers and

transmitters occupying the same bandwidth. Security from Eavesdropping. If an eavesdropper does not know the

modulation code of a spread spectrum transmission, all the eavesdropper

will see is random electrical noise rather than something to eavesdrop

(i) Original signal to be transmitted.(ii) The sender spreads the signal and converts the narrow-band

signal to broadband (Power level can be much lower without losing data)

(iii) During transmission, narrow and broadband noise gets added.(iv) The receiver despreads the given signal, narrow band

interference is spread, leaving the broadband as it is.(v) Receiver applies a band pass filter cutting off left & right of

narrow band signal.

Spreading the spectrum can be achieved in two different ways

Direct Sequence Spread Spectrum (DSSS) Data signal is multiplied by a spreading code, and resulting signal

occupies a much higher frequency band Spreading code is a pseudo-random sequence

Frequency Hopping Spread Spectrum (DSSS) Data signal is modulated with a narrowband signal that hops from

frequency band to frequency band, over time The transmission frequencies are determined by a spreading, or

hopping code (a pseudo-random sequence)

DSSS DS modulation is achieved by modulating the carrier wave with

a digital code sequence which has a bit rate much higher than

that of the message to be sent. This code sequence is typically a

pseudorandom binary code (often termed "pseudo-noise" or PN),

specifically chosen for desirable statistical properties. The time period of a single bit in the PN code is termed a chip,

and the bit rate of the PN code is termed the chip rate.

tc = Chip Periodtb = Bit Period

Spreading factor S = tb/tc

S*original bandwidth is the new bandwidth.It determines the BW of the resulting signal

XOR of the signal with pseudo-random number (chipping sequence)

many chips per bit (e.g., 128) result in higher bandwidth of the signal

• Most civil applications need a spreading factor of 10 to 100.

• Military applications use a speeding factor of around 10,000.

• Barker codes exhibit a good robustness against interference and insensitivity to multipath propagation. 10110111000 (802.11 wireless LANS), 11, 110, 1110, 11101, 1110010, 1111100110101

Let the code to be transmitted be 110.

Let the Chip Barker Code be 10110111000

Hence the transmitted code is: 11111111111 11111111111 00000000000 XOR 10110111000 10110111000 10110111000 - 01001000111 01001000111 10110111000

Example

At the Receiver : The transmitted signal is XORed with the same chip sequence.

01001000111 01001000111 10110111000 XOR 10110111000 10110111000 10110111000

Resulting in : 11111111111 11111111111 00000000000This is the original signal 110

DSSS Transmitter & Receiver

FHSS Total available BW is split into many channels of smaller BWs Transmitter & Receiver stay in one of these channels and hop to

another channel The implementation is a combination of FDM & TDM Hopping sequence defines the transition sequence amongst channels Dwell Time : Time spent on a channel with certain frequency The bandwidth of a frequency hopping signal is simply w times the

number of frequency slots available, where w is the bandwidth of each

hop channel. In Slow Hopping FHSS, one frequency is used for several bit periods In Fast Hopping FHSS, transmitter changes frequency several times

during single bit transmission. Slow Hopping FHSS are not as immune to narrow band interference as

the fast hopping FHSS

Slow and Fast Hopping

FHSS Transmitter & Receiver

Comparison Spreading is simpler in FHSS

FHSS uses only a portion of the Bandwidth at any given time.

DSSS are more resistant to fading and multi-path effects.

DSSS signals are had to detect in the absence of the knowledge of spreading code.

Cellular SystemsImplements space division multiplex: base station covers a certain transmission area (cell)Mobile stations communicate only via the base stationAdvantages of cell structures:

higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc.

locallyProblems:

• fixed network needed for the base stations• handover (changing from one cell to another) necessary• interference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

4 cell cluster with 3 sector antennas

• To avoid interference, different transmitters within each other’s interference range use FDM.

• Cells are combined in clusters – on the left side three cells form a cluster, on the right side seven cells form a cluster. All cells within a cluster use

• disjointed sets of frequencies.

• To reduce interference even further, sectorized antennas can be used. The figure shows the use of three sectors per cell in a cluster with four cells.

Channel Allocation

• Fixed Channel Allocation (FCA) means fixed assignment of frequencies to cell clusters and cells respectively which is not very efficient if traffic load varies. It is used in GSM and requires careful traffic analysis

• Fixed allocation of frequency channels do not optimize channel usage, if the traffic pattern is varying.

• Cells with higher traffic pattern can borrow frequencies from its neighbors (which do not carry heavy traffic) and this scheme is termed BCA (Borrowing Channel Allocation)

• In DCA (Dynamic Channel Allocation), channels (frequencies) will be allocated dynamically. Frequencies can only be borrowed. The borrowed frequency can be blocked for the surrounding cells, to reduce interference.

Cell Breathing• CDM cells are commonly said to ‘breathe’.

• While a cell can cover a larger area under a light load, it shrinks if the load increases. The reason for this is the growing noise level if more users are in a cell.

• CDM systems: cell size depends on current load

• Additional traffic appears as noise to other users

• If the noise level is too high users drop out of cells