IT351: Mobile & Wireless Computing Wireless Radio Communications

Objectives:

– To study the wireless radio communication medium, spectrum and signals.

– To study antennas and their role in wireless communications.

– To study the process of wireless signal propagation.

– To introduce basic issues in signal processing and signal modulation. – To study signal modulation techniques including ASK, FSK and PSK.

– To detail the concept of spread spectrum and study its techniques; FHSS, DSSS.

– To study issues in radio resource management and detail the cellular concept of channel allocation.

Outline

• The radio spectrum • Signals • Antennas • Signal propagation problems • Multiplexing • Modulation • Spread spectrum • Radio Management

Wireless communications

• The physical media – Radio Spectrum – There is one finite range of frequencies over which radio waves can exist – this is the

Radio Spectrum

– Spectrum is divided into

bands

for use in different systems, so Wi-Fi uses a different band to GSM, etc.

– Spectrum is (mostly) regulated to ensure fair access

• • • • •

Frequencies for communication

VLF = Very Low Frequency LF = Low Frequency (submarine) UHF = Ultra High Frequency (DAB, dig-TV, mobile phone, GSM) SHF = Super High Frequency (satellite) MF = Medium Frequency (radio AM) link) HF = High Frequency (radio FM & SW) EHF = Extremely High Frequency (direct UV = Ultraviolet Light VHF = Very High Frequency (analog TV broadcast) • Frequency and wave length –  = c/f – wave length  , speed of light c  3x10 8 m/s, frequency f 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 visible light UV

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, beam forming – large bandwidth available • Wireless LANs use frequencies in UHF to SHF range – 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.

Frequencies and regulations

• ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Examples Cellular phones Cordless phones Wireless LANs Other RF systems Europe GSM 880-915, 925 960, 1710-1785, 1805-1880 UMTS 1920-1980, 2110-2170 CT1+ 885-887, 930 932 CT2 864-868 DECT 1880-1900 802.11b/g 2412 2472 27, 128, 418, 433, 868 USA

AMPS, TDMA,

CDMA, GSM 824 849, 869-894

TDMA, CDMA, GSM,

UMTS 1850-1910, 1930-1990 PACS 1850-1910, 1930-1990 PACS-UB 1910-1930 802.11b/g 2412 2462 315, 915 Japan PDC, FOMA 810-888, 893-958 PDC 1429-1453, 1477-1501 FOMA 1920-1980, 2110-2170 PHS 1895-1918 JCT 245-380 802.11b 2412-2484 802.11g 2412-2472 426, 868

Wireless communications

• Signals – – – Physical representation of data is the

signal

Signals are function of time and location In wireless

sine waves

are used as the basic signal: • • • Amplitude: strength of the signal Frequency: no of waves generated per second Phase shift: where the wave starts and stops – These factors are transformed into the exactly required signal by

Fourier transforms

(complicated equations that parameterise the sine wave)

Signals

• Sine wave representation – signal parameters: parameters representing the value of data – 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) = A t sin(2  f t t +  t ) Frequency Amplitude Phase Shift

t

Fourier representation of periodic signals

1 0 t ideal periodic signal 1 0 t real composition (based on harmonics) • It is easy to isolate/ separate signals with different frequencies using filters

Antennas

• Sending and receiving signals is performed via antennas • Role: Radiation and reception of electromagnetic waves, coupling of wires to space and vice versa for radio transmission • Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna z y z x y x ideal isotropic radiator

Antennas

• Real antennas do not produce radiate signals in equal power in all directions. They always have directive effects (vertically and/or horizontally) • Radiation pattern: measurement of radiation around an antenna • Most basic antenna is the

dipole

– Two antennas both of length  /4 (  /2 in total) – Small gap between the two antennas – Produces an

omni-directional

signal in one plane of the three dimensions Source: Wikipedia

Antennas

• • • • Omni-Directional Antennas are wasteful in areas where obstacles occur (e.g. valleys)

Directional antennas

reshape the signal to point towards a target, e.g. an open street – Placing directional antennas together can be used to form cellular reuse patterns

Antennas arrays

can be used to increase reliability (strongest one will be received)

Smart antennas

use signal processing software to adapt to conditions – e.g. following a moving receiver (known as

beam forming

), these are some way off commercially

Antennas

 /4 y y z side view (xy-plane) x side view (yz-plane) z top view (xz-plane) x simple dipole y y z side view (xy-plane) x side view (yz-plane) z top view (xz-plane) x directed antenna  /2

Signal propagation

• In perfect conditions (a vacuum) wireless signals will weaken predictably –

Transmission range

: receivers can understand enough of the signal (i.e. low error) for data –

Detection range

: receivers hear the signal but cannot recover the data (i.e. high error) –

Interference range

: there is a signal but it is indistinguishable from other noise • Wireless is less predictable since it has to travel in unpredictable substances – air, dust, rain, bricks sender transmission detection interference distance

Signal propagation: Path loss (attenuation)

• In free space signals propagate as light in a straight line (independently of their frequency).

• If a straight line exists between a sender and a receiver it is called line-of-sight (LOS) • Receiving power proportional to 1/d² in vacuum (free space loss) – much more in real environments (d = distance between sender and receiver) • Situation becomes worse if there is any matter between sender and receiver especially for long distances – Atmosphere heavily influences satellite transmission – Mobile phone systems are influenced by weather condition as heavy rain which can absorb much of the radiated energy

Signal propagation

• Radio waves can penetrate objects depending on frequency. The lower the frequency, the better the penetration – Low frequencies perform better in denser materials – High frequencies can get blocked by, e.g. Trees • Radio waves can exhibit three fundamental propagation behaviours depending on their frequencies: – Ground wave (<2 MHz): follow the earth surface and can propagate long distances – submarine communication – Sky wave (2-30 MHz): These short waves are reflected at the ionosphere. Waves can bounce back and forth between the earth surface and the ionosphere, travelling around the world – International broadcast and amateur radio – Line-of-sight (>30 MHz): These waves follow a straight line of sight – mobile phone systems, satellite systems

Additional signal propagation effects

• Receiving power additionally influenced by • fading (frequency dependent): signals can change as the receiver moves • Blocking/ Shadowing: large objects may block signals (building,..etc) • Reflection: waves can bounce off dense objects • Refraction: waves can bend through objects depending on the density of a medium • scattering : small objects may reflect multiple weaker signals • 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,… LOS pulses multipath pulses signal at sender signal at receiver • Different signals use different length paths • The difference is called delay spread – Systems must compensate for the delay spread – Interference with “neighbor” symbols, Inter Symbol Interference (ISI) • Symbols may cancel each other out • Increasing frequencies suffer worse ISI

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) power long term fading • Additional changes in – distance to sender – obstacles further away –  slow changes in the average power received (long term fading) short term fading t

Multiplexing

• Multiplexing describes how several users can share a medium with minimum or no interference • It is concerned with sharing the frequency range amongst the users • Bands are split into channels • Four main ways of assigning channels – Space Division Multiplexing (SDM) : allocate according to location – Time Division Multiplexing (TDM): allocate according to units of time – Frequency Division Multiplexing (FDM): allocate according to the frequencies – Code Division Multiplexing (CDM) : allocate according to access codes • Guard Space: gaps between allocations

Multiplexing

• Multiplexing in 4 dimensions – space (s i ) – time (t) – frequency (f) – code (c) channels k i k s 1 1 c k 2 t k 3 f k 4 s 2 c k 5 • Goal: multiple use of a shared medium c t s 3 • Important: guard spaces needed!

f k 6 t f

Space Division Multiplexing (SDM)

• Space Division – This is the basis of frequency reuse – Each physical space is assigned channels – Spaces that don’t overlap can have the same channels assigned to them – Example: FM radio stations in different countries channels k i k 1 k 2 s 1 c t s 3 k 3 c f k 4 s 2 t c k 5 f k 6 t f

Frequency Division Multiplexing (FDM)

• Separation of the whole spectrum into smaller non overlapping frequency bands (guard spaces are needed) • A channel gets a certain band of the spectrum for the whole time – receiver has to tune to the sender frequency • Advantages k 1 k 2 k 3 k 4 k 5 – no dynamic coordination necessary – works also for analog signals c • Disadvantages – waste of bandwidth if the traffic is distributed unevenly – inflexible t k 6 f

Time Division Multiplexing (TDM)

• A channel gets the whole spectrum for a certain amount of time • Guard spaces (time gaps) are needed • Advantages – only one carrier in the medium at any time – throughput high even for many users c k 1 k 2 k 3 k 4 k 5 k 6 • Disadvantages – precise clock synchronization necessary t f

Time and frequency multiplexing

• Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example: GSM • Advantages – better protection against tapping – protection against frequency selective interference c k 1 k 2 k 3 k 4 k 5 • but: precise coordination required t k 6 f

Code Division Multiplexing (CDM)

• Code Division – Instead of splitting the channel, the receiver is told which channel to access according to a pseudo-random code that is synchronised with the sender – The code changes frequently – Security: unless you know the code it is (almost) impossible to lock onto the signals – Interference: reduced as the code space is huge – Complexity: very high

Code multiplexing

• Each channel has a unique code k 1 k 2 k 3 k 4 k 5 k 6 • All channels use the same spectrum at the same time • Advantages – bandwidth efficient – no coordination and synchronization necessary – good protection against interference and tapping • Disadvantages – precise power control required – more complex signal regeneration t • Implemented using spread spectrum technology c f

Modulation

• • •

Definition: transforming the information to be transmitted into a format suitable for the used medium The signals are transmitted as a sign wave which has three parameters: amplitude, frequency and phase shift.

These parameters can be varied in accordance with data or another modulating signal

•

Two types of modulation

– Digital modulation: digital data (0, 1) is translated into an analog signal (baseband signal) – Analog modulation: the center frequency of the baseband signal generated by digital modulation is shifted up to the radio carrier

Why we need digital modulation?

• Digital modulation is required if digital data has to be transmitted over a medium that only allows analog transmission (modems in wired networks).

• Digital signals, i.e. 0/1, can be sent over wires using voltages • Wireless must use analogue sine waves • This translation is performed by digital modulation – digital data is translated into an analog signal (baseband) – Shift Keying is the translation process – Amplitude, Freq., Phase Shift Keying (ASK/FSK/PSK) – differences in: • spectral efficiency: how efficiently the modulation scheme utilizes the available frequency spectrum • power efficiency: how much power is needed to transfer bits • Robustness: how much protection against noise, interference and multi path propagation

Why we need analogue modulation ?

• Analogue modulation then moves the signal into the right part of the channel – Motivation • smaller antennas (e.g.,  /4) • Frequency Division Multiplexing • medium characteristics – path loss, penetration of objects, reflection,..etc

– Basic schemes • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM)

Modulation and demodulation

digital data 101101001 digital modulation analog baseband signal analog modulation radio carrier

radio transmitter

analog demodulation radio carrier analog baseband signal synchronization decision digital data 101101001

radio receiver

Digital Modulation - Amplitude Shift Keying (ASK)

• Amplitude Shift Keying (ASK) – 0 and 1 represented by different amplitudes • i.e. a basic sine wave – Problem: susceptible to interference – Constant amplitude is hard to achieve – ASK is used for optical transmissions such as infra-red and fibre (simple + high performance) – In optical  light on = 1 light off = 0

Digital Modulation - Frequency Shift Keying (FSK)

• Frequency Shift Keying (FSK) – 0 and 1 represented by different frequencies – Switch between two oscillators accordingly – Twice the bandwidth but more resilient to error

Digital Modulation - Phase Shift Keying (PSK)

• Phase Shift Keying (PSK) – 0 and 1 represented by different (longer) phases – Flip the sine wave 180 to switch between 0/1 – Better still than FSK but more complex • Other modulation schemes are mostly complex variants of ASK, FSK, or PSK…

Digital modulation - summary

• Modulation of digital signals known as Shift Keying • Amplitude Shift Keying (ASK): 1 0 1 – very simple – low bandwidth requirements – very susceptible to interference 1 0 1 t • Frequency Shift Keying (FSK): – needs larger bandwidth – more error resilience than AM 1 0 1 t • Phase Shift Keying (PSK): – more complex – robust against interference t

Analog modulation

•

Definition: Impress an information-bearing analog waveform onto a carrier waveform for transmission

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 – Advantage: protection against narrow band interference signal power interference spread signal power detection at receiver spread interference f f • Side effects: – coexistence of several signals without dynamic coordination – tap-proof

Spread spectrum

• Basic idea – Spread the bandwidth needed to transmit data – Lower signal power, more bandwidth, same energy – Resistant to narrowband interference – Steps • Apply spreading (convert narrow band to broadband) • Send low power spread signal • Signal picks up interference • Receiver can de-spread signal • Signal is more powerful than remaining interference • Signal is therefore able to be interpreted

Effects of spreading and interference

i) iii) dP/df dP/df dP/df ii) f sender dP/df iv) f receiver f f v) user signal broadband interference narrowband interference dP/df f

Spreading and frequency selective fading

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

Spread spectrum problems

• Spread spectrum problems: – Increased complexity of receivers – Raising background noise • Spread spectrum can be achieved in two different ways: – Direct Sequence – Frequency Hopping

Spread Spectrum – Direct Sequence Spread Spectrum (DSSS)

• Each bit in original signal is represented by multiple bits in the transmitted signal • Spreading code spreads signal across a wider frequency band • XOR of the signal with pseudo-random number (chipping sequence) – many chips per bit (e.g., 128) result in higher bandwidth of the signal • Advantages – reduces frequency selective fading

DSSS

• Chipping sequence appears like noise, to others • Spreading factor S = t b /t c • If the original signal needs a bandwidth w, the resulting signal needs s*w • The exact codes are optimised for wireless – E.g. for Wi-Fi 10110111000 (Barker code) – For civil application spreading code between 10 and 100 – For military application the spreading code is up to 10,000 t b 0 t c 1 user data

XOR

0 1 1 0 1 0 1 0 1 1 0 1 0 1 chipping sequence

=

resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 t b : bit period t c : chip period

DSSS

• New modulation process: – Sender: Chipping  Digital Mod.  Analog Mod.

– Receiver: Demod.  Chipping  Integrator  Decision?

• At the receiver after the XOR operation (despreading), an integrator adds all these products, then a decision is taken for each bit period • Even if some of the chips of the spreading code are affected by noise, the receiver may recognize the sequence and take a correct decision regarding the received message bit.

DSSS (Direct Sequence Spread Spectrum)

user data X chipping sequence spread spectrum signal modulator transmit signal radio carrier

transmitter

received signal demodulator correlator lowpass filtered signal X products integrator sampled sums decision data radio carrier chipping sequence

receiver

Spread Spectrum – Frequency Hopping Spread Spectrum (FHSS)

• Uses entire bandwidth for signals • Signal is broadcast over seemingly random series of radio frequencies – A number of channels allocated for the FH signal – Width of each channel corresponds to bandwidth of input signal • Signal hops from frequency to frequency at fixed intervals – Transmitter operates in one channel at a time – At each successive interval, a new carrier frequency is selected. Pattern of hopping is the

hopping sequence

– Time on each frequency is the dwell time – Fast hopping = many hops per bit – Slow hopping = many bits per hop – Fast hopping is more robust but more complex – FHSS is used in Bluetooth - 1600 hops/s, 79 channels

Frequency Hopping Spread Spectrum (FHSS)

• Process 1 - Spreading code modulation – The frequency of the carrier is periodically modified (hopped) following a specific sequence of frequencies.

– In FHSS systems, the

spreading code is this list of frequencies

to be used for the carrier signal, the “hopping sequence” – The amount of time spent on each hop is known as dwell time and is typically in the range of 100 ms.

• Process 2 - Message modulation – The message modulates the (hopping) carrier, thus generating a narrow band signal for the duration of each dwell, but generating a wide band signal if the process is regarded over periods of time in the range of seconds.

FHSS

• Discrete changes of carrier frequency – sequence of frequency changes determined via pseudo random number sequence • Two versions – Fast Hopping: several frequencies per user bit – Slow Hopping: several user bits per frequency • 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

f f 3 f 2 f 1 f f 3 f 2 f 1

FHSS

0 t d t b 1 t d t b : bit period 0 1 t d : dwell time 1 t user data t t slow hopping (3 bits/hop) fast hopping (3 hops/bit)

FHSS (Frequency Hopping Spread Spectrum)

user data narrowband signal spread transmit signal modulator modulator

transmitter

frequency synthesizer hopping sequence received signal demodulator narrowband signal demodulator data hopping sequence frequency synthesizer

receiver

Resource Management

• Radio Resource Management

– Channel Access – Channel Assignment

• Power Management • Mobility Management

– Location Management – Handoff/Handover: the term

handover

or

handoff

refers to the process of transferring an ongoing call or data session from one channel to another

• Example: The cellular System

The cellular system: cell structure

• Channel allocation: Implements space division multiplexing (SDM) – base station covers a certain transmission area (cell) – Cellular concept: channel reuse across the network prevents interference, improves the likelihood of a good signal in each cell • Mobile stations communicate only via the base station • Advantages of cell structures – higher capacity, higher number of users f 4 f 5 f 3 f 6 – less transmission power needed – more robust, decentralized f 3 f 1 f 2 f 7 – base station deals with interference, transmission area etc. locally • Problems – Expensive – fixed network needed for the base stations – handover (changing from one cell to another) necessary – interference with other cells f 2 f 4 f 5 f 1

Frequency planning

• Frequency reuse only with a certain distance between the base stations • Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies • Cells are combined in clusters • All cells within a cluster use disjointed sets of frequencies f 3 • The transmission power of a sender has to be limited to avoid interference f 4 f 3 f 5 f 1 f 6 f 7 f 2 • Standard model using 7 frequencies • To reduce interference further, sectorized antennas can be used especially for larger cell radii f 2 f 4 f 5 f 1

Frequency planning

f 3 f 1 f 2 f 3 f 2 f 3 f 1 f 3 f 1 f 2 f 3 f 2 f 3 f 1 f 3 f 1 f 2 f 3 3 cell cluster f 1 f 2 f 3 g 2 h 1 g 1 g 3 h 2 h 3 f 1 g 1 f 2 f 3 g 2 h 1 h h 3 2 g 3 f 1 f f 2 3 g 1 g 2 g 3 3 cell cluster with 3 sector antennas f 2 f 4 f 3 f 6 f 5 f 1 f 2 f 3 f 6 f 7 f 5 f 2 f 4 f 3 f 7 f 5 f 1 f 2 7 cell cluster

Radio resource management

• Channel Allocation – Channel Allocation is required to optimise frequency reuse • Fixed Channel Allocation • Dynamic Channel Allocation • Hybrid Channel Allocation 3 6 6 1 1 5 7 Frequency Reuse 7 2 2 4 3

Radio resource management : Channel allocation

• Fixed Channel Allocation (FCA) – Permanent or semi-permanent allocation – Certain frequencies are assigned to a certain cell – Problem: different traffic load in different cells – Methods: • Simple: all cells have same number of channels • Non-uniform: optimise usage according to expected traffic • Borrowing: channels can be reassigned if underused (BCA) Frequency Reuse 1 6 5 7 3 1 2 2 4 3 6 7

Radio resource management

• Dynamic Channel Allocation (DCA) – Gives control to base stations / switches to adapt – Channels are assigned as needed, not in advance – Base station chooses frequencies depending on the frequency already used in neighbour cells – Channels are returned when user has finished – More capacity in cells with more traffic – Assignment can also be based on interference measurements – Affecting factors include: • Blocking probability • Usage patterns and reuse distance • Current channel measurement

Radio resource management

• Hybrid Channel Allocation (HCA)

– Fixed schemes are not flexible enough – Dynamic schemes are too complex / difficult – Hybrid Schemes: • Split resources into pools of fixed and dynamic channels • Assign core of fixed channels then allocate rest dynamically • Altering the ratio may optimise the system – E.g. produce the lowest blocking rate

Radio resource management

• Overlapping Cells – Cells are naturally overlap (ideal shape is circular) – System may push some users into adjacent cells – Cost: increased handoff rate • Handoff – Two types of channel assignment: new calls, handoff – New calls have lower priority than handoff calls – QoS Channel Access Control should favour handoff over new

Radio resource management

• Macrocell/Microcell Overlay – Smaller cells increases frequency of handoff – Overlaying large cells on top of small ones: • Fast moving terminals are assigned channels in Macrocells • Slow moving terminals can use microcells • Overlap can be used to handoff during congestion • Increases the capacity (area) • But, increases the complexity

CDM cellular systems: Cell breathing

• CDM instead of FDM. Do not need elaborate channel allocation schemes and complex frequency planning. • Cell size depends on current load: cell breathe • Additional traffic appears as noise to other users • If the noise level is too high users drop out of cells