WIRELESS COMMUNICATIONS

Presenter :- Melkamu Deressa

(B.Sc, M.Sc, PhD Candidate)

March , 2015

Xidian University

Xi’an ,P.R of China

OUTLINE

Introduction

Point-to-Point Communication

Multiuser Systems

Conclusion

04/15/2023 04:13:55 PM

Decoding

Sender

AWGN

Receiver

Feedback

Channel

Encoding

MessageNoise

Process of communication

Introduction

A group of connected devices that communicate voice and data through the air by means of electromagnetic waves, such as Radio, television, mobile telephone, and satellite communications in open space.

What is a Wireless Communication?

Types of Wireless Communication

Cellular

Wireless computer networkRadio service

Wireless Applications (services)

• In vehicles and traffic lights.• In aircrafts and underwater.• In Military and security needs • Hospitals• Street lamps• Petroleum and Chemical industries

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Advantages:• Wireless• Speed• Cost• Durability• Flexibility• Place of device

Disadvantages:• security vulnerabilities• Power consumption• Compatibility issues • high costs for setting the

infrastructure• Influenced by physical

obstructions, climatic conditions, interference from other wireless devices

Advantages and Disadvantages of Wireless Communication

04/15/20237

Evolution of Current Systems

• Wireless Systems today2G Cellular: ~30-70 Kbps.3G running 300Kbps and Cellular in testing: 384KbpsWLANs: ~10 Mbps and planned to up grad ~70 -100Mbps 4G Cellular: 100Mbps-1Gbps

• Next Generation Expected as 5 G : 1 Gbit/s to be offered, simultaneously

Coverage should be improvedSignaling efficiency enhanced.

• Technology Enhancements Hardware: Better batteries. Better circuits/processors.Link: Antennas, modulation, coding, adaptively, DSP,Network: Dynamic resource allocation. Mobility support.Application: Soft and adaptive QoS.

Wireless Data Vision• Geographically to expand to all needed application areas

TAXI

Region

Campus

City

In-Building

laptops, PDAs

• Technically to make it:- Faster and More Persistent More Secure More Deployable and Manageable Ease At Home

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Wireless Applications Devices

• Laptops• Cellular phones• Headphones• Keyboards• Printers• Speakers• Global Positioning

Systems (GPS)

• Laser Bridges• Emergency Services• Robotics• Biotechnology• Nanotechnology• Radio Frequency

Identification (RFID) transponders

The possibilities are endless!

OUTLINE

Introduction

Point-to-Point Communication

Multiuser Systems

Conclusion

Point-to-Point Communication

• Point-to-point communication is a networking technique for linking or connecting to physical networking devices with the help of wireless technology.

Cont.…..

• Many devices such as bridges, routers, connectors and transmitters work collectively in point to point wireless networks.

• It is platform independent and works efficiently and effectively for receiving and transmitting data from one place to another.

Supplementary Chapter 3Communication Channel Technology

S3-13

Communication Channel

•  Communication channel is a connection between transmitter and receiver through which data can be transmitted.

• Communication channel also called as communication media or transmission media

General Communication Model

Transmitter Communication channel Receiver

Shannon’s Wireless Communication System

SourceSourceEncoder

ChannelEncoder

Mod-ulator

UserSourceDecoder

ChannelDecoder

Demod-ulator

MessageSignal

Channel code word

Estimate ofMessage signal

Estimate of channel code word

ReceivedSignal

ModulatedTransmitted Signal

Wireless Channel

AWGN Channel• Additive white Gaussian noise (AWGN) is a basic noise

model used in information theory to mimic the effect of many random processes that occur in nature.

• This channel is assumed to corrupt the signal that n(t), which denotes a sample function of the additive white Gaussian noise process with zero-mean and two-sided power spectral density.

Thus the channel capacity   for the AWGN channel is given by:

1log(1 )

2

Pc

N

2) • In this channel, the transfer function assumed for the m’th user can be represented as:

• The random magnitude is assumed to be Rayleigh random variables for all users and sub-carriers, where Rayleigh distribution is:

Rayleigh Fading Channel

𝐻𝑚 [ 𝑓 𝑐+𝑖𝐹𝑇𝑏

]=𝜌𝑚 , 𝑖𝑒𝑗𝜃𝑚 ,𝑖

2,

2,

,

2,, 2

,

( )m i

m i

m i

m ip m i

m i

f e

Rician Fading Channel

• If we consider line-of-sight (LOS) with magnitude b0=const, the transfer function assumed for the m’th user can be represented as:

• The NLOS magnitude factors are assumed to have the following Rician distribution:

,

,

[ ]m ije

m cm ib

FH f i

T

2 2, ,

2 2, ,

,

(2,

, 2,

( )m i o o m i

om i m i

m i

b bI

m im i

m i

f

• Channel capacity(C): It is the maximum capacity at which data can be transmitted at a given communication Path, or cannel under a given conditions.

• Data Rate (BPs): The rate at which data can be communicated, impairments ,such as noise ,limit data rate that can be achieved.

• Band width (B): the band width of transmitted signal as constrained by the transmitter and the nature of the transmission medium (herzth).

• Noise (N): Impairments on communication path.

• Error rate - Rate at which errors occurs (BER) error = transmit 1 and receive 0; transmit 0 and receive 1

Channel Capacity

Cont.….

• Shannon during WWII, defines the notion of channel capacity and provides a mathematical model.

• The key result states that the capacity of the channel, as defined by mutual information between the input and output of the channel, where the maximization is with respect to the input distribution.

Shannon–Hartley theorem states the channel capacity C, meaning the theoretical tightest upper bound on the information rate.

whereC is the channel capacity in bits per second.B is the Bandwidth of the channel in hertz (passband bandwidth in case of a modulated signal);S is the average received signal power over the bandwidth (in case of a modulated signal, often denoted C, i.e.

modulated carrier), measured in watts (or volts squared);N is the average noise or interference power over the bandwidth, measured in watts (or volts squared); andS/N is the signal-to-noise ratio(SNR) or the carrier-to-noise ratio (CNR) of the communication signal to the Gaussian

noise interference expressed as a linear power ratio (not as logarithmic decibels).

2log (1 )S

C BN

• The block diagram on the top shows the blocks common to all communication systems

• Communication System: Components/subsystems act together to accomplish information transfer/exchange.

Communication systems

Digital

Analog

Communication is the transfer of information from one place to another.

21

Modulation and Demodulation

BDG(xx)

• Modulation is the process of changing one or more properties ( Amplitude, frequency or phase) of the analog carrier in proportion with the information signal.

• Reverse process of modulation and converting the modulated carrier back to the original information is known as demodulation.

Modulation

Analog Modulation

Digital modulation

• Modulation :- Converting digital to analog information to wave form suitable for transmission over a given medium.

• It involves varying ome parameter of carrier wave (sinusoidal waveform) at agiven frequency as a function of the message signal.

• General sinusoid:-

Modulation

¿

Amplitude FrequencyPhase

• If the information is digital changing parameters is called “keying “(e.g. ASK, PSK,FSK)

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Why Modulation is necessary?

1. It is difficult to radiate Low Frequency (LF) signal from antenna in the form of Electro Magnetic (EM) energy.

2. Information signal often occupy the same frequency band that would interfere with each other.

Types of Analog Modulation Amplitude Modulation (AM)

• Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant

Frequency Modulation (FM)• Frequency modulation is the process of varying the frequency of a carrier

wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant

Phase Modulation (PM)• Another form of analog modulation technique which we will not discuss

The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.

Frequency modulationAmplitude modulation

The total bandwidth required for FM can be determined from the bandwidth of the audio signal:

BFM = 2(1 + β)B. Where is usually 4.

Phase modulation

The total bandwidth required for PM can be determined from the bandwidth and maximum

amplitude of the modulating signal: BPM = 2(1 + β)B. Where = 2 most often.

• In digital wireless communication systems, the modulating signal may be represented as a time sequence of symbols or pulses, where each symbol has m finite states. Each symbol represents n bits of information where n = log2m bits/symbol.

Digital Modulation

Types of Digital modulation Techniques

(a) If the information signal is digital and amplitude of carrier is varied proportional to information signal, a digitally modulated signal known as Amplitude Shift Keying (ASK) is produced.

(b) If frequency of the carrier is varied, Frequency shift Keying (FSK) is produced.

(c) If phase of the carrier is varied , Phase Shift Keying (PSK) is produced.

Digital Modulation and Demodulation

signal x(t) = A cos(2πft + Φ)• A – amplitude• f – frequency• Φ – phase (initial angle of the sinusoidal function at its origin

Digital Modulation Techniques

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

Amplitude Shift Keying (ASK)

• In ASK, the two binary values are represented by to different amplitudes of the carrier frequency.

• The resulting modulated signal for one bit time is

0,0

1),2cos()(

binary

binarytfAts c

• Susceptible to noise• Inefficient modulation technique• used for

up to 1200bps on voice grade linesvery high speeds over optical fiber

Frequency Shift Keying (FSK)• The most common form of FSK is Binary FSK (BFSK)• Two binary values represented by two different frequencies ( f1 and f2 )

0),2cos(

1),2cos()(

2

1

binarytfA

binarytfAts

• less susceptible to noise than ASK• used for

up to 1200bps on voice grade lineshigh frequency radio (3 to 30MHz) even higher frequency on LANs using coaxial cable

Phase Shift Keying (PSK)

• Phase of carrier signal is shifted to represent data• Binary PSK (BPSK): two phases represent two binary digits

1)(),2cos()(

0),2cos(

1),2cos(

0),2cos(

1),2cos()(

tdtftAd

binarytfA

binarytfA

binarytfA

binarytfAts

c

c

c

c

c

(C)PSK

Quadrature Amplitude Modulation (QAM)

• QAM is described by a constellation consisting of combination of phase and amplitudes

• The rule governing bits-to-symbols are the same, i.e. n bits are mapped to M=2n symbols

• QAM used on asymmetric digital subscriber line (ADSL) and some wireless standards

• logical extension of QPSK• send two different signals simultaneously on same carrier

frequencyuse two copies of carrier, one shifted by 90°

each carrier is ASK modulated

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16-QAM Constellation Using Gray Coding

• 16-QAM has the following constellation• Note gray coding where adjacent symbols differ by only 1

bit0010001100010000

1010

1110

0110

1011

1111

0111

1001

1101

0101

1000

1100

0100

Advantages digital modulation technique

• High data rate• High spectral efficiency (minimum bandwidth occupancy)• High power efficiency (minimum required transmit power)• Robustness to channel impairments (minimum probability

of bit error)• Low power/cost implementation

• Amplitude/Phase modulation and Frequency modulation.

Spectral properties

• Techniques, more efficient multiple access strategies, and better security and privacy.

OUTLINE

Introduction

Point-to-Point Communication

Multiuser Systems

Conclusion

Multiuser Systems

• In this system resources (power, bandwidth, etc.) must be divided among the multiple users.

• It is a system that jointly estimate the channel response and detect all the user’s bits.

• Shown to have better performance as well as reduced computational complexity.

Multiuser channel

• A multiuser channel refers to any channel which must be shared among multiple users.

• There are two different types of multiuser channels: the broadcast channel and the multiple access channel.

Multiuser Channel

A Broadcast Channel

• It is a channel which has one transmitter sending to many receivers, and thus the bandwidth and power of the transmitter must be divided accordingly. Examples of broadcast channels include all radio and television transmissions, the downlink (satellite-to-earth station) of a satellite system, and the base station-to-mobile transmission of a cellular system.

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Multiple Access

• Allow multiple users to share same medium.• A multiple access channel has many transmitters sending

signals to one receiver. Share a finite amount of radio spectrum(limited bandwidth).High performanceDuplexing generally required

• 3 Main access techniques:Frequency Division MA (FDMA).Time Division MA (TDMA).Code Division MA (CDMA).

Police station

Public school

City Hall

Public Library

Fire station

Medical Center

Internet

Frequency Division

• Two bands of frequencies for every user• Forward band• Reverse band• Duplexer needed• Frequency separation between forward band and reverse

band is constant

frequency separation

reverse channel forward channelf

Time Division

• Uses time for forward and reverse link• Multiple users share a single radio channel• Forward time slot• Reverse time slot• Noncontiguous transmission• Digital data• No duplexer is required

time separationt

forward channelreverse channel

• Unique code to differentiate all users• Sequence used for spreading have low cross-correlations • Allow many users to occupy all the frequency/bandwidth

allocations at that same time• Receivers detect only the desired codeword. All others

appear as noise.• Receivers must know transmitter’s codeword. • Processing gain is the system capacity

How many users the system can support

Code-Division

Random Access

• Assumes dedicated channels wasteful - no dedicated channel assigned to each user

• Users contend for channel when they have data to send• Very efficient when users rarely active; very inefficient

when users have continuous data to send• Scheduling and hybrid scheduling used to combine

benefits of multiple and random access• Random Access Techniques

Aloha (Pure and Slotted)Carrier sensingTypically include collision detection or avoidancePoor performance in heavy loading

Random Access protocols

Random access MAC protocol specifies• How to detect collisions• How to recover from collisions (e.g., via delayed

retransmissions)

Random access MAC protocols:-Slotted ALOHAALOHACSMA and CSMA/CD

• Time is divided into equal size slots (= sending one frame)

• a newly arriving station transmits at the beginning of the next slot

• Highly decentralized: only slots in nodes need to be in sync and simple.

• It has difficulties with collisions, wasting slots, idle slots and nodes may be able to detect collision in less than time to transmit packet

Slotted Aloha

Pure (unslotted) ALOHA

• Slotted ALOHA requires slot synchronization• A simpler version, and no slot synchronization• A node transmits without awaiting for the beginning of a

slot (when frame first arrives transmit immediately)

CSMA (Carrier Sense Multiple Access)

• CSMA: listen before transmit. If channel is sensed busy, defer transmission

• Persistent CSMA: retry immediately when channel becomes idle (this may cause instability)

• Non persistent CSMA: retry after random interval of time• Note: collisions may still exist, since two stations may

sense the channel idle at the same time ( or better, within a “vulnerable” window = round trip delay)

CSMA/CD (Collision Detection)

• CSMA/CD: like in CSMAcollisions are detected within a few bit timesTransmission is then aborted, reducing the channel

wastage considerablypersistent retransmission is implemented

• Collision detection is easy in wired LANs: can measure signal strength on the line

• Collision detection cannot be done in wireless LANs :• CSMA/CD can approach channel utilization =1 in LANs:

low ratio of propagation over frame transmission time

Conclusions

• Communication is the process of meaningful interaction among human beings and also currently among various networked devices.

• The wireless vision encompasses many exciting systems and applications with technical challenges transcend across all layers of the system design.

• During Point-to-point communication one process sends a message and another process receives through effective communication channel according to the capacity of the information whether analog or digital data.

• A multiuser systems can be accessed randomly (ALOHA ,

CSMA,..) or deterministically (TDMA, FDMA, CDMA) which many transmitters sending signals on dedicated channels.

The ENDThank you for attention!

谢谢