Wireless Digital Communication System

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Wireless Digital Communication System

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Wireless Digital Radio System-SDR

Wireless Digital Radio - SDR



“Life is beautiful if powered by Wireless

Technology”




Communication




Communication

“Communication is the process of transfer of information from a sender to a receiver who understands the message from the sender.”




Block Diagram of Digital Communication System

InputTransducer

OutputTransducer

SourceEncoder

SourceDecoder

ChannelEncoder

ChannelDecoder

DigitalModulator

DigitalDemodulator

Input Signal

Output Signal

Channel




Software Defined Radio




The SDR Forum defines the ultimate software radio (USR) as a radio that accepts fully programmable traffic and control information and supports a

broad range of frequencies, air-interfaces, and application software.

A good working definition of a software radio is a radio that is substantially defined in software and whose physical layer behavior can be significantly

altered through changes to its software.

Software defined radios are “radios that provide software control of a variety of modulation techniques, wideband or narrowband operation,

communications security functions (such as frequency hopping), and waveform requirements of current and evolving standards over a broad

frequency range”.



SDR – Five Tire Definition

Frequency Modulation/Demodulation Security,

Digitization Signal Processing etc.

Tier 0 HR

Tier 1 SCR

Tier 2

Tier 3

Tier 4

SDR

ISR

USR

Radio

Antenna

Earphone

Mouthpiece

Display/ Keyboard




Practical Model

Smart Antenna

Control

Channelization and

Sample rateConversion

IF OutputFlexible

RFHardwar

e

ADC

DAC

Input

Software• Algorithms• Middleware• CORBA• Virtual Radio Machine

Hardware• FPGAs• DSPs• ASICs

Processing




Technical Parameters




Data Bits to Symbol

Symbols are represented by the possible states of digital modulation. Higher order modulation allows more bits per symbol. What does that mean?

Mapping symbols to I and Q.

Compression, Error Correction, Encryption

011010100101 Symbol Mapping[Convert to Symbols]

01 10 10 10 01 01

Symbol rate = bit rate

the number of bits transmitted with each symbol




Bandwidth

Bandwidth is a measure of the frequency content of a signal.

Bandwidth can be imagined as a frequency width.

Sort of the fatness of the signal.

Why Carrier signal BW is zero?

The more information in a signal, the larger the BW of the information signal.

The span of information frequency content is called BW.

Example:- Human Voice – Spans in frequency from 30 Hz to 10,000 Hz.

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Message Bandwidth = WWireless Digital Radio - SDR



Transmission Bandwidth = 2WWireless Digital Radio - SDR



Shannon’s Theorem (1950’s) Relates Bit Rate, Bandwidth, & Signal to Noise

Bit Rate = BW * log2(1 + SNR)

Bit Rate (Bits/Sec.) = BR Signal bandwidth = BW SNR = Signal to Noise Ratio

Bit Rate is limited by S/N Symbol rate is a function of Bandwidth

Bit Rate / Symbol Rate is a function of signal complexity Complex modulations optimize Bit Rates/BW

Higher BR/BW requires higher Signal to Noise

In a noiseless system Infinite complexity and Bit Rate is theoretically attainable

Shannon Theoretical limit has never been reached C grows linearly with B, but only logarithmically with S/N




Filter Effect

Filtering allows the transmitted bandwidth to be significantly reduced without losing the content of the digital data. This improves the spectral efficiency of the signal. Any fast transition in a signal, whether it be amplitude, phase, or frequency, will require a wide occupied bandwidth.

Any technique that helps to slow down these transitions will narrow the occupied bandwidth.

Filtering serves to smooth these transitions (in I and Q). Filtering reduces interference because it reduces the tendency of one signal or one transmitter to interfere with another in a transmission system.




Root-Raised Cosine Filters

Time Domain Response of a Raised Cosine Filter Filter Bandwidth Parameters ()




Filter Bandwidth Parameter – Alpha (Roll-off Factor)The sharpness of a raised cosine filter is described by alpha () or roll-off factor. Alpha gives a direct measure of the occupied bandwidth of the system and is calculated as:

) (1 rate Symbol bandwidth Occupied

If the filter has a perfect (brick wall) characteristic with sharp transitions and an alpha of zero, the occupied bandwidth would be:

rate Symbol 0) (1 rate Symbol bandwidth Occupied 0, for

In a perfect world, the occupied bandwidth would be the same as the symbol rate, but this is not practical. An alpha of zero is impossible to implement. Alpha is sometimes called the “excess bandwidth factor” as it indicates the amount of occupied bandwidth that will be required in excess of the ideal occupied bandwidth (which would be the same as the symbol rate). At the other extreme, take a broader filter with an alpha of one, which is easier to implement.

rate Symbol 2 1) (1 rate Symbol bandwidth Occupied 1, for

An alpha of one uses twice as much bandwidth as an alpha of zero. In practice, it is possible to implement an alpha below 0.2 and make good, compact, practical radios. Typical values range from 0.35 to 0.5, though some video systems use an alpha as low as 0.11.




Filter Bandwidth Parameter – Alpha (Roll-off Factor)Wireless Digital Radio - SDR



EYE Diagram Recovered Pulse must avoid the RED area

RED area is an error in the amplitude or time



Link Budget

Vacation PlanMode : Railway with Sleeper Estimate : Rs 5000/- for hotel, restaurants, food etc.Watch money get spent at each stop.Get back to home : Still Rs 500/- left in your wallet

Communication Link : Link budgetTraveler : SignalMoney : PowerTravel : wire or wireless

Credit Card along the way for extra money : Extra power using repeaters for telephone link & transponder in satellite link.




Link Budget

First Class Vacation Plan Mode : Flight Five star hotel, restaurants, food etc.Budget of Rs 5000/- will not be enough and possibly will need instead Rs 25000/-.

With Signal, the quality is measured by the BER.

If we want our signal to have a low BER, we would start it out with higher power.

Quality of the trip desired determines how much money we need to take along.




Modulation and

its application




“Life is beautiful if powered by Wireless

Technology”



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Wireless Digital Communication System

Technology

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