Characteristics of wireless channel

Wireless Communication

The wireless radio channel poses a severe challenge 

as a medium for reliable high-speed communication.

It is not only susceptible to noise, interference, and 

other channel impediments, but these impediments 

change over time in unpredictable ways due to user 

movement. In this chapter we will characterize the 

variation in received signal power over distance due to

path loss , shadowing and multipath. Path loss is 

caused by dissipation of the power radiated by the 

transmitter as well as effects of the propagation channel.

Path loss models generally assume that path loss

is the same at a given transmit-receive distance. 

Shadowing is caused by obstacles between the 

transmitter and receiver that attenuate signal power 

through absorption, reflection, scattering, and 

diffraction. When the attenuation is very strong, 

the signal is blocked. Variation due to path loss

occurs over very large distances (100-1000 meters),

whereas variation due to shadowing occurs over 

distances proportional to the length of the obstructing 

object (10-100 meters in outdoor environments and 

less in indoor environments). Since variations due to

path loss and shadowing occur over relatively large 

distances, this variation is sometimes referred to as

large-scale propagation effects. Variation due to 

multi-path occurs over very short distances, on the order

of the signal wavelength, so these variations are 

sometimes referred to as small-scale propagation 

effects. Multi-path is caused by various signal

components come from reflections , these replicas

can be received in phase which leads to strong 

reception or out of phase which leads to weak signal. 

Fig.1.1 illustrates the ratio of the received-to-transmit 

power in dB versus log-distance for the combined 

effects of path loss, shadowing, and multi-path.

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Adaptive Modulation 

Wireless Communications

 

 I-The Use of Adaptive Modulation Not Fixed Modulation:

The reason is as the Fading channels confront us with the problem
of lost Packets and the need for frequent re-transmissions and 
Wireless communication systems are affected by changing weather conditions.

And if those effects not handled correctly, such systems may not be
able to cope with issues such as signal degradation thereby seriously 
endangering a Carrier’s ,quality of service obligations.
To overcome this effects making coding and interleaving to reduce
the effect of channel and weather effects but it reduces the 
throughput of  the channel, but we can “Dynamic Adaptive Modulation” 
which ensures maximum bandwidth most of the time with guaranteed 
critical services all the time .

II- Idea of Adaptive Modulation:

The basic idea of Adaptive Modulation is to adapt different 

order modulations that allow sending more bits per symbol

and thus achieving higher throughput or better spectral 

efficiencies However, it must also be noted that when using

a modulation technique such as 64-QAM, better signal-to-noise

ratios (SNRs) are needed to overcome any interference and 

maintain a certain bit error ratio (BER).

The use of adaptive modulation allows a wireless system to

choose the highest order modulation depending on

,the channel conditions then when

*The channel condition is good use 16-QAM or 64-QAM

*Bad channel condition use QPSK

*The channel is very bad do not send the data or use BPSK

In one word we can define Adaptive Modulation as 

the automatic modulation adjustment that a wireless system

can make to prevent weather-related fading from causing 

communication on the link to be disrupted.

From the figure, knowing the mean of the adaptive modulation

As the channel changes the modulation schemes also changes

(to maintain a constant BER (here its 10^-5

 The adaptive modulation try to use the time -variation in 

 channel conditions to increase the capacity of the system

not try to fight this effects only.

or can be  obvious from the following figures that the modulation

scheme changes due to weather changes or the distance form the a

antenna

The Block diagram of adaptive modulation: 

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Adaptive or Fixed Modulation 

Wireless communications

Which better Adaptive or Fixed Modulation ?

This experiment to made to know which is the better  adaptive modulation or fixed modulation 

We assume that the channel conditions are known for a few milliseconds ahead in time.The experiment is applicable to both TDD and FDD systems,provided that accurate predictions of the channel conditions exist.For each prediction of the SNR at the receiver, the modulation scheme is selected for 512 consecutive symbols.This implies that the channel estimator and the predictor work at a rate of (BW/512) where BW is the channel bandwidth.The data bit stream is then modulated and transmitted over the noisy channel.white Gaussian Noise (AWGN) with varying variance is added to simulate good and bad channel conditions.

At the receiver, the signal is demodulated and the obtained bit stream is compared to the original one. The number of errors is counted, as well as the number of transmitted bits. 

The first three columns in the figure belong to three transmissions using 

adaptive modulation with different error probability Thresholds. As expected, the adaptive modulation approach results in a relatively constant (adjustable) error rate. On the other hand, the use of non-adaptive modulation results in high peaks in the error rate when the receiver encounters a fading dip.

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Switching Between Modulation Schemes In Adaptive Modulation

Adaptive Modulation

The Idea of Switching:

if the performance of the modulation schemes as the next figure, Switching between modulation schemes is done to maintain a BER less than a certain BER the next figure as the desired BER less than 10^-3

From the previous figure, to maintain a BER less than 10^-3 with higher throughput, the switching between modulation schemes is due to the SNR of the channel. 

If the SNR less than 7dB don’t send data, 

8<SNR<17 use DPSK ,  

20<SNR<22.5 use 32-QAM 

17<SNR<20 use    16-QAM 

22.5<SNR<25 use  64-QAM 

Larger than 25 use 128-QAM

After switching the performance curve will be

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Cycle prefix In Wireless Communications

Wireless Communications

Cycle prefix :

The cyclic prefix means introduction of guard interval between each symbol to reduce the inter-symbol interference (ISI) caused by delay spread in the transmission channel, the CP achieved by taking a copy of the last portion of the data symbol appended to the front of the
symbol during the guard interval & this data portion must be at least 15% of the data length 
As shown in the figure. The CP adds redundancy through repetition of the signal rather than by adding any new information. When the CP is added , it's guarantees that the symbol will be undistorted for at least its nominal symbol in the presence of multipath and this allows the receiver to avoid the frequency domain ICI while at the same time avoiding all time domain
ISI due to multipath. And also by adding the CP to our original signal and transmitting through the same channel, we can obtain the desired circular convolution which makes it easier to recover the signal after the FFT at the receiver.

Cyclic prefix advantages:

1- Increases transmitted power.
2- Lowers symbol rate.
Disadvantages of cyclic prefix
1- Affect on data rate due to addition of redundant symbols.
2- Cause power loss due to the transmission power loss to send the redundant symbols.

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Basics of OFDM In Wireless Communications

Wireless Communications

Basics of OFDM:

OFDM Transmitter:

OFDM Transmitter is shown as follows:

The bit sequence is first subjected to channel encoding to reduce the probability of error at the receiver due to the channel effects. 
The bits are mapped to symbols of either 16-QAM or QPSK.
The symbol sequence is converted to parallel format IFFT (OFDM modulation) is applied to convert the block of frequency data to a block of time data that modulates the carrier.
The sequence is once again converted to the serial format. 
Guard time is provided between the OFDM symbols and the guard time is filled with the cyclic extension of the OFDM symbol.
Windowing is applied to the OFDM symbols to make the fall-off rate of the spectrum steeper.
The resulting sequence is converted to an analog signal using a DAC and
passed on to the RF modulation stage. 
The resulting sequence is converted to an analog signal using a DAC and passed on to the RF modulation stage.

OFDM receiver:

OFDM receiver is shown as follows:

At the receiver, first RF demodulation is performed. Then the signal is digitized using an
ADC. Timing and frequency synchronization are performed. The guard time is removed from
each OFDM symbol. The sequence is converted to parallel format.FFT (OFDM
demodulation) is applied to get back to the frequency domain. The output is then serialized.
Symbol de-mapping is done to get back the coded bit sequence. Channel decoding is, then,
done to get the user bit sequence.

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Comparison Between FDM and OFDM

Wireless Communications

 

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Orthogonality of OFDM

Wireless Communications

In OFDM, the spectra of subcarriers overlap but remain orthogonal to each other.
This means that at the maximum of each subcarrier spectrum, all the spectra of other subcarriers are zero. 
The receiver samples data symbols on individual subcarriers at the maximum points and demodulates them free from any interference from the other subcarriers and hence no ICI. 
The orthogonality of subcarriers can be viewed in either the time domain or in frequency domain.
From the time domain perspective, each subcarrier is a sinusoid with an integer number of cycles within one FFT interval. From the frequency domain perspective, this corresponds to each subcarrier having the maximum value at its own center frequency and zero at the center frequency of each of the other subcarriers.

  

Fig .1 shows the spectrum of an individual data subcarrier. The OFDM signal multiplexes in the individual spectra with frequency spacing equal to the transmission bandwidth of each subcarrier.

Fig.2 shows that at the center frequency of each subcarrier, there is no cross talk from other channels. 

Therefore, if a receiver performs correlation with the center frequency of each subcarrier, it can recover the transmitted data without any cross talk.

In addition, using the DFT-based multicarrier technique, frequency division multiplexing is achieved by baseband processing rather than the bandpass processing. 

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Wireless Communications (4)

Wireless Communications

Block diagrams of a wireless communication system

Block diagram of a receiver:

System block diagram of a direct conversion receiver and transmitter. An antenna is followed by a band pass filter, used as a band select filter. This eliminates out-of-band noise and presents the signal to the low-noise amplifier (LNA). The LNA then amplifies the desired signal, adding a minimum amount of inherent noise. The signal processed by the LNA is then down-converted to the desired IF frequency by a set of mixers operating in quadrature. These mixers are often image-reject mixers and have some gain as well. After down-conversion, the low-frequency IF signal is low pass -filtered to remove aliasing components and converted to digital samples by the analog-to-digital converter. In the digital domain, more filtering is applied for channel selection, and plenty of signal processing is performed to remove any channel effect before the detection stage.

Block diagram of transmitter:

On the transmitter side, the digital I and Q data – which are already processed by a digital-to-analog converter, filtered and amplified – are up-converted by quadrature mixers to the carrier frequency of interest. After combining, the signal is again filtered to contain the spectral content of the signal in the required bandwidth stipulated by the emission mask. After that, it is applied to the power amplifier and transmitted over the air with an antenna.

Details for each of the blocks in the transmitter and receiver:

ü Antennas

Antennas are coupling circuits to space that radiate or receive information-bearing electromagnetic waves.

 In a receiving antenna, the EM wave impinging on the surface produces currents, which in a 50-ohm system are applied to an LNA for amplification and subsequent processing.

On the transmitter side, the surface current density on the antenna produces a magnetic field around the antenna.

ü Filters

Filters remove the effect of broadband noise and thereby increase the SNR of a desired signal. They are also used to select channels in multiple transmission environments and to remove image frequencies in broadband services and other out-of-band interference.

 In the transmitter, digital pulse-shaping filters are used for efficient utilization of the RF spectrum and externally to suppress RF splatter in adjacent channels.

ü Amplifiers

The RF signal at a receiver’s antenna is very small in magnitude. The IEEE 802.15.4 standard defines a minimum signal of -85 dBm = 3.16 pW, whose voltage in a 50-ohm system is 12.6 µV. At the detector, the typical signal requirement is at 1 mVp-p for detection and decoding of digital waveforms.

On the transmitter side, power amplifiers (PAs) are used to transmit the EM wave. PAs come in various classes and can be linear and nonlinear. They usually employ matching circuits between the output and the load.

ü Mixers

Mixers are fundamental building blocks that translate frequencies from one band to another for further processing without changing the information content.

On the transmitter side, they up-convert a baseband signal for efficient transmission over a channel.

At the receiver, they down-convert to a suitable intermediate frequency for the extraction of information.

ü Oscillator

Oscillators produce sinusoidal signals that up-convert or down-convert an RF signal to the required frequency, where subsequent processing might begin. They are designed to operate at a specified frequency. Generally, there is an amplifier and feedback circuit that returns a portion of the amplified signal back to the input. When feedback is aligned in phase, sustained oscillators occur. In practice, they are not perfect, and drift in frequency from time to time. They are also susceptible to phase noise. Due to this, many transceivers operate them in a phase-locked loop (PLL) that can provide frequency stability and lower phase noise.

ü Analog-to-digital converter

Analog-to-digital converters are required to convert analog signals to digital signals for baseband processing. After digitizing, signal channel selection can occur in the digital domain, as can equalization.

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Wireless Communications (3)

Wireless Communications

Advantages of wireless communication:

Wireless communication has the following advantages:

i. Communication has enhanced to convey the information quickly to

 the consumers.

ii.Working professionals can work and access Internet anywhere and  

 anytime without carrying cables or wires wherever they go. This

also helps to complete the work anywhere on time and improves

 the productivity.

iii. Doctors, workers and other professionals working in remote areas

can be in touch with medical centres through wireless communication.

iv.Urgent situation can be alerted through wireless communication.

The affected regions can be provided help and support with the

help of these alerts through wireless communication.

v.Wireless networks are cheaper to install and maintain.

Disadvantages of wireless communication:

The growth of wireless network has enabled us to use personal devices anywhere and anytime. This has helped mankind to improve in every field of life but this has led many threats as well.

Wireless network has led to many security threats to mankind. It is very easy for the hackers to grab the wireless signals that are spread in the air.

 It is very important to secure the wireless network so that the information cannot be exploited by the unauthorized users. This also increases the risk to lose information. Strong security protocols must be created to secure the wireless signals like WPA and WPA2. Another way to secure the wireless network is to have wireless intrusion prevention system.

Applications of wireless technologies:

ü Mobile telephones

One of the best-known examples of wireless technology is the mobile phone, also known as a cellular phone, with more than 4.6 billion mobile cellular subscriptions worldwide as of the end of 2010. These wireless phones use radio waves to enable their users to make phone calls from many locations worldwide. They can be used within range of the mobile telephone site used to house the equipment required to transmit and receive the radio signals from these instruments.

ü Wireless data communications

      Wireless data communications are an essential component of mobile computing. The various available technologies differ in local availability, coverage range and performance, and in some circumstances, users must be able to employ multiple connection types and switch between them. To simplify the experience for the user, connection manager software can be used, or a mobile VPN deployed to handle the multiple connections as a secure, single virtual network. Supporting technologies include:

o   Wi-Fi

o   Cellular data service

o   Mobile Satellite Communications

ü Wireless energy transfer

Wireless energy transfer is a process whereby electrical energy is transmitted from a power source to an electrical load that does not have a built-in power source, without the use of interconnecting wires.

ü Computer interface devices

Answering the call of customers frustrated with cord clutter, many manufacturers of computer peripherals turned to wireless technology to satisfy their consumer base Originally these units used bulky, highly limited transceivers to mediate between a computer and a keyboard and mouse; however, more recent generations have used small, high-quality devices, some even incorporating Bluetooth. These systems have become so ubiquitous that some users have begun complaining about a lack of wired peripherals. Wireless devices tend to have a slightly slower response time than their wired counterparts; however, the gap is decreasing.

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