Wireless Communications

Characteristics of wireless channel

alexeng_ismailelsayed@yahoo.com (Anonymous) — Sat, 20 Apr 2013 14:16:00 +0000

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

alexeng_ismailelsayed@yahoo.com (Anonymous) — Sat, 20 Apr 2013 14:07:00 +0000

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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Which Is the Better Adaptive or Fixed Modulation

alexeng_ismailelsayed@yahoo.com (Anonymous) — Sat, 20 Apr 2013 13:48:00 +0000

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

alexeng_ismailelsayed@yahoo.com (Anonymous) — Sat, 20 Apr 2013 13:44:00 +0000

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

alexeng_ismailelsayed@yahoo.com (Anonymous) — Thu, 18 Apr 2013 13:49:00 +0000

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

alexeng_ismailelsayed@yahoo.com (Anonymous) — Thu, 18 Apr 2013 13:39:00 +0000

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 In Wireless Communications

alexeng_ismailelsayed@yahoo.com (Anonymous) — Thu, 18 Apr 2013 12:42:00 +0000

Comparison Between FDM and OFDM

Wireless Communications

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Orthogonality of OFDM In Wireless Communications

alexeng_ismailelsayed@yahoo.com (Anonymous) — Thu, 18 Apr 2013 12:26:00 +0000

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)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Wed, 17 Apr 2013 15:54:00 +0000

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)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Wed, 17 Apr 2013 14:34:00 +0000

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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Wireless Communications (2)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Wed, 17 Apr 2013 13:46:00 +0000

Wireless Communications (2)

Wireless Communications

Categories of wireless implementations, devices and standards:

· Radio communication system

· Broadcasting

· Amateur radio

· Land Mobile Radio or Professional Mobile Radio: TETRA, P25, Open Sky, EDACS, DMR, dPMR

· Cordless telephony: DECT (Digital Enhanced Cordless Telecommunications)

· Cellular networks: 0G, 1G, 2G, 3G, Beyond 3G (4G), Future wireless

· List of emerging technologies

· Short-range point-to-point communication : Wireless microphones, Remote controls, IrDA, RFID (Radio Frequency Identification), Transfer Jet, Wireless USB, DSRC (Dedicated Short
Range Communications), En Ocean, Near Field Communication

· Wireless sensor networks: ZigBee, En Ocean; Personal area networks, Bluetooth, Transfer Jet, Ultra-wideband (UWB from WiMedia Alliance).

· Wireless networks: Wireless LAN (WLAN), (IEEE 802.11 branded as Wi-Fi and Hiper LAN), Wireless Metropolitan Area Networks (WMAN) and (LMDS, WiMAX, and Hiper MAN)

Wireless network:

Wireless network refers to any type of computer network that uses wireless (usually, but not always radio waves) for network connections.

It is a method by which homes, telecommunications networks and enterprise (business) installations avoid the costly process of introducing cables into a building, or as a connection between various equipment locations.

Types of Wireless networks:

ü Wireless PAN

Wireless personal area networks (WPANs) interconnect devices within a relatively small area that is generally within a person's reach. For example, both Bluetooth radio and invisible infrared light provides a WPAN for interconnecting a headset to a laptop.

ü Wireless LAN

A wireless local area network (WLAN) links two or more devices over a short distance using a wireless distribution method, usually providing a connection through an access point for Internet access. The use of spread-spectrum or OFDM technologies may allow users to move around within a local coverage area, and still remain connected to the network.

ü Wireless mesh network

A wireless mesh network is a wireless network made up of radio nodes organized in a mesh topology. Each node forwards messages on behalf of the other nodes. Mesh networks can "self heal", automatically re-routing around a node that has lost power.

ü Wireless MAN

Wireless metropolitan area networks are a type of wireless network that connects several wireless LANs.

ü Wireless WAN

Wireless wide area networks are wireless networks that typically cover large areas, such as between neighboring towns and cities, or city and suburb. These networks can be used to connect branch offices of business or as a public internet access system.

ü Cellular network

A cellular network or mobile network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver, known as a cell site or base station. In a cellular network, each cell characteristically uses a different set of radio frequencies from all their immediate neighboring cells to avoid any interference.

Electromagnetic spectrum of wireless:

Light, colors, AM and FM radio and electronic devices make use of the electromagnetic spectrum.

The frequencies of the radio spectrum that are available for use for communication are treated as a public resource and are regulated by national organizations such as the Federal Communications Commission in the USA, or Of com in the United Kingdom.

This determines which frequency ranges can be used for what purpose and by whom. In the absence of such control or alternative arrangements such as a privatized electromagnetic spectrum, chaos might result if, for example, airlines didn't have specific frequencies to work under and an amateur radio operator were interfering with the pilot's ability to land an aircraft. Wireless communication spans the spectrum from 9 kHz to 300 GHz.

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Wireless Communications

alexeng_ismailelsayed@yahoo.com (Anonymous) — Wed, 17 Apr 2013 13:18:00 +0000

Wireless Communications (1)

Wireless Communications

Definition:

Wireless communication is the transfer of information between two or more points that are not connected by an electrical conductor.

Wireless technologies:

The most common wireless technologies use electromagnetic wireless telecommunications, such as radio. With radio waves distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications.

It encompasses various types of fixed, mobile, and portable applications, including two-way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking.

Other examples of applications of radio wireless technology include GPS units, garage door openers, wireless mice, keyboards and headsets, headphones, radio receivers, satellite television, broadcast television and cordless telephones.

Wireless networks:

Wireless networking (e.g., the various types of unlicensed 2.4 GHz WiFi devices) is used to meet many needs. Perhaps the most common use is to connect laptop users who travel from location to location. Another common use is for mobile networks that connect via satellite. A wireless transmission method is a logical choice to network a LAN segment that must frequently change locations. The following situations justify the use of wireless technology:

· To span a distance beyond the capabilities of typical cabling.

· To provide a backup communications link in case of normal network failure

· To link portable or temporary workstations.

· To overcome situations where normal cabling is difficult or financially impractical.

· To remotely connect mobile users or networks.

Wireless modes:

Wireless communications can be via:

· Radio communication.

· microwave communication, for example long-range line-of-sight via highly directional antennas, or short-range communication

· Light, visible and infrared (IR) for example consumer IR devices such as remote controls or via Infrared Data Association (IrDA).

· Sonic, especially ultrasonic short range communication.

· electromagnetic induction short range communication and power

Applications may involve point-to-point communication, point-to-multipoint communication, broadcasting, cellular networks and other wireless networks.

· Wi-Fi technology.

Wireless services:

· Telemetry control and traffic control systems.

· Infrared and ultrasonic remote control devices.

· Professional LMR (Land Mobile Radio) and SMR (Specialized Mobile Radio) typically used by business, industrial and Public Safety entities.

· Consumer Two way radio including FRS Family Radio Service, GMRS (General Mobile Radio Service) and Citizens band ("CB") radios.

· The Amateur Radio Service (Ham radio).

· Consumer and professional Marine VHF radios.

· Air band and radio navigation equipment used by aviators and air traffic control.

· Cellular telephones and pagers: provide connectivity for portable and mobile applications, both personal and business.

· Global Positioning System (GPS): allows drivers of cars and trucks, captains of boats and ships, and pilots of aircraft to ascertain their location anywhere on earth.

· Cordless computer peripherals: the cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless using technology such as Wireless USB or Bluetooth.

· Cordless telephone sets: these are limited-range devices, not to be confused with cell phones.

· Satellite television: Is broadcast from satellites in geostationary orbit. Typical services use direct broadcast satellite to provide multiple television channels to viewers.

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4G Wireless Communications

alexeng_ismailelsayed@yahoo.com (Anonymous) — Tue, 16 Apr 2013 17:57:00 +0000

4G Wireless Communications

Wireless Communications

4G is the name given to the fourth generation of mobile phone mobile communication technology standards. It is a successor of the third generation (3G) standards. It provides mobile ultra-broadband Internet access

4G is around five times faster than existing 3G services. Theoretically it can provide download speeds of up to 100Mbps and 50Mbps for upload. This makes 4G is more than twice as fast as the latest 3G technology and many more times faster than previous versions. but these speeds are theoretical, and we won't achieve this in real-world use.

However, that doesn't mean 4G isn't twice as quick. In our tests, which we'll get to later, we saw speeds around three times faster on 4G compared to 3G, and an even larger improvement with uploads.

The faster speeds mean websites load quicker, and that you'll be able to stream videos and podcasts without first waiting for them to buffer. Plus, you'll be able to download large email attachments or other content from the web faster. Apps which need to download data, such as maps, will work more smoothly, especially when zooming in or out as this generally requires a lot of data. The speed differential should be akin to switching from 3G to Wi-Fi.

For video streaming and similar tasks, where you would typically require Wi-Fi for smooth performance, 4G should allow you to have a 'home broadband' experience on the move. EE expects the average speed to be between 8- and 12Mbps, potentially faster than the 5.9Mbps average for ADSL home broadband.

Faster upload speeds will also be a boon. If you hate waiting for pictures to be posted to Facebook or Twitter, for example, then this should be a much faster process over 4G.

4G networks use different frequencies to transmit data than 3G so we need a handset which has a modem that supports these new frequencies.

Two forms of 4G been developed and are in use: WiMAX and LTE.

In fact, we may recognize the first technology, as WiMAX was trialled in the UK in 2009. However, the first WiMAX network was launched by South Korean firm KT in 2006.

'LTE' Stands for Long Term Evolution and is a type of 4G technology

The first LTE network was deployed in Scandinavia in 2009. However, it was debatable whether the speeds on offer back then were really 4G or not.

4G LTE aims to offer users faster, more reliable mobile broadband internet for devices like smart phones, tablets and laptops.

What is 4G technology?

The main reason 4G is faster than 3G is because of Orthogonal Frequency-Division Multiplexing (OFDM). It sounds complicated, but it's the same technology used in Wi-Fi, ADSL broadband, digital TV and radio.

OFDM is a technique for squeezing more data onto the same amount of radio frequency. It also reduces latency and interference. Data is split up and sent via small chunks of frequency in parallel, therefore increasing the capacity of the network.

Multiple-input and multiple-output, or MIMO, is another reason 4G is able to provide faster speeds. It is simply the use of multiple antenna arrays at both the transmitter and receiver to improve communication performance.

This allows more data to be transferred without requiring additional bandwidth or drawing more power. The most common configuration currently is a 2x2 MIMO, found in many smart phones and some tablets. A 4x4 setup is also possible and promises even faster speeds but is still a little way off making its way onto devices. Since different setups are possible, one phone could provide faster 4G speeds than another.

With 3G handsets, most of us take roaming for granted. We take our phones travelling around the world, and expect to be able to pick up emails and browse websites as soon as we land. Things are different with 4G.

Although there are 4G networks in many countries around the world, your UK 4G Smartphone won't necessarily work wherever you go. The reason is that 4G doesn't operate on the same frequencies in every country.

What are 4G frequency bands?

Technology	Frequency spectrum
3G	850MHz	900MHz	1900MHz	2100MHz
4G	800MHz	1800MHz	2600MHz
Wi-Fi	2400MHz	5000MHz
Bluetooth	2400MHz

What is 4G coverage?

EE says it will provide 4G coverage in a total of 10 UK cities at launch, with 16 switched on by Christmas. The firm says it means 20 million users will be able to get the faster speeds before the year is over. The lucky cities to get 4G before the rest of the country are:

Birmingham, Bristol, Cardiff, Edinburgh, Glasgow, Leeds, Liverpool, London, Manchester and Sheffield.

4G improvements than 3G:

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Bluetooth

alexeng_ismailelsayed@yahoo.com (Anonymous) — Tue, 16 Apr 2013 17:17:00 +0000

Bluetooth

Wireless Communications

Definition:

a wireless technology standard for exchanging data over short distances (using short-wavelength radio transmissions in the ISMband from 2400–2480 MHz)

from fixed and mobile devices, creating personal area networks (PANs) with high levels of security. It can connect several devices, overcoming problems of synchronization.

Communication and connection:

A master Bluetooth device can communicate with a maximum of seven devices in a piconet (an ad-hoc computer network using Bluetooth technology), though not all devices reach this maximum. The devices can switch roles, by agreement, and the slave can become the master (for example, a headset initiating a connection to a phone will necessarily begin as master, as initiator of the connection; but may subsequently prefer to be slave).

The Bluetooth Core Specification provides for the connection of two or more piconets to form a scatternets , in which certain devices simultaneously play the master role in one piconet and the slave role in another.

At any given time, data can be transferred between the master and one other device (except for the little-used broadcast mode. The master chooses which slave device to address; typically, it switches rapidly from one device to another in a round-robin fashion. Since it is the master that chooses which slave to address, whereas a slave is (in theory) supposed to listen in each receive slot, being a master is a lighter burden than being a slave. Being a master of seven slaves is possible; being a slave of more than one master is difficult the specification is vague as to required behavior in scatternets.

Many USB Bluetooth adapters or "dongles" are available, some of which also include an IrDA adapter. Older Bluetooth dongles, however, have limited capabilities, offering only the Bluetooth Enumerator and a less-powerful Bluetooth Radio incarnation. Such devices can link computers with Bluetooth with a distance of 100 meters, but they do not offer as many services as modern adapters do.

Usage:

Bluetooth is a standard wire-replacement communications protocol primarily designed for low power consumption, with a short range (power-class-dependent, but effective ranges vary in practice; see table below) based on low-cost transceiver microchips in each device. Because the devices use a radio (broadcast) communications system, they do not have to be in visual line of sight of each other, however a quasi optical wireless path must be viable.

Applications:

* Wireless control and communication between a mobile phone and a handsfree headset. This was one of the earliest applications to become popular.

* Wireless control of and communication between a mobile phone and a Bluetooth compatible car stereo system

* Wireless Bluetooth headset and Intercom.

* Wireless networking between PCs in a confined space and where little bandwidth is required.

* Wireless communication with PC input and output devices, the most common being the mouse, keyboard and printer.

* Transfer of files, contact details, calendar appointments, and reminders between devices with OBEX.

* Replacement of previous wired RS-232 serial communications in test equipment, GPS receivers, medical equipment, bar code scanners, and traffic control devices.

* For controls where infrared was often used.

* For low bandwidth applications where higher USB bandwidth is not required and cable-free connection desired.

* Sending small advertisements from Bluetooth-enabled advertising hoardings to other, discoverable, Bluetooth devices

* Wireless bridge between two Industrial Ethernet (e.g., PROFINET) networks.

* Three seventh and eighth generation game consoles, Nintendo's Wii and Sony's PlayStation 3, PSP Go and PS Vita, use Bluetooth for their respective wireless controllers.

* Dial-up internet access on personal computers or PDAs using a data-capable mobile phone as a wireless modem.

* Short range transmission of health sensor data from medical devices to mobile phone, set-top box or dedicated telehealth devices.

* Allowing a DECT phone to ring and answer calls on behalf of a nearby mobile phone

* Real-time location systems (RTLS), are used to track and identify the location of objects in real-time using “Nodes” or “tags” attached to, or embedded in the objects tracked, and “Readers” that receive and process the wireless signals from these tags to determine their locations.

* Personal security application on mobile phones for prevention of theft or loss of items. The protected item has a Bluetooth marker (e.g. a tag) that is in constant communication with the phone. If the connection is broken (the marker is out of range of the phone) then an alarm is raised.

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MIMO In 4G Wireless Communications(2)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Tue, 16 Apr 2013 14:24:00 +0000

MIMO In 4G Wireless Communications(2)

4G Communications

MIMO Schemes in LTE-Advanced:

In order to support downlink peak spectrum efficiency of 30 bps/Hz and

uplink peak spectrum efficiency of 15 bps/Hz according to LTE-Advanced

requirement, the spatial multiplexing with antenna configuration of 8 × 8 for

downlink transmission and 4 × 4 for uplink transmission is being investigated.

Here N × N denotes a configuration of N transmit antennas and N receive

antennas. In addition to meeting the peak spectrum efficiency, further

improvement of the average cell throughput as well as the cell edge performance is also an important aspect of the LTE-advanced study.

Coordinated multipoint transmission/reception is a candidate technology

where antennas of multiple cell sites are utilized in a way such that the

transmit/receive antennas of the serving cell as well as the neighboring cells can contribute in improving quality of the received signal at the UE/eNodeB, as well as in reducing the co-channel interferences from neighboring cells.

The application of coordinated multipoint transmission, For a downlink transmission scenario. An example scheme is to form a beam to the schedule UE by using the transmit antennas of the cells 1, 2, and 3, where each cell transmits the same data to the scheduled UE and the UE-specific reference signal is used for support of demodulation at the UE. For the cells 1, 2, and 3 to jointly form the transmit signal matching to the composite channel

experienced by the UE, it may be necessary to provide feedback representing the downlink spatial channel of each cell without any pre-assumption on operation at the eNodeB transmitter and the UE receiver. The feedback for explicit representation of the spatial channel of each cell may naturally require much larger overhead than the feedback defined in LTE such as CQI, PMI

Performance:

LTE downlink SU-MIMO performance

Relation between data throughput & coverage in different number of ranks

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MIMO In 4G Wireless Communications(1)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Tue, 16 Apr 2013 13:53:00 +0000

MIMO In 4G Wireless Communications(1)

4G Communications

Definition:

Multiple-input and Multiple-Output, or (MIMO), is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology.

Importance:

C=B log₂(1+SNR) …………. Shannon's law

Two major limitations in communications channels can be multipath

interference, and the data throughput limitations as a result of Shannon's Law.

MIMO provides a way of utilizing the multiple signal paths that exist between a

transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth.

By using multiple antennas at the transmitter and receiver along with some complex digital signal processing,

MIMO technology enables the system to set up multiple data streams on the

same channel, thereby increasing the data capacity of a channel.

MIMO is being used increasingly in many high data rate technologies

including Wi-Fi and other wireless and cellular technologies to provide improved levels of efficiency.

Essentially MIMO employs multiple antennas on the receiver and transmitter to utilize the multi-path effects that always exist to transmit additional data, rather than causing interference.

The schemes employed in LTE again vary slightly between the uplink and downlink. The reason for this is to keep the terminal cost low as there are far more terminals than base stations and as a result terminal works cost price is far more sensitive.

For the downlink, a configuration of two transmit antennas at the base station and two receive antennas on the mobile terminal is used as baseline, although configurations with four antennas are also being considered.

Applications:

It is used in Mobile radio telephone standards such as recent 3GPP and 3GPP2. In 3GPP, High-Speed Packet Access plus (HSPA+) and Long Term Evolution (LTE)standards take MIMO into account.

It also can be used in non-wireless communications systems, Wi-Fi and other wireless and cellular technologies.

DOWNLINK SU-MIMO in LTE:

The SU-MIMO scheme is applied to the Physical Downlink Shared Channel (PDSCH), which is the physical layer channel that carries the information data from the network to the UE. With SU-MIMO spatial multiplexing, the LTE system provides a peak rate of 150 Mbps for two transmit antennas and 300 Mbps for four transmit antennas.

Modes of SUMIMO:

There are two operation modes in SUMIMO spatial multiplexing:

The closed-loop spatial multiplexing mode and the open-loop spatial multiplexing mode.

In the closed-loop spatial multiplexing mode, the base station (also known as E-Node B) applies the spatial domain precoding on the transmitted signal taking into account the pre-coding matrix indicator (PMI) reported by the UE so That the transmitted signal matches with the spatial channel experienced by the UE.

MU-MIMO in LTE:

MU-MIMO scheme is supported in both the uplink and downlink of the LTE

standard.

In the uplink, the eNodeB can always schedule more than one UEs to

transmit in the same time frequency resource, which forms a MU-MIMO

transmission configuration However, in order for the eNodeB to be able to

correctly differentiate and demodulate these UEs’ signals eNodeB needs to

assign orthogonal reference signals for these UEs scheduled for the MU-MIMO transmission.

The following Figure shows the uplink slot structure where the reference signal is transmitted using the fourth symbol and the data is transmitted using the others. For a given slot and sub-frame in each cell, a Zadoff-Chu sequence is defined as the base sequence for uplink reference signals.

The cyclically shifted versions of a given Zadoff-Chu sequence form an orthogonal set of sequences.

Each UE scheduled for MUMIMO transmission is assigned a distinctive cyclic

shift value, and the UE combines this cyclic shift value with the knowledge of the base Zadoff-Chu sequence to form a reference signal sequence that is

orthogonal to other UEs’ reference signal sequences. It is noted that the cyclic

shift value is always contained in the control signaling, which the UE has to

receive for data transmission on uplink, regardless whether the MU-MIMO is

operated or not.

In the downlink, if a UE is configured to be in the MU-MIMO transmission

mode, only rank-1 transmission can be scheduled to the UE. The eNodeB can

schedule multiple UEs, which are configured to be in the MU-MIMO transmission mode, in the same time-frequency resource using different rank-1 pre-coding matrices at LTE Standard for two transmit antennas and another matrix at LTE Standard for four transmit antennas. Note that the UE receives only the information about its own pre-coding matrix.

The scheduled UE then decodes the information data utilizing the common reference signal together with the precoding information obtained from the control signaling.

The UE generates the PMI/CQI (pre-coding matrix indicator / Channel Capacity indicator) feedback without any knowledge about other simultaneously scheduled UEs. Hence, there could be mismatch between

The UE’s CQI report and the actual CQI experienced due to lack of knowledge of interference caused by another UEs scheduled simultaneously.

In LTE, for support of receiving higher-order modulation signal such as 16QAM and 64QAM without causing too much complexity in the UE, the transmit power level for each UE is configured in a long-term manner.

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WI-FI Wireless Communication System (3)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Mon, 15 Apr 2013 12:49:00 +0000

WI-FI Wireless Communication System (3)

Wireless Communication

Network security:

The main issue with wireless network security is its simplified access to the network compared to traditional wired networks such as Ethernet, with wired networking one must either gain access to a building (physically connecting into the internal network) or break through an external firewall.

To enable Wi-Fi, one merely needs to be within the wireless range of the Wi-Fi network. Most business networks protect sensitive data and systems by attempting to disallow external access. Enabling wireless connectivity reduces security if the network uses inadequate or no encryption.

An attacker who has gained access to a Wi-Fi network router can initiate a DNS spoofing attack against any other user of the network by forging a response before the queried DNS server has a chance to reply.

Securing methods:

A common measure to deter unauthorized users involves hiding the access point's name by disabling the SSID broadcast. While effective against the casual user, it is ineffective as a security method because the SSID is broadcast in the clear in response to a client SSID query.

Another method is to only allow computers with known MAC addresses to join the network, but determined eavesdroppers may be able to join the network by spoofing an authorized address.

Wired Equivalent Privacy (WEP) encryption was designed to protect against casual snooping but it is no longer considered secure. Tools such as AirSnort or Aircrack-ng can quickly recover WEP encryption keys. Because of WEP's weakness the Wi-Fi Alliance approved Wi-Fi Protected Access (WPA) which uses TKIP.

WPA was specifically designed to work with older equipment usually through a firmware upgrade. Though more secure than WEP, WPA has known vulnerabilities.

The more secure WPA2 using Advanced Encryption Standard was introduced in 2004 and is supported by most new Wi-Fi devices. WPA2 is fully compatible with WPA.

A flaw in a feature added to Wi-Fi in 2007, called Wi-Fi Protected Setup, allows WPA and WPA2 security to be bypassed and effectively broken in many situations. The only remedy as of late 2011 is to turn off Wi-Fi Protected Setup, which is not always possible.

Hotspot (Wi-Fi):

A hotspot is a site that offers Internet access over a wireless local area network through the use of a router connected to a link to an Internet service provider. Hotspots typically use Wi-Fi technology.

Hotspots may be found in coffee shops and various other public establishments in many developed urban areas throughout the world.

Types of Hotspot:

ü Free Wi-Fi hotspots

Free hotspots operate in two ways:

· Using an open public network is the easiest way to create a free hotSpot. All that is needed is a Wi-Fi router. Private users of wireless routers can turn off their authentication requirements, thus opening their connection, intentionally or not, for sharing by anyone in range.

· Closed public networks use a HotSpot Management System to control the HotSpot. This software runs on the router itself or an external computer. With this software, operators can authorize only specific users to access the Internet, and they often associate the free access to a menu or to a purchase limit. Operators are also now able to limit each user's available bandwidth - each user is therefore restricted to a certain speed to ensure that everyone gets a good quality service. Often this is done through Service Level Agreements.

ü Commercial hotspots

A commercial hotspot may feature:

· A captive portal / Login Screen that users are redirected to

for authentication and payment

· A payment option using credit card, PayPal, iPass, or other

payment service

· A walled garden feature that allows free access to certain

sites

· Service oriented provisioning to allow for improved revenue

ü Software Hotspots:

Many Wi-Fi adapters built into or easily added to consumer computers include the functionality to operate as hotspots.

Manufacturers can enable this functionality through driver-level support. Modern consumer operating systems, including Windows 7 and later and Apple OS X 10.6 and later added features to support this.

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WI-FI Wireless Communication System (2)

alexeng_ismailelsayed@yahoo.com (Anonymous) — Mon, 15 Apr 2013 12:45:00 +0000

WI-FI Wireless Communication System (2)

Wireless Communication

Range:

Wi-Fi networks have limited range. A typical wireless access point using 802.11b or 802.11g with a stock antenna might have a range of 32 m (120 ft) indoors and 95 m (300 ft) outdoors. IEEE 802.11n, however, can more than double the range. Range also varies with frequency band.

Wi-Fi in the 2.4 GHz frequency block has slightly better range than Wi-Fi in the 5 GHz frequency block which is used by 802.11a and optionally by 802.11n.

On wireless routers with detachable antennas, it is possible to improve range by fitting upgraded antennas which have higher gain in particular directions.

Outdoor ranges can be improved to many kilometers through the use of high gain directional antennas at the router and remote device(s).

In general, the maximum amount of power that a Wi-Fi device can transmit is limited by local regulations, such as FCC Part 15 in the US.

Long-range Wi-Fi:

It is used for low-cost, unregulated point-to-point computer network connections, as an alternative to other fixed wireless, cellular networks or satellite Internet access.

Wi-Fi networks have a range that's limited by the transmission power, antenna type, the location they're used in, and the environment. A typical wireless router in an indoor point-to-multipoint arrangement using 802.11b or 802.11g and a stock antenna might have a range of 32 meters (105 ft). Outdoor point-to-point arrangements, through use of directional antennas, can be extended with many kilometers between stations.

Interference:

Wi-Fi connections can be disrupted or the internet speed lowered by having other devices in the same area. Many 2.4 GHz 802.11b and 802.11g access-points default to the same channel on initial startup, contributing to congestion on certain channels. Wi-Fi pollution, or an excessive number of access points in the area, especially on the neighboring channel, can prevent access and interfere with other devices' use of other access points, caused by overlapping channels in the 802.11g/b spectrum, as well as with decreased signal-to-noise ratio (SNR) between access points. This can become a problem in high-density areas, such as large apartment complexes or office buildings with many Wi-Fi access points.

Additionally, other devices use the 2.4 GHz band: microwave ovens, ISM band devices, security cameras, ZigBee devices, Bluetooth devices, video senders, cordless phones, baby monitors, and (in some countries) Amateur radio all of which can cause significant additional interference. It is also an issue when municipalities or other large entities (such as universities) seek to provide large area coverage.

Data security risks:

The most common wireless encryption-standard, Wired Equivalent Privacy (WEP), has been shown to be easily breakable even when correctly configured. Wi-Fi Protected Access (WPA and WPA2) encryption, which became available in devices in 2003, aimed to solve this problem. Wi-Fi access points typically default to an encryption-free (open) mode.

Novice users benefit from a zero-configuration device that works out-of-the-box, but this default does not enable any wireless security, providing open wireless access to a LAN.

To turn security on requires the user to configure the device, usually via a software graphical user interface (GUI). On unencrypted Wi-Fi networks connecting devices can monitor and record data (including personal information). Such networks can only be secured by using other means of protection, such as a VPN or secure Hypertext Transfer Protocol (HTTPS) over Transport Layer Security.

Multiple access points:

Increasing the number of Wi-Fi access points provides network redundancy, support for fast roaming and increased overall network-capacity by using more channels or by defining smaller cells.

Except for the smallest implementations (such as home or small office networks), Wi-Fi implementations have moved toward "thin" access points, with more of the network intelligence housed in a centralized network appliance, relegating individual access points to the role of "dumb" transceivers. Outdoor applications may use mesh topologies.

Embedded systems:

Increasingly in the last few years (particularly as of 2007), embedded Wi-Fi modules have become available that incorporate a real-time operating system and provide a simple means of wirelessly enabling any device which has and communicates via a serial port. This allows the design of simple monitoring devices.

An example is a portable ECG device monitoring a patient at home. This Wi-Fi-enabled device can communicate via the Internet.

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