Amplitude Shift Keying (ASK) Modulation

Amplitude-shift keying (ASK) is a kind of digital modulation that represents digital data as variations in the amplitude of a carrier wave.The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal) where  frequency and phase are keeping constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given. 

Here is a diagram showing the ideal model for a transmission system using an ASK modulation:

It can be divided into three blocks. The first one represents the transmitter, the second one is a linear model of the effects of the channel, the third one shows the structure of the receiver.

What is Digital Modulation ?

In digital modulation, an analog carrier signal is modulated by a digital bit stream. The aim of digital modulation is to transfer a digital bit stream over an analog passband channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.

Phase Modulation

In the case of PM(phase modulation) the information or AF(Audio Frequency) changes the phase of the carrier wave without changing its frequency or amplitude. Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Main advantages of PM are improved signal to noise ratio and less radiated power.

The carrier signal is represented by

Frequency Modulation

In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency.In a frequency modulated system,the amplitude of the modulated carriers kept constant,while its frequency is varied in accordance with the variations of the modulating signal. . Main advantages of PM are improved signal to noise ratio and less radiated power.

Mathematically,the process can be expressed as

Amplitude modulation

Amplitude modulation(AM) is the simplest form of modulation .It is an analog-to-analog conversion method. Here the information or AF(Audio Frequency) signal changes the amplitude of the carrier wave without changing its frequency or phase. Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent.

Mathematically,the process can be expressed as

What is Analog Modulation ??

In analog modulation, the modulation is applied continuously in response to the analog information signal.The representation of analog information by an analog signal.There are two principle reasons for analog modulation of analog signals:

• ·         A higher frequency may be needed for effective transmission.

• ·         Modulation permits frequency division multiplexing.

The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal, over an analog passband channel, for example a limited radio frequency band or a cable TV network channel.

            Here the main analog modulation are

• ·         Amplitude Modulation

• ·         Frequency Modulation

• ·         Phase Modulation

Modulation technique in wireless communication

 Modulation has been defined as the process of combining an input signal and a carrier signal with a specified frequency,to produce a modulated signal whose bandwidth is centered on carrier signal frequency.Modulation is the process of transforming a message signal to make it easier to work with. Modulation is the process of changing one or more properties of high frequency signal or carrier with respect to modulating signal. It usually involves varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. In radio communications for instance, electrical signals are best received when the transmitter and receiver are tuned to resonance. Therefore keeping the frequency content of the message signal as close as possible to the resonant frequency of the two is ideal. Often a high-frequency sinusoid waveform is used as carrier signal to convey a lower frequency signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.

Cellular telephone system

Cellular telephone system is one of the most important developments in the area of voice networks. Cellular technology supports geographically mobile users, which is not possible in hard-wired network. Cellular Systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum. In cellular networks, the distance between a transmitter and a receiver is unpredictable because it depends on the location of the users. However, cellular telephone systems provide high quality service that is often comparable to that of the land-line telephone systems. High capacity is achieved  by limiting the coverage of each base station transmitter to a small geographic area called a cell, so that the same radio channels may be reused by another base station located some distance away. A sophisticated  switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.

 The figure below shows a basic cellular system which consists of mobile stations, base stations and a switching center. The mobile switching center sometime called MTSO (mobile telephone switching office), Since it is responsible for connecting all mobiles to the PSTN in a cellular system. Communication between the base station and the mobile stations is defined by a standard  common air interface (CAI) that specifics four different channel.

 

 The channels used for voice transmission from the base station to mobiles are called forward voice channels (FVC), and the channels used for voice transmission from the mobiles to the base stations are called revers voice channels (RVC). The two channels responsible for initiating mobile calls are the forward control channel (FCC) and revers control channel (RCC). Control channels often called setup channels because they are only involving in setting up a call and moving it to an unused voice channel.

Third Generation (3G) wireless networks

The third-generation(3G) concept started in 1992.The main goal of third-generation cellular telephony is to provide universal personal communication.The third-generation of cellular telephony is the combination of technologies that provide variety of services. In this generation, it provides both digital data and voice communication. In 1992 when ITU issues a blueprint called Internet Mobile Telephone Communication for year 2000 (IMT-2000). The blueprint define some criteria for 3G technology as outline below:

1. High voice quality comparable to the existing public telephone network or PSTN system.
2. Data rate of 144 kbps for access in a moving vehicle, 384 kbps for access as the user walks and 2 Mbps for the stationary users.
3. It supports for packet-switched and circuit-switched service.
4. Bandwidths of 2 MHz.
5. A band of 2GHz.

A person can download and watch move, can download and listen to music, can play interactive games, can have video conference and can do much more with third-generation(3G) network.

One of the interesting characteristics of 3G system is that the portable device is always connected, we do not need to dial a number to connect to the internet.

What is WiMAX ?

In the world of wireless networks WiMAX is a dominant technology. WiMAX (Worldwide Interoperability for Microwave Access) technology is a worldwide wireless networking standard that addresses interoperability across IEEE 802.16* standard-based product. WiMAX is a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL(Digital Subscriber Line).Mainly it is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access.

WiMAX in Bangladesh

Perspective of Bangladesh WiMAX technology is very effective, because Bangladesh is a small but largely populated country and rivers are situated here like a net. Buildup a cable network all over the country is very difficult because of those rivers. Though WiMAX technology is not cheap, but after installation its maintenance cost is low. So if we install it one time then cost becomes less. So country like Bangladesh can buildup network with WiMAX technology and can provide better service in telecommunication and high speed Internet.

Applications of WiMAX

The main application of WiMAX technology is transmition of data at high rate over a long distance. It is used for high speed broad band internet service. WiMAX is a possible replacement candidate for cellular phone technologies such as GSM and CDMA, or can be used as an overlay to increase capacity. It has also been considered as a wireless backhaul technology for 2G, 3G, and 4G networks in both developed and poor nations. Multi-player Interactive Gaming, VOIP and Video Conference are also some application of WiMAX technology

How WiMAX works ?

In practical terms, WiMAX would operate similar to WiFi but at higher speeds, over greater distances and for a greater number of users. WiMAX could potentially erase the suburban and rural blackout areas that currently have no broadband Internet accessbecause phone and cable companies have not yet run the necessary wires to those remote locations.WiMAX is designed to support high speed broadband services. It has two parts:
  WiMAX base station:      WiMAX base station is similar to a cellular network base station which consists of a  WiMAX tower and indoor electronics. Base station performs the MAC and PHY  features. It also handles the signaling and user scheduling. It is also responsible for uplink and downlink bandwidth management on a real time basis and frequency reuse.  
  WiMAX receiver: WiMAX receiver could be a WiMAX enabled computer, PCMCIA card, WiMAX modem, mobile internet devices or a standalone box. It works like a Wi-Fi network but in a broader coverage area.
            A WiMAX tower station can connect directly to the Internet using a high-bandwidth wired connection. It can also connect to another WiMAX tower using a line-of-sight, microwave link. This connection to a backhaul, along with the ability of a single tower to cover up to 3,000 square miles, is what allows WiMAX to provide coverage to remote rural areas. WiMAX actually can provide two forms of wireless service:There is the non-line-of-sight, where a small antenna on your computer connects to the tower. In this mode, WiMAX uses a lower frequency range 2 GHz to 11 GHz.Lower-wavelength transmissions are not as easily disrupted by physical obstructions,they are better able to diffract, or bend, around obstacles.
There is line-of-sight service, where a fixed dish antenna points straight at the WiMAX tower from a rooftop or pole. The line-of-sight connection is stronger and more stable, so it's able to send a lot of data with fewer errors. Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz. At higher frequencies, there is less interference and lots more bandwidth. WiFi-style access will be limited to a4-to-6 mile radius. Through the stronger line-of- sight antennas, the WiMAX transmitting station would send data to WiMAX enabled computers or routers set up within the transmitter's 30-mile radius. The fastest WiMAX handles up to 70 megabits per second, which, according to WiMAX proponents, is enough bandwidth to simultaneously support more than 60 businesses with T1-type connectivity and well over a thousand homes at 1Mbit/s DSL-level connectivity

Some drawbacks and Suggested Solutions of WiMax

There are some drawbacks of this Technology; we must consider those when going to the phase of implementation of this technology. Some important objections to WiMAX are:

Interference: The interference from other broadcasters may degrade the quality of the WiMAX service.

1. Quality of Service (QoS): Wireless is inherently unstable so this can affect voice and video services.

2. Security: As it is believed that Wireless medium is less secure so Security of WiMax can be big question.

3. Reliability: Also reliability of any wireless technology isn’t that much as compare to cable network, so reliability of Wimax can be a question too.

The solutions to these drawbacks are best understood through the Physical (PHY) and Medium Access Control (MAC) Layers. The WiMax Working Group no doubt were aware of these objections based on experiences with earlier wireless technologies (Wi-Fi, LMDS, MMDS,CDMA, GSM) and they made Wimax to fix failures of past wireless technologies.

Spectrum Allocation Problem

A major problem with WiMax is Spectrum Allocation; there are two main types of spectrum Allocation. Licensed and unlicensed, the unlicensed bands such as 2.4 GHz and 5.8 GHz, which are used for some applications, may reduce the performance and QoS of WiMax, which may affect the commercial viability of the service. And for the licensed bands 2.5 GHz and 3.5 GHz  bands, they are currently occupied by some other services such as fixed-satellite, broadcasting satellite, etc. in some countries. The scarcity of suitable spectrum for WiMax may not be a problem which can be satisfactorily resolved in the short term.

Peak to average power Ratio

One problem with OFDM systems is the high peak-to-average power Ratio (PAPR), which significantly reduces the efficiency of the transmitter high power amplifier (HPA). Many solutions have been suggested to reduce the peak power of OFDM signals. Using typical HPA models and spectral masks, these PAPR reduction methods are evaluated in terms of the total system degradation.

Limitation or disadvantages of WiMAX

One of the major problem with WiMax is Spectrum Allocation; there are two main types of spectrum Allocation. Licensed and unlicensed, the unlicensed bands such as 2.4 GHz and 5.8 GHz, which are used for some applications, may reduce the performance and QoS of WiMax, which may affect the commercial viability of the service. And for the licensed bands 2.5 GHz and 3.5 GHz bands, they are currently occupied by some other services such as fixed-satellite, broadcasting satellite, etc. in some countries. The scarcity of suitable spectrum for WiMax may not be a problem which can be satisfactorily resolved in the short term. 

 One problem with OFDM systems is the high peak-to-average power Ratio (PAPR), which significantly reduces the efficiency of the transmitter high power amplifier (HPA). Many solutions have been suggested to reduce the peak power of OFDM signals. Using typical HPA models and spectral masks, these PAPR reduction methods are evaluated in terms of the total system degradation.
 

Advantages of WiMAX

WiMAX technology can reach theoretically 30-mile coverage radius and achieve data rates up to 70 Mbps. This technology is very efficient, because it provide non-line-of-sight communication. As a result line-of-sight not needed between base station and user. WiMAX technology supports both of licensed and unlicensed frequency bands. There are some limitations of this technology that must be considered before implementation. WiMAX suffer from interference from other broadcasters of wireless devices. Wireless is inherently unstable so this can affect voice and video services. Also reliability of any wireless technology isn’t that much as compare to cable network, so reliability of WiMAX can be a question. As it is believed that Wireless medium is less secure so Security of WiMAX can be big question too. A commonly-held misconception is that WiMAX will deliver 70 Mbit/s over 50 kilometers (~31 miles). In reality, WiMAX can either operate at higher bitrates or over longer distances but not both: operating at the maximum range of 50 km increases bit error rate and thus results in a much lower bitrate. Conversely, reducing the range (to <1 km) allows a device to operate at higher bitrates.