researches done on E-commerce

Research Design

Pursuit of knowledge requires a scientific inquiry, which intends to find the truth with most accuracy. Researcher’s epistemological positions are affected by their ontological stance. It is believed that methods of research are directly linked with epistemology. So a researcher’s philosophy and her practices of interacting with the world shape up the content of her research.

This section is going to begin with the affect of epistemological practices on this research.

 

Three World Framework

The purpose of this thesis is to add value to an already established body of knowledge. Mouton (1996) writes that there are three models of knowledge; knowledge of lay individuals; knowledge of everyday life; and scientific knowledge. Any scientific research involves all three of these parameters.

In relation to the context of this study, the analogy of e-commerce in a marketplace with the three models of knowledge can be represented as:

• The research questions are related to everyday life
• To answer these questions, an interaction must be developed with knowledge of the ‘Layman’ with respect to e-commerce.
• The third model is used to review the first two and form a conclusion

There are no definite research and theoretical models to give a definite answer as to the balance of opportunities and barriers provided by e-commerce so this research is based on drawing a conclusion from a mixture of business experiences, individual thinking on the matter and opinion of various theorists.

Background and Orientation for the discipline of e-commerce

A fair amount of research has already been done on the subject, generally using a positivistic approach. One of the reason for this that e-commerce integrates business concepts and technical concepts. It is a simple concept but with a wide application. And for this reason the electronic commerce society of theorist have presented various arguments and methods on the barriers and opportunities for this field.

The study of e-commerce is based on a variety of subjects ranging from marketing to IT. The following figure shows the classification of various related fields.;

So it is quite apparent that researchers need to reassess the methods of their research to take advantage of the current methodological thinking.

Nature of this Research

Before analyzing the methods of research, discussing the context of this research would be useful. Idea for e-commerce only came into existence in the past two decades. This is still not as much developed as economics and medicine are. So with respect academic research on e-commerce, this is still considered to be an “a new kid on the block” (Hirschheim & Klein, 2003, 282). E-commerce is still not being taught in colleges and universities as a separate course. So researchers of this field prefer to call this a field of study as opposed to a discipline.

Critical Realism approach for this study

The reasons for choosing critical realism approach for this study are:

• This technique allows a method of bridging the gap between the interpretive and positivistic research philosophies, which is exactly what is needed for this study as both of these method are represented in the topic for discussion. Adoption of a critical realist approach gives flexibility to the research paper.
• There is a need to be thoughtful about the intransitive elements of this thesis, and the informants need to be aware of all the dimensions of this knowledge in order to be unbiased. (Henning, 2004)
• The critical realist makes use of both qualitative and quantitative techniques and could prove to offer in depth explanations of the reasons of the current patterns of e-commerce. (Henning, 2004)

• Critical Realist is persistent with multiple fields of knowledge, which is ideal for a analyzing a subject matter like e-commerce. (Henning, 2004)

• The research requires for its author to act as an observer in a social setting. Using this approach, the intransitive and transitive concept are given preference. (Henning, 2004)

Qualitative Research

The next task is to define a strategy for conduction research. The factors that need to be considered before selecting a research method are the type of research questions and the nature of the problem. In context with the critical realist concepts, a qualitative research technique seemed to be the correct approach. (Henning, 2004)

 

Quantitative Vs Qualitative Approach

According to Snape and Spencer (2003) a qualitative research practice make use of a number of illustrations in the shape of photographs, notes, audio and video recordings and interviews. This is considered to be a naturalistic approach, which means that this method analyzes objects in their natural order and drawing deductions from them through the interpretations of people. (Henning, 2004)

On the other hand, quantitative method focuses on collecting information in a numeric form. A researcher employing quantitative approach believes that quantitative measurements are best way of observing a given problem. Precautions for variables, that might add error in to the findings of the research, should be taken and mentioned.

Justification of choosing Qualitative Approach

In this document qualitative approach have been chosen for the following reasons:

• This method is usually less structured compared to other methods and the researchers tackles the problem with more openness and in the process learn more from it
• Conducting interviews, understanding the social paradigm and reading document forms a good platform for inquiring the problem.
• Qualitative approach can be implemented by using various data collection techniques. And studying the consequences of e-commerce, which itself is a very vast phenomenon, this approach would give more dividends
• Samples are very precise according to the selected criteria.
• Close contact with the participants is maintained for data collection. This leads to an interactive relationship between the researcher and the participant, which helps in the research process
• The analysis is open to new ideas.

Strategies for conducting Qualitative research for this study

The focus should be on being as objective and unbiased as possible. The various methods that can be employed by a qualitative researcher are ethnography, surveys, case studies and this section will explain the reasons for choosing the selected strategies for this study. (Hirschheim & Klein, 2003)

The use of the interview technique and surveys

The selection of the research questions has the greatest input on the choice of design technique. Other factors include the budget available to the researchers, time constraints and the skills of the researchers. For this study, researchers choose to use the survey and interview method due to the nature of the research questions. It required a practical example of individuals and organizations to analyze the problems and opportunities of e-commerce. (Hirschheim & Klein, 2003)

The in-depth survey allowed the researcher to acquire detailed evidence by using a relatively few participants in a series of surveys and interviews.

This method supported the objectives of the qualitative methods and allowed the author to obtain a perspective of the e-commerce marketplace and was able to generate more understanding of the research problem. (Henning, 2004)

The research questions which were posed required a complete literature review and a concluding theory based on logical and practical experiences.

Other choices included utilizing, focus groups, questionnaire, interviews could also have been considered to gain understanding of the topic. But the author decided to utilize a semi-structured interview as the primary technique for evidence compilation. This technique is recommended and accepted in practicing social researches. There were several other doctoral level studies on e-commerce that have used the same technique, e.g. Griffiths (2005), Bannister (2001).

In social research, the required evidence does not just lie around, but in fact they have to be gathered by the interaction of the informants and the researcher. As a researcher states “…stroll through the landscape and enters through discussion with the people come across. The traveler discover with the people's encounters. The traveler see the sights of many spheres of the country, as unidentified terrain or with maps, nomadic around the territory…The interviewer mosey along with the local residents, asks questions that guide the residents to tell their stories of their lived world” (Kvale, 1996: 4)

Electronic commerce and its benifits

Introduction

Electronic commerce a term describes as the trade of commodities and services using electronic mediums like Internet. E-commerce is a concept which was not in practice just a decade ago but now this trend has gained popularity amongst organization to develop internet based operating tools in order to conform to their business strategies (Christopher, 2005).

In recent years, many new e-commerce companies have challenged the traditional retailers and other service providers, acquiring most of the market share. This trend has seen the established players to invest in their own e-commerce websites or a new strategy to gain market share. Now organizations develop online systems to perform their business operations including better customer relationship management, supply chain management and main operations of the business. E-business helps not just organization to grow their business but it benefits the customers to utilize the services and make their purchases from the company from their homes.  E-business is beneficiary for the organization to protract its position in the growing market and stand against any threat of new competitor in the market (Ashrafi and Baghdadi, 2008). As technology is evolving rapidly nowadays, there is no way other than establishing e-business facilities to increase market share and strengthen customer base (Christopher, 2005).

With the massive opportunities there with the e-commerce business model, some challenges are also associated with it. To implement an E-business plan effectively, it is far much important to develop a strategic plan as to how e-business activities would be conducted and how business would work on internet (Ashrafi and Baghdadi, 2008). As sophisticated technology and professional expertise is required for designing an e-business model, it is significant to have a viable framework for the whole procedure which is then put into real practice (Christopher, 2005). Virtual establishment of business enables an organization to optimize demand forecast, process orders properly and manage distribution channels and make decisions accordingly.

This research will focus on three aspects:

·         Opportunities and challenges of setting up a new e-commerce business model

·         Opportunities and challenges for converting an existing retail business model to e-commerce market place

·         Feasibility of keeping both a retail outlet and electronic business

Aims and Objectives

In light of the research questions posed, the objectives and aims of this study are:

·         To review the current developments in the e-commerce marketplace.

·         Investigate the research questions by showing empirical evidence from different organizations

·         Validate the literature by referencing knowledgeable stakeholders for this topic.

Research design and Methodology

The design of this research makes use of qualitative paradigm to aid the  objective of this document and formation of theory. Empirical evidence was acquired from different practitioners of e-commerce to a point where it was enough to develop an analytical review. (Benbasat, Ives & Piccoli)

A total of 20 interviews were conducted from multiple businesses. Furthermore four case studies were prepared using information taken from a business website, business documentation and field notes. The data was collected and analyzed with hermeneutics and reflexive interpretation. This was an iterative process that leads to similar results, creating an informed pattern of results. With the help of these findings the document demonstrates the opportunities and barriers in the e-commerce market place.

Importance of this Document

The document demonstrates with the help of case studies the success stories and the opportunity for growth in the field of e-commerce. So both local and foreign business personnel can benefit from this research and might be inclined to shift their existing businesses by integrating e-commerce or create new e-commerce businesses as most of the countries already have the technological infrastructure and the commercial capability of the Internet is always growing.

The research also highlights the barriers and disadvantages of taking such steps and how these barriers can be overcome. This document gives all the ‘pros and cons’ of setting up a new e-commerce business, converting an existing one to an on line one or keeping both a physical outlet and also taking advantage of an on line methods. (Garrity, Glassberg, Kim, Sanders & Shin, 2005)

The academic community would also find this research very useful. There has been an ongoing debate on success and failures of the e-commerce market place. This research uses a fragmented approach to demonstrate all the key components accounting for the success and failure of e-commerce. And also observes the general trend with respect to geography and economy.

GROWTH OF E-COMMERCE:

E-commerce has altered the scenario of a business is demeanor by the use of computer networks. In recent years, the advancement in computer technologies and the telecommunications sector have made computer networks an important and influential component the economic setup. The trend is to favor more and more transactions over the Internet. The advertising campaigns have been made as such to aim every computer vendor has an Internet connection. Researchers suppose business-to-business transactions generate good revenue in the e-commerce marketplace, but the major share of e-commerce holds specifically in developed countries like England and The US is generated from the transactions between consumers and businesses. The growth of e-commerce is directly attributed to the multiple benefits it provides to the customers in form of giving them a wide range of goods available at lower costs and help save a lot of their time. Now purchasing can be done from the comfort of home or office, and it is this ease and leisure that has enabled the progress of the e-commerce trend. Now hotel room booking, payments of utility bills, ticketing (bus, shows, and airways) and even banks are making use of online transactions. And with the availability of these services, e-commerce has created a new marketplace of its own and the consumers have indeed benefited tremendously. Many theorists are of the judgment that e-commerce will increase exponentially in the future. It is considered that business to business transactions will definitely increase but the field of online vending will also progress. All online businesses like grocery shopping, entertainment, travel services and financial services are all predicted to establish themselves as a need of the people rather than just a facility. (Kraemer, 2006) 

Even in developing countries like the Indian sub-continent or some African countries, e-commerce proffer with enormous opportunities. E-commerce in the under developed countries is in a emerging stage, but researchers, even the most- gloomy ones, foresee a boost in the foreseeable future. There are a lot of factors aiding this theory, such as the continuously lowering costs of laptops and personal computers, more Internet users emerging, competition between the various internets services providers have all accounted for the growth in e-commerce in these countries. Amid all the Asian countries, the e-commerce blooms highest in India between 1999 and 2007. The population of India (second largest in the world) gives a lot of emphasis on the growth of e-commerce market. Theorists have forecasted that India will generate round about 70 million Internet users by 2011 and that the latent Internet market will cross the number of 57 million households by the same year. According to a research report by McKinsey-Nasscom, in the year 2009, e-commerce trade of commodities in just India is predictable to reach around the $100 billion mark. One of the causes for that in India middle class is around the 300 million people, which is larger the U.S. consumer base. This, fact  along with great work done in improving the transactions of the e-commerce market place (in terms of delivery and payment services) makes India and similar developing countries a very eye-catching opportunity for the growth of e-commerce. Even though there has been a clear increase in the number of organizations opting for e-commerce in  recent years, some challenges still need to overcome for the developing countries to replicate the growth made in the e-commerce by the countries in the West. There is diversity in the range of products but challenges such as small credit card population and varying policies adopted by the credit card companies make payment difficult. Even liberation of products to customers by private couriers or postal services has been very erratic especially in small towns and rural areas. However, in recent years it has been seen that more and more Banks in the developing countries have started offering of Internet banking in place for the improvement of e-commerce community. Even the post and courier system has seen a lot of improvement just in the last two years. Protection against fraud and theft has been minimized with the advent of the secured socket layer system over the Internet. And people have become more comfortable sharing their personal information over the Internet than a few years ago. With such patterns business to business relation as well as business to consumer relations has blossomed over the Internet. If the progress of the e-commerce market continues in its present state than researchers have no doubt that soon even the developing counties would hold a major share of the global e-commerce market. (Laudon & Traver, 2004)

While many organizations, societies and companies from all around the world are starting to make use of the impending of e-commerce, some major confronts still require to be conquer for masses becomes a part of this global phenomenon. (Laudon & Traver, 2004)

Radio Signaling Channels

 

Radio Signaling Channels

I am explaining Radio signaling channels how they work
Below are the main types of signaling Channels used for radio communication

• TCHF - Full rate traffic channel.
• TCHH - Half rate traffic channel.

Common Control Channels (CCH)

Used for signaling between the BTS and the MS and to request and grant access to the network.

Broadcast Channels (BCH)

Transmitted by the BTS to the MS. This channel carries system parameters needed to identify the network, synchronize time and frequency with the network, and gain access to the network.

Standalone Dedicated Control Channels (SDCCH)

Used for call setup.

Associated Control Channels (ACCH)

Used for signaling associated with calls and call-setup. An ACCH is always allocated in conjunction with a TCH or a SDCCH.

The above signaling channels can be further divided into the following logical channels:

Broadcast Channels (BCH)
     Broadcast Control Channel (BCCH)
     Frequency Correction Channel (FCCH)
     Synchronization Channel (SCH)
     Cell Broadcast Channel (CBCH)

Common Control Channels (CCCH)
     Paging Channel (PCH)
     Random Access Channel (RACH)
     Access Grant Channel (AGCH)

Standalone Dedicated Control Channel (SDCCH)
     Associated Control Channel (ACCH)
     Fast Associated Control Channel (FACCH)
     Slow Associated Control Channel (SACCH)

Let's discuss each type of logical channel individually.

Broadcast Channels (BCH)

Broadcast Control Channel (BCCH)

BCCH is a downlink channel. This channel contains system parameters needed to identify the network and gain access. These parameters include the Location Area Code (LAC), the Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters.

Frequency Correction Channel (FCCH)

FCCH is a downlink channel.  This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts.

Synchronization Channel (SCH)

 SCH is a downlink channel. This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know what TDMA frame they are on within the hyper frame.

Cell Broadcast Channel (CBCH)

CBCH is a downlink channel. This channel is not truly its own type of logical channel. The CBCH is for point-to-omnipoint messages. It is used to broadcast specific information to network subscribers; such as weather, traffic, sports, stocks, etc. Messages can be of any nature depending on what service is provided. Messages are normally public service type messages or announcements. The CBCH isn’t allocated a slot for itself; it is assigned to an SDCCH. It only occurs on the downlink. The CBCH usually occupies the second sub slot of the SDCCH. The mobile will not acknowledge any of the messages.

Common Control Channels (CCCH)

Paging Channel (PCH)  

PCH is a downlink channel. This channel is used to inform the MS that it has incoming traffic. The traffic could be a voice call, SMS, or some other form of traffic.

Random Access Channel (RACH)

RACH is a Uplink channel. This channel is used by a MS to request an initial dedicated channel from the BTS. This would be the first transmission made by a MS to access the network and request radio resources. The MS sends an Access Burst on this channel in order to request access.

Access Grant Channel (AGCH)

AGCH is a downlink channel. This channel is used by a BTS to notify the MS of the assignment of an initial SDCCH for initial signaling.

Standalone Dedicated Control Channel (SDCCH)  

SDCCH is used as both uplink and downlink. This channel is used for signaling and call setup between the MS and the BTS.

Associated Control Channels (ACCH)

Fast Associated Control Channel (FACCH)

 Used as both UPLINK/DOWNLINK - This channel is used for control requirements such as handoffs. There is no TS and frame allocation dedicated to a FAACH. The FAACH is a burst-stealing channel; it steals a Timeslot from a Traffic Channel (TCH).

Slow Associated Control Channel (SACCH)

Used as both UPLINK/DOWNLINK - This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.

Signaling Channel Mapping

 Normally the first two timeslots are allocated to signaling channels.

Need to remember that Control Channel composed of 51 TDMA frames.

On a time slot within the multi-frame, the 51 TDMA frames are divided up and allocated to the various logical channels.

There are several channel combinations allowed in GSM. Some of the more common ones are:

FCCH + SCH + BCCH + CCCH
BCCH + CCCH
FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)
SDCCH/8(0 .7) + SACCH/C8(0 . 7)

Difference between Modulation(Analog and Digital), multiplexing and multiple access

 The contents of this article are taken from different sources.

Difference between Modulation(Analog and Digital), multiplexing and multiple access

Modulation, keying, Multiplexing and Multiple Access are the basic terms used for any type of network. Usually some of us get confused by these terms. In this post lets discuss about what exactly these terms means and how they are different from each other.

MODULATION:

Usually, the signal that we want to transmit, say a speech signal with 4000 Hz frequency, will require a very big antenna. For any signal the frequency f is related to wavelength L as

c = L * f ………………………… (i)

Where c is velocity of light. And antenna length is generally taken as L/2 which simply means for our case antenna length is 75000 m, obviously this size of antenna is too big to use on day to day basis. That is why we take our speech signal or the desired signal and take another high frequency signal known as carrier (carrier can be any signal but should have high frequency and in practice we use a simple continuous wave signal), now we alter one or more parameters of this career signal in accordance with our desired signal, this parameters can be any one or combination of parameters. The basic parameters are amplitude, frequency, and phase of the signal. The result of this alteration we get is known as modulated signal, the desired signal which we wanted to transmit is known as modulating signal also known as base band signal and modulated signal is also known as band pass signal. The whole process is known as MODULATION.

Two forms of modulation are generally distinguished, although they have many properties in common: If the modulating signal's amplitude varies continuously with time, it is said to be an analog signal and the modulation is referred to as analog. In the case where the modulating signal may vary its amplitude only between a finite number of values and the change may occur only at discrete moments in time, the modulating signal is said to be a digital signal and the modulation is referred to as digital or keying.

In most applications of modulation the carrier signal is a sine wave, which is completely characterized by its amplitude, its frequency, and its phase relative to some point in time. Modulating the carrier then amounts to varying one or more of these parameters in direct proportion to the amplitude of the modulating signal. In analog modulation systems, varying the amplitude, frequency, or phase of the carrier signal results in amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM), respectively. Since the frequency of a sine wave expressed in radians per second equals the derivative of its phase, frequency modulation and phase modulation are sometimes subsumed under the general term “angle modulation” or “exponential modulation.”

If the modulating signal is digital, the modulation is termed amplitude-shift keying (ASK), frequency-shift keying (FSK), or phase-shift keying (PSK), since in this case the discrete amplitudes of the digital signal can be said to shift the parameter of the carrier signal between a finite number of values. For a modulating signal with only two amplitudes, “binary” is sometimes added before these terms.
Digital modulating signals with more than two amplitudes are sometimes encoded into both the amplitude and phase of the carrier signal. For example, if the amplitude of the modulating signal can vary between four different values, each such value can be encoded as a combination of one of two amplitudes and one of two phases of the carrier signal. Quadrature amplitude modulation (QAM) is an example of such a technique.

In certain applications of modulation the carrier signal, rather than being a sine wave, consists of a sequence of electromagnetic pulses of constant amplitude and time duration, which occur at regular points in time. Changing one or the other of these parameters gives rise to three modulation schemes known as pulse-position modulation (PPM), pulse-duration modulation (PDM), and pulse-amplitude modulation (PAM), in which the time of occurrence of a pulse relative to its nominal occurrence, the time duration of a pulse, or its amplitude are determined by the amplitude of the modulating signal

MULTIPLEXING:

Basically there are two types of system, time domain and frequency domain. In time domain we transmit frames, and in frequency domain we transmit in accordance with frequency. Now if there is more than one source of signal and we want to transmit them together then we implement multiplexing. In multiplexing we mix the source signals (off course with some precautions) say if we want to mix them in time domain then our frame will contain some packets form source A and some packets from source B and so on depending upon the constraints of the channel and time frame. The signals that source are generating can either be modulated signals or we can even send our multiplexed signal to the modulator and then modulate the signal. At the receiving end be de-multiplex the signals. In multiplexing we do not provide a dedicated resource to a single source. I.e. we do not dedicate the complete time frame to a single source (in our case it is time frame). Multiplexing is also seen as you are travelling on a four lane road and suddenly it get narrower and turned to single lane, at this point the traffic police will allow one car from each lane to drive through that narrow single lane, this is what we called MULTIPLEXING.

MULTIPLE ACCESS:

As the name suggest, multiple access means multiple users can access the channel or link. Multiple access provides dedicated resources to the user (with a time constraint) in comparison to the multiplexing which does not provide any type of resources. There are many type of Multiple access schemes like FDMA frequency division multiple access, TDMA time division multiple access, CDMA code division multiple access, SDMA space division multiple access etc. take the example of FDMA, the whole frequency band is divided into small frequency bands called channels, now each channel is having certain capacity to take the traffic say a channel can accommodate single user at time, then the whole frequency bandwidth can be access by as many users as there are channels, mathematically if we are having a bandwidth of 200 KHz and channel bandwidth is 50 KHz then it means we can accommodate 4 users at a time by giving 50 KHz channel to each. This is so called multiple access, i.e. multiple users can access the bandwidth simultaneously and we do not require any additional hardware at the receiving end to separate the desired user from the other users as we do in Multiplexing. In reality the concept of Multiple Access is more complicated and In GSM each channel can accommodate 8 users at a time and each channel has 200 KHz bandwidth.

Orthogonal Frequency Division Multiplexing (OFDM)

The contents of this article are taken from different sources.

 OFDM

Introduction

Digital multimedia applications as they are getting common lately create an ever increasing demand for broad band communication systems. Although the technical requirements for related products are very high the solutions must be cheap to implement since we are basically talking about consumer products.

Whereas for the satellite channel and for the cable channel such cost-efficient solutions already exist for the terrestrial link (i.e. classical TV broadcasting) the requirements are so high that the 'standard' solutions are no longer feasible or lead to sub optimal results. Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows to transmit high data rates over extremely hostile channels at a comparable low complexity. OFDM has been chosen as the transmission method for the European radio (DAB) and TV (DVB-T) standard. Due to its numerous advantages it is under Discussion for future broadband application such as wireless ATM as well.

OFDM and the orthogonality principle

The general problem: Data transmission over multipath channels

Differently from satellite communication where we have one single direct path from transmitter to receiver in the classical terrestrial broadcasting scenario we have to deal with a multipath- channel: The transmitted signal arrives at the receiver in various paths (see figure 1) of different length. Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.

  
Figure 1: Multipath transmission in a broadcasting application

The common representation of the multipath channel is the channel impulse response (cir) of the channel which is the signal at the receiver if a single pulse is transmitted (figure 2).

  
Figure 2: Effective length of cir

Let's assume a system transmitting discrete information in time intervals T. The critical measure concerning the multipath-channel is the delay of the longest path with respect to the earliest path. A received symbol can theoretically be influenced by previous symbols. This influence has to be estimated and compensated for in the receiver, a task which may become very challenging.

Single carrier approach

In figure 3 the general structure of a single carrier transmission system is depicted. The transmitted symbols are pulse formed by a transmitter filter. After passing the multipath channel in the receiver a filter matched to the channel is used to maximize signal to noise ratio a device used to extract the data.

  
Figure 3: Basic structure of a single carrier system

The scenario we are dealing with in DVB-T is characterized by the following conditions:

           Transmission Rate:

           Maximum channel delay:

For the single carrier system this results in an ISI of: 

The complexity involved in removing this interference in the receiver is tremendous. In the scenario under consideration here, using such an approach will only lead to sub-optimal results. This is the main reason why the multi carrier approach is used.

Multi carrier approach

Figure 4 shows the general structure of a multicarrier system. 

  
Figure 4: Basic structure of a multicarrier system

The original data stream of rate R is multiplexed into N parallel data streams of rate 

each of the data streams is modulated with a different frequency and the resulting signals are transmitted together in the same band. Correspondingly the receiver consists of N parallel receiver paths. Due to the prolonged distance in between transmitted symbols the ISI for each sub system reduces to 

In the case of DVB-T we have N=8192 leading to an ISI of 

Such little ISI can often be tolerated and no extra counter measure such as an equalizer is needed. Alas as far as the complexity of a receiver is concerned a system with 8192 parallel paths still isn't feasible. This asks for a slight modification of the approach which leads us to the concept of OFDM.

Orthogonal Frequency Division Multiplexing

In OFDM the subcarrier pulse used for transmission is chosen to be rectangular. This has the advantage that the task of pulse forming and modulation can be performed by a simple Inverse Discrete Fourier Transform (IDFT) which can be implemented very efficiently as a I Fast Fourier Transform (IFFT). Accordingly in the receiver we only need a FFT to reverse this operation. According to the theorems of the Fourier Transform the rectangular pulse shape will lead to a sin(x)/x type of spectrum of the subcarriers (see figure 5).

  
Figure 5: OFDM and the orthogonality principle

Obviously the spectrums of the subcarriers are not separated but overlap. The reason why the information transmitted over the carriers can still be separated is the so called orthogonality relation giving the method its name. By using an IFFT for modulation we implicitly chose the spacing of the subcarriers in such a way that at the frequency where we evaluate the received signal (indicated as arrows) all other signals are zero. In order for this orthogonality to be preserved the following must be true:

1. The receiver and the transmitter must be perfectly synchronized. This means they both must assume exactly the same modulation frequency and the same time-scale for transmission (which usually is not the case).
2. The analog components, part of transmitter and receiver, must be of very high quality.
3. There should be no multipath channel.

In particular the last point is quite a pity, since we have chosen this approach to combat the multipath channel. Fortunately there's an easy solution for this problem: The OFDM symbols are artificially prolonged by periodically repeating the 'tail' of the symbol and precede the symbol with it (see figure 5). At the receiver this so called guard interval is removed again. As long as the length of this interval is longer than the maximum channel delay all reflections of previous symbols are removed and the orthogonality is preserved. Of course this is not for free, since by preceding the useful part of length by the guard interval we lose some parts of the signal that cannot be used for transmitting information. Taking all this into account the signal model for the OFDM transmission over a multipath channel becomes very simple: The transmitted symbols at time-slot l and subcarrier k are only disturbed by a factor which is the channel transfer function (the fourier transform of the cir) at the subcarrier frequency, an by additional white Gaussian noise n

 

 

The influence of the channel can easily be removed dividing by.

As far as the analog components are concerned experience has shown that in the broadcasting applications under consideration here, they are not so critical. What remains is to establish 'perfect' synchronization. This requires a very sophisticated receiver. The general structure and the receiver of such a receiver which we have developed for the DVB-T application.

An OFDM receiver for DVB-T

Tasks of the inner receiver and receiver structure

As mentioned before in order for a digital transmission system to work, receiver and transmitter have to be synchronized. This involves the following tasks:

• Timing synchronization: Since it is unknown to the receiver, to which exact (absolute) time instant the symbol has been transmitted and how long the dispersion of the channel is, one essential task is to find the 'beginning' of a received OFDM symbol. Thus the time scales of transmitter and receiver are synchronized and the removal of the guard interval can be done with the required accuracy.
• Frequency synchronization: The signal is usually not transmitted in baseband but modulated with a radio carrier at a frequency assigned by the standard. Though this frequency is known to the receiver the tolerance of the RF components usually applied is so large that there will be a frequency-deviation. In many cases this deviation will be too large for a reliable data transmission. It therefore must be estimated and compensated at the receiver.
• Sampling-clock synchronization: The signal produced by the FFT will be converted into an analog signal assuming a certain span of time between two values. At the receiver the down converted RF signal is sampled in order to obtain a discrete time signal for further (digital) processing. The sampling times assumed in the receiver must match very accurately in order to avoid a degradation of the performance. A possible deviation between transmitter and receiver must again be estimated and compensated.
• Channel estimation: If a coherent modulation scheme is used (which must not be necessarily the case) according to equation (0.4) the channel transfer function must be estimated and compensated.

A receiver structure that allows to estimate and compensate all parameters required is depicted in figure 6. 

  
Figure 6: Receiver structure for a DVB-T receiver

In addition to the elementary tasks found in single carrier receivers too for the receiver under consideration here two further tasks can be identified:

1. TPS detection: So called TPS (transmission parameter signaling) data is provided in DVB-T to inform the receiver about the modulation and coding scheme used. This information is provided via selected subcarriers that are modulated in a robust differential BPSK.
2. CPE detection (and correction): The common phase error (CPE) is a phenomena that results from imperfections of the oscillators used for modulation and demodulation. Instead of providing a stable frequency real oscillators tend to provide a frequency that is slowly changing in time. This change in time leads to an additional modulation of the OFDM signal which in some cases must be estimated and compensated. For the constellations used in DVB-T it can be shown that due to other reasons the quality of the oscillators must be so high that this effect can be neglected.

We will not go into detail as far as the implementation of the single components. What proves to be the most critical component of the receiver is the channel estimation unit. We will therefore go a little more into detail.

Channel estimation for OFDM

The method of channel estimation implied by the frame structure of DVB-T is channel estimation via interpolation. The basic principle is depicted in figure 7. 

  
Figure 7: Principle of channel estimation via interpolation

Embedded into the OFDM data stream are training symbols (depicted as arrows) that can be used to obtain samples of the channel transfer function.

 

The values of the channel in between the samples can then be obtained via a interpolation procedure. Generally we have a two dimensional interpolation problem. Fortunately the problem can be separated into a interpolation in time and in frequency. The most critical task is the design of the interpolation filters used. Both interpolations must agree with the sampling theorem:

• The interpolation in time is bandlimited by the time-variant behavior of the channel. This is cause by a movement of the receiver and by uncompensated synchronization errors. The maximum allowable bandwidth of these disturbances is determined by the number of training symbols in one subcarrier.
• Due to the duality of time and frequency the interpolation in frequency is bandlimited by the length of the cir. The maximum allowable cir-length thus is not only determined by the length of the guard interval but also by the number of training symbols in one OFDM symbol. If we use fixed filters for implementation where the maximum dispersion to be assumed is given by the length of the guard interval this implies that for short guard intervals the channel can be estimated with a higher accuracy than with a larger guard interval.

For interpolation in frequency a interpolation filter optimized according to the Wiener filter theory is used. For interpolation in time a linear interpolation is sufficient.

 Performance of the complete receiver

Figure 8 shows the results achievable with the channel-estimator described in the previous section. The application is a DVB-T receiver according to the European standard operating in 8k mode.

  
figure 8: Achievable performance for different channel estimators

As we can see the achievable system performance very much depends on the achievable quality of the channel estimator. Since it is higher for small cirs thus the performance of the receiver will be better. The loss with respect to the performance with ideal channel estimation ranges from about 0.5 dB for the smallest guard interval up to 1.6 dB for the largest guard interval. Also included are the results for a dynamic channel. Using linear interpolation in time will not further degrade the system. Alas if we try do without any interpolation in time the additional loss in performance is significant.

Key Terms and Details for Refference

DAB

Digital Audio Broadcasting (DAB) is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format.

The DAB standard was initiated as a European research project in the 1980s, and the BBC launched the first DAB digital radio in 1995. DAB receivers have been available in many countries since the end of the nineties. DAB may offer more radio programmes over a specific spectrum than analogue FM radio. DAB is more robust with regard to noise and multipath fading for mobile listening, since DAB reception quality first degrades rapidly when the signal strength falls below a critical threshold, whereas FM reception quality degrades slowly with the decreasing signal.

An "informal listening test" by Professor Sverre Holm has shown that for stationary listening the audio quality on DAB is lower than FM stereo, due to most stations using a bit rate of 128 kbit/s or less, with the MP2 audio codec, which requires 160 kbit/s to achieve perceived FM quality. 128 kbit/s gives better dynamic range or signal-to-noise ratio than FM radio, but a more smeared stereo image, and an upper cutoff frequency of 14 kHz, corresponding to 15 kHz of FM radio. However, "CD sound quality" with MP2 is possible "with 256..192 kbps".

DAB+

An upgraded version of the system was released in February 2007, which is called DAB+. DAB is not forward compatible with DAB+, which means that DAB-only receivers will not be able to receive DAB+ broadcasts. DAB+ is approximately twice as efficient as DAB due to the adoption of the AAC+ audio codec, and DAB+ can provide high quality audio with as low as 64kbit/s. Reception quality will also be more robust on DAB+ than on DAB due to the addition of Reed-Solomon error correction coding.

More than 20 countries provide DAB transmissions, and several countries, such as Australia, Italy, Malta and Switzerland, have started transmitting DAB+ stations. See Countries using DAB/DMB. However, DAB radio has still not replaced the old FM system in popularity.

DVB-T

DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first publicated in 1997 and first broadcast in the UK in 1998. This system transmits compressed digital audio, video and other data in an MPEG transport stream, using coded orthogonal frequency-division multiplexing (COFDM or OFDM) modulation.

FFT

A fast Fourier transform (FFT) is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse. There are many distinct FFT algorithms involving a wide range of mathematics, from simple complex-number arithmetic to group theory and number theory; this article gives an overview of the available techniques and some of their general properties, while the specific algorithms are described in subsidiary articles linked below.

A DFT decomposes a sequence of values into components of different frequencies. This operation is useful in many fields (see discrete Fourier transform for properties and applications of the transform) but computing it directly from the definition is often too slow to be practical. An FFT is a way to compute the same result more quickly: computing a DFT of N points in the naive way, using the definition, takes O(N2) arithmetical operations, while an FFT can compute the same result in only O(N log N) operations. The difference in speed can be substantial, especially for long data sets where N may be in the thousands or millions—in practice, the computation time can be reduced by several orders of magnitude in such cases, and the improvement is roughly proportional to N / log(N). This huge improvement made many DFT-based algorithms practical; FFTs are of great importance to a wide variety of applications, from digital signal processing and solving partial differential equations to algorithms for quick multiplication of large integers.

The most well known FFT algorithms depend upon the factorization of N, but (contrary to popular misconception) there are FFTs with O(N log N) complexity for all N, even for prime N. Many FFT algorithms only depend on the fact that is an Nth primitive root of unity, and thus can be applied to analogous transforms over any finite field, such as number-theoretic transforms.

Since the inverse DFT is the same as the DFT, but with the opposite sign in the exponent and a 1/N factor, any FFT algorithm can easily be adapted for it.

DFT

The discrete Fourier transform (DFT) is a specific kind of Fourier transform, used in Fourier analysis. It transforms one function into another, which is called the frequency domain representation, or simply the DFT, of the original function (which is often a function in the time domain). But the DFT requires an input function that is discrete and whose non-zero values have a limited (finite) duration. Such inputs are often created by sampling a continuous function, like a person's voice. Unlike the discrete-time Fourier transform (DTFT), it only evaluates enough frequency components to reconstruct the finite segment that was analyzed. Using the DFT implies that the finite segment that is analyzed is one period of an infinitely extended periodic signal; if this is not actually true, a window function has to be used to reduce the artifacts in the spectrum. For the same reason, the inverse DFT cannot reproduce the entire time domain, unless the input happens to be periodic (forever). Therefore it is often said that the DFT is a transform for Fourier analysis of finite-domain discrete-time functions. The sinusoidal basis functions of the decomposition have the same properties.

The input to the DFT is a finite sequence of real or complex numbers (with more abstract generalizations discussed below), making the DFT ideal for processing information stored in computers. In particular, the DFT is widely employed in signal processing and related fields to analyze the frequencies contained in a sampled signal, to solve partial differential equations, and to perform other operations such as convolutions or multiplying large integers. A key enabling factor for these applications is the fact that the DFT can be computed efficiently in practice using a fast Fourier transform (FFT) algorithm.

FFT algorithms are so commonly employed to compute DFTs that the term "FFT" is often used to mean "DFT" in colloquial settings. Formally, there is a clear distinction: "DFT" refers to a mathematical transformation or function, regardless of how it is computed, whereas "FFT" refers to a specific family of algorithms for computing DFTs. The terminology is further blurred by the (now rare) synonym finite Fourier transform for the DFT, which apparently predates the term "fast Fourier transform" (Cooley et al., 1969) but has the same initialism.

ISI (intersymbol interference)

intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable. ISI is usually caused by multipath propagation or the inherent non-linear frequency response of a channel causing successive symbols to "blur" together. The presence of ISI in the system introduces errors in the decision device at the receiver output. Therefore, in the design of the transmitting and receiving filters, the objective is to minimize the effects of ISI, and thereby deliver the digital data to its destination with the smallest error rate possible. Ways to fight intersymbol interference include adaptive equalization and error correcting codes.