SIX SIGMA CONTROL IN TOTAL QUALITY MANAGEMENT

Abstract

six sigma programs are raging through corporations worldwide, with some corporations citing savings in the $US billions resulting from six sigma implementation. Six sigma has both proponents and detractors with some arguing that nothing new is involved and others identifying it as revolutionary. The view espoused herein argues for six sigma as a methodology within the larger framework of total quality management - a blend of old and new in the sense that the tools of six sigma are often familiar ones, but are applied with an eye that is more strategically focused than historic use of those tools ordinarily indicates. 

TOTAL QUALITY MANAGEMENT:- (CONCEPT)

             TQM is a management strategy with a customer focus, deploying suitable technique to eliminate waste. In all activities of an organization and seeking continuous improvement. It is management technique. It is a leadership philosophy with a customer focus. It is a way of doing business & it is not just a programme. It is a management strategy. Any strategy has a policy detailed by objectives, a method to meet the objectives and where the method deploys different techniques and the techniques supported by goals. 

*Continuous improvement

*Continuous wide programme

*Management as leader

                                                          

OBJECTIVES OF TOTAL QUALITY MANAGEMENT:-

       1. Meeting the customer requirement.

       2. Continuous improvement of quality at the every level at every state and at every place.

       3. Participative problems solving process.

       4. Focused and continuous cost reduction.

       5. Interlink and integrate various subsystem of the organization.

                         

TQM IMPLIMENTATION / PDCA:-

One of the guru of the quality ‘WE Deming’ explained PDCA cycle (plan-Do-Check and ACT) for implementation of TQM in any of the organization. 

Plan:                      

(1) Lay down and plan policies and objectives of TQM.

(2)Plan method to achieve the objectives of TQM. 

DO   

(3) Provide education and training to workers and managers to Achieve Objectives.

(4)Implement TQM by introduction never things.

 CHECK:       

(5) Check the result by observing them and find cause of the nonConformance

(6) Analyze the results.

ACT:     

(7) try to act for the preventing undesired effects. (8) Measure the improvement and design for future.

                                                                                                                                                                                                                                                                                                                                                                    

     

BLOCK DIAGRAM NO:- 1 (PDCA CIRCLE)

INTRODUCTION TO SIX SIGMA:-

                      Six Sigma was pioneered by Bill Smith at Motorola in the 1986 and popularized by General Electric (GE) in the 1990’s. Bill Smith did not really "invent" Six Sigma in the 1980s; rather, he applied methodologies that had been available since the 1920s developed by luminaries like Shewhart, Deming, Juran, Ishikawa, Ohno, Shingo, Taguchi and Shainin.Organizations including Honeywell, Citigroup, Motorola, Starwood Hotels, DuPont, Dow Chemical, American Standard, Kodak, Sony, IBM, Ford have implemented Six Sigma programs across diverse business operations ranging from highly industrial or high-tech manufacturing to service and financial operations . Although not yet widespread in Vietnam, several foreign invested manufacturing companies in Vietnam such as American Standard, Ford, LG and Samsung in Vietnam have introduced Six Sigma programs. Jack Welch, the energetic chairman of GE, has been Six Sigma's most influential advocate. Other companies, notably Motorola and Allied Signal, have been incubators and proponents of the movement. Mikel Harry is its most colorful champion. In 1891, British physicist Lord Kelvin wrote, “When you can measure what you are speaking about, and express it in numbers, you know something about it.” Mikel Harry, a noted Six Sigma authority, extends the thought as, “we don't know what we don’t know; we can’t act on what we don’t know; we won’t know until we search; we won’t search for what we don’t question; we don’t question what we don’t measure.” Both imply that if you failed to quantify the results of what you were doing, in a way, it means that you might not understand what you were really doing.

What is Six Sigma: -

                  First, what it is not. It is not a secret society, a slogan or a cliche. Six Sigma is a highly disciplined process that helps us focus on developing and delivering near-perfect products and services. Six Sigma is a statistically-based process improvement methodology that aims to reduce defects to a rate of 3.4 defects per million defect opportunities by identifying and eliminating causes of variation in business processes. Six Sigma focuses on developing a very clear understanding of customer requirements and is therefore very customer focused.

Why the name Six Sigma: -

              Why "Sigma"? The word is a statistical term that measures how far a given process deviates from perfection. The term "six sigma process" comes from the notion that if one has six standard deviations between the mean of a process and the nearest specification limit, he will make practically no items that exceed the specifications. The central idea behind Six Sigma is that if you can measure how many "defects" you have in a process, you can systematically figure out how to eliminate them and get as close to "zero defects" as possible. To achieve Six Sigma Quality, a process must produce no more than 3.4 defects per million opportunities. The "3.4 Defects Per Million Opportunities (DPMO)" is a gross confusion of the following situation, for example: A population of 1,100,000 units is manufactured. A perfect inspection/test process removes the 100,000 defective units. The remaining 1,000,000 units are thus the intended population and contain no defective units. Approximately three to four of these measure at Short-term Mean ±4.5 Sigma or more extreme. The "3.4 per million" is thus a characteristic of the Normal Distribution that is true of the intended population, not the defects or defective units.

Key themes in Six Sigma: -

      Some of the key themes of Six Sigma can be summarized as follows:

·         Continuous focus on the customer’s requirements

·         Using measurements and statistics to identify and measure variation in the production process and

                other business processes

·         Identifying the root causes of problems

·         Emphasis on process improvement to remove variation from the production process or other

                Business processes and therefore lowers defects and improves customer satisfaction

·         Pro-active management focusing on problem prevention, continuous improvement and constant

                Striving for perfection

·         Cross-functional collaboration within the organization; and

·         Setting very high targets.

Methodologies of Six Sigma: -

   Six Sigma uses two methodologies named ‘DMAIC’ (Define, Measure, Analyze, Improve, Control) and ‘DFSS’                               (Design For Six Sigma).

    1.  DMAIC: - The Six Sigma DMAIC process is an improvement system for existing processes falling below specification and    looking for incremental improvement.                          

   2.   DFSS: - The Six Sigma DFSS methodology has two variations named DMADV (Define, Measure, Analyze,   Design, Verify)       and DMADOV process (Define, Measure, Analyze, Design, Optimize, Verify). DFSS is used to develop new processes or products at Six Sigma quality levels. It can also be employed if a current process requires more than just incremental improvement. Both Six Sigma processes are executed by Six Sigma Green Belts and Six Sigma Black Belts and are overseen by Six Sigma Master Black Belts.

Besides this the two key methodologies commonly used are: -

1.DMADV: -      

               

Basic Methodology  consists of following five steps: -

·         Define the goals of the design activity of the consistent with customer demands and enterprise strategy.

·         Measure and identify CTQs (critical to qualities), product capabilities, production process capability, and risk assessments.

·         Analyze to develop and design alternatives, create high-level design and evaluate design capability to select the best design.

·         Design details, optimize the design, and plan for design verification. This phase may require simulations.

·         Verify the design, set up pilot runs, implement production process and handover to process owners.

2. DMAIC: -

               Basic methodologies consists of following five steps: -

• Define the process improvement goals that are consistent with customer demands and enterprise strategy.
• Measure the current process and collect relevant data for future comparison.
• Analyze to verify relationship and causality of factors. Determine what the relationship is, and attempt to ensure that all factors have been considered.
• Improve or optimize the process based upon the analysis using techniques like Design of Experiments.
• Control to ensure that any variances are corrected before they result in defects. Set up pilot runs to establish process capability, transition to production and thereafter continuously measure the process and institute control mechanisms.

                                                                         

Which quality management systems process improvement tools have yielded the greatest results?

Six Sigma                                      - 53.6 %                           Problem Solving                         - 23.2 %     

Process mapping                           - 35.3 %                            ISO 9001                                    - 21.0 %    

Root cause analysis                       - 33.5 %                            Process Capability                     - 20.1 %

Cause-and-effect analysis             - 31.3 %                             Statistical  Process Control       - 20.1 %

Lean thinking/manufacturing        - 26.3 %                            Performance Metrics                 - 19.2 %

Benchmarking                               - 25.0 %                            Control Charts                           - 19.2 %

Costs of Six Sigma Projects: -

                                                Although Six Sigma projects can have many benefits and help the company to save money over the long run, there are also costs associated with Six Sigma projects. They typically include the following:

·         Direct Payroll - Payroll expenses for individuals dedicated to the Six Sigma project on a full time basis.

·         Indirect Payroll – The cost of time devoted by senior executives, team members, process owners and others in the implementation of the Six Sigma project.

·         Training and Consulting – The cost of teaching people Six Sigma skills

·         Improvement Implementation Costs – The costs of improving the production process to eliminate the sources of variation identified in the Six Sigma project. This might involve new equipment, new software, additional personnel costs for newly formed positions, etc.

·         Software – Some software such as Minitab Inc.’s Minitab statistical software or Microsoft’s Visio, for generating flow-charts, may also be required. More advanced software tools sometimes include Popkin’s System Architect, Proforma’s Provision or Corel’s iGrafx Process 2006 for Six Sigma.

 Six Sigma Roadmap: -
                       The six sigma methodology utilizes the Define-Measure-Analyze-Improve-Control (DMAIC) cycle to achieve process excellence.

                                                                                                             

BLOCK DIAGRAM NO :- 2

Six Sigma origins

The history of Six Sigma is a well-documented one and hence we note only briefly here that its origin as a quality improvement approach in the 1980s can be traced to the American electronic giant, Motorola where a goal of improving all products - goods as well as services - by an order of magnitude (e.g. a factor of ten) within five years was established. This provided an important focus on the improvement rate and, in particular, that simply “better” may not be sufficient, but that the critical consideration is that of becoming sufficiently better expeditiously. Six Sigma clearly focused resources at Motorola, including human effort, on reducing variation in all processes, that is to say manufacturing processes, administrative processes and all other processes. To set a clear measure on the improvement work, the program called Six Sigma was launched in 1987

defects at a rate of 3.4 defects per million opportunities (DPMO) for defects to arise. Note that this almost certainly implies more than 3.4 defective units per one million units, since typically any given unit is sufficiently complex so as to allow multiple

.It is generally possible to calibrate the “cost of quality” or - more accurately - the “cost of poor quality” (CPQ) with the sigma level at which processes perform. Six Sigma performance levels are generally considered to be world class with the CPQ being less than 1 per cent of sales. By contrast sigma levels of three, four, and five produce DPMO rates of 66,807, 6,210, and 233, and corresponding CPQ ranges of 25-40 percent, 15-25 per cent, and 5-15 percent. These numbers substantiate the importance of reducing process variation across all key primary and support processes in an organization as well as variation of that obtained from suppliers. Without significant divergence from our discussion this clearly illustrates sound reasoning behind reduction in the number of suppliers used by an organization that extends beyond negotiation/relationship issues into statistical ones. A straightforward example of process sigma level estimation is provided in the first chapter of Harry and Schroeder (2000).

Signs of significant success at Motorola quickly became apparent. In fact, from 1987 to 1997 Motorola achieved a fivefold growth in sales with profits climbing nearly 20 percent per year, cumulative savings at $US14 billion and stock price gains compounded to an annual rate of 21.3 percent. Motorola was also cited as the first winner of America's Malcolm Baldrige National Quality Award in 1988.

Soon other companies became interested in the program and successively more companies were able to demonstrate good results. As examples, AlliedSignal attained savings of $US2 billion during a five-year period while General Electric saved $US1 billion over a two year window. Indeed, “big dollar impact” is one of five key reasons cited by Hoerl (1998) for the success of Six Sigma. The other four reasons cited by Hoerl for Six Sigma success are ones that any quality advocate should embrace: continued top management support and enthusiasm, emphasis on a quantitative and disciplined approach to process improvement, value placed on understanding and satisfying customer needs, and the manner in which it combines right projects with the right people and tools.

employees, the year 2000 saw 14 people attending a seven month Black Belt (deep knowledge in Six Sigma philosophy and methods) education program on a half-time basis, 20 more people attending a two-day course on Six Sigma, and ten people in the top management group attending a one-day course on Six Sigma. Six Sigma applications at this factory saved about $US0.5 million during the first ten months of 2000 - about $500 per employee over the entire employee base, but closer to $10,000 per employee trained in Six Sigma methods. In the early stages of Six Sigma program implementation these figures indicate something on the order of simple cost recovery with return on investment promised for the near future, since We believe that the “new” of Six Sigma is its explicit linking of the tactical with the many companies have reported savings on the order of $150,000 per Black Belt project with each Black Belt completing four to six such projects annually.

Is Six Sigma really something new?

While typically applied consistently within a company, the content of the Six Sigma approach varies from company to company, consultant to consultant, and author to author. Generally, however, Six Sigma programs do have some common features, among which are the following:

It is a top-down, rather than bottom-up approach.

It is a highly disciplined approach that typically includes four stages: measure, analyse, improve and control.

It is a data-oriented approach, making sound and heavy use of various statistical decision tools.

It is our position that from a content perspective Six Sigma does not, in principle, contain anything new. Its focus on processes and variation is central to what is historically thought of as “quality control” and can be found in works by W. Edwards Deming and Walter A. Shewhart. Design of experiments and statistical process control, both of which are featured in Six Sigma programs, are not new - though their proactive use to improve processes and products is certainly laudable. Systematic application of quality tools such as Pareto diagrams and Ishikawa diagrams in Six Sigma is praiseworthy, but it is with good reason that these are counted among the so-called “old tools” of quality as these were developed by the late Kaoru Ishikawa of Japan during the 1950s.

So what is it, if anything, that is new about Six Sigma?

Reed (2000) contends that there is nothing at all new about Six Sigma and that it “has been around for many years, just called something else”. She goes on to say that Six Sigma “could be called problem solving, team building, SPC, plan, act, do, check, whatever you want ...”.

Carnell and Lambert (2000) assert what we all know - that Six Sigma is no silver bullet and that like most change processes involving people it is difficult to institutionalise. The perspective offered by these two “in the trenches” Six Sigma professionals is that it is a tactical tool of great value in achieving operational excellence. Operational excellence is, of course, required for the overall attainment of business excellence - something that also requires customer-related, financial, and marketplace performance excellence (Edgeman, 2000).

Strategic. That is, what is new in Six Sigma is that efficient, often statistical

Techniques are used in a systematic way to reduce variation and improve processes and there is a focus on results - including customer-related ones that lead to enhanced marketplace performance and hence improved bottom-line financial results.

The point is that Six Sigma is of great value in attainment of business excellence and measurement of that progress so that appropriately configured and deployed Six Sigma programs may be highly consistent with the results-orientation underlying various international quality awards, such as the European Quality Award, America's Malcolm Baldrige National Quality Award, the Canada Excellence Awards, and the Australian Quality Award. Mikel Harry[1], key developer and proponent of the Six Sigma program at Motorola, has defined Six Sigma as “a disciplined method of using extremely rigorous data gathering and statistical analysis to pinpoint sources of errors and ways of eliminating them”. Well-known statistician and quality consultant Ron Snee (2000) has indicated that “Six Sigma should be a strategic approach that works across all processes, products, company functions and industries” and Bajaria (2000) reinforces this idea - a “nuts and bolts” point-counterpoint discussion of each of 14 key Six Sigma ideas.

In order to efficiently use statistical tools to base decisions on fact, substantial effort and resources are dedicated to education and training of staff members. Responsibility and authority are distributed in a structured way by using a “belt” system similar to that used in Korean karate, to identify experience and mastery of Six Sigma tools and application thereof. Indeed, the terms “green belt”, “black belt” and “master black belt” convey particular meanings within the Six Sigma vocabulary.

Here Six Sigma has actively contributed to the creation of a comprehensive infrastructure within the practising organization that includes clear routines for control and reporting. It is a non-trivial issue, however, as regards how to get more people - not only Black Belts and other formalised problem solvers - involved in conquering mental barriers and using statistical methods more routinely in daily work. For example, understanding of variation has been pointed out as an important aspect for successful implementation of a Six Sigma program and has been featured within the Deming management approach for several decades.

Six Sigma seen contextually

Six Sigma provides a structured means of pushing product and process improvement, but we do not see it as an alternative to TQM. It is important, instead, to position Six Sigma in a larger context. As illustrated in BLOCK DIAGRAM NO.4, we regard TQM as a management system consisting of values, methodologies and tools that aims to improve customer satisfaction with a reduced amount of resources. TQM starts in most descriptions from values such as the six provided in BLOCK DIAGRAM NO.4 focus on customers, focus on processes, base decisions on facts, let everybody be committed, improve continuously and top management commitment[2]. These values contribute to creation of organizational culture. To attain this, the values have to be supported, systematically and continuously, by suitable methodologies and tools.

Of course, “everybody's commitment” cannot be obtained by simply proclaiming that “it is one of our deeply held organizational values”. As is commonly said, the proof is in the pudding and organizational values reflect organizational practices so that “everybody's commitment” and other values can be made part of the organizational fabric through use of suitable methodologies in such a way that the values permeate the work being done, whether that work is performed by improvement groups or through goal deployment to individual goals. Robust and sturdy tools are also needed to support, systematize and facilitate the work. As examples, Ishikawa diagrams and Pareto diagrams are tools commonly used by improvement groups

whereas deployment of goals might well be facilitated through use of matrix diagrams. These are reflective of approaches that may assist in embedding a value for fact-based decision-making in the organization's culture. An organizational value of focusing on processes can be obtained through use of Process Management, but within that methodology tools such as process maps and control charts are needed to map and control key organizational processes.

In building or transforming an organizational culture we must identify those values that we desire. We should then choose methodologies supporting those values and finally tools supporting those methodologies. Methodologies are not unambiguous but naturally some steps within a methodology may differ depending on the situation or organization. Of course, some methodologies may support several values and in general we need to employ multiple methodologies to support the various values. More or less random acquisition and employment of methodologies and tools that do not support organizational values should be avoided.

With this view it is obvious to us that Six Sigma is a methodology within TQM. The reason why this methodology has been so successful is that it is structured and systematic and uses several efficient tools. But it is also important to note that Six Sigma in fact supports all six values in Figure 1. Methodologies, supporting several values are important to the success of TQM. Six Sigma also illustrates that the management system is dynamic. New methodologies and new tools will appear and be developed and Six Sigma is an excellent example of this. But still Six Sigma is a methodology within - not an alternative to - TQM in much the same way as business process reengineering launched in the 1990s by Hammer and Champy

The main conclusion of this article related to application of Six Sigma methodology by an organisation is the following: Six Sigma is a methodology that might cut costs for your organisation; however, think about how the Six Sigma methodology supports the values of your organization and how you choose the tools and, above all, do not forget the totality of TQM.

Notes

1. Six Sigma is sometimes called “cowboy quality” in part because of the ranch and rodeo lifestyle of its main proponents Mikel Harry and Richard Schroeder.

2. The number of values varies between different sources. For example the European Quality Award is based on eight values and the Malcolm Baldrige National Quality Award is said to be based on 11 values. The six values in Figure 1 is the basis applied by Bergman and Klefsjö (1994).

                                                                                                     

    

                                                                                  BLOCK DIAGRAM NO.:-3

Six Sigma is a highly disciplined process that helps us focus on developing and delivering near-perfect products and services. Six Sigma is a statistically-based process improvement methodology that aims to reduce defects to a rate of 3.4 defects per million defect opportunities by identifying and eliminating causes of variation in business processes. Six Sigma focuses on developing a very clear understanding of customer requirements and is therefore very customer focused.

       Why "Sigma"? The word is a statistical term that measures how far a given process deviates from perfection. The term "six sigma process" comes from the notion that if one has six standard deviations between the mean of a process and the nearest specification limit, he will make practically no items that exceed the specifications. The central idea behind Six Sigma is that if you can measure how many "defects" you have in a process, you can systematically figure out how to eliminate them and get as close to "zero defects" as possible. To achieve Six Sigma Quality, a process must produce no more than 3.4 defects per million opportunities. The "3.4 Defects Per Million Opportunities (DPMO)" is a gross confusion of the following situation. 

Key themes in Six Sigma: -

      Some of the key themes of Six Sigma can be summarized as follows:

·         Continuous focus on the customer’s requirements

·         Using measurements and statistics to identify and measure variation in the production process and

                other business processes

·         Identifying the root causes of problems

·         Emphasis on process improvement to remove variation from the production process or other

                Business processes and therefore lowers defects and improves customer satisfaction

·         Pro-active management focusing on problem prevention, continuous improvement and constant

                Striving for perfection

·         Cross-functional collaboration within the organization; and

·         Setting very high targets.

Six Sigma is that efficient, often statistical, techniques are used in a systematic way to reduce variation and improve processes and there is a focus on results - including customer-related ones that lead to enhanced marketplace performance and hence improved bottom-line financial results.

The point is that Six Sigma is of great value in attainment of business excellence and measurement of that progress so that appropriately configured and deployed Six Sigma programs may be highly consistent with the results-orientation underlying various international quality awards, such as the European Quality Award, America's Malcolm Baldrige National Quality Award, the Canada Excellence Awards, and the Australian Quality Award. Mikel Harry[1], key developer and proponent of the Six Sigma program at Motorola, has defined Six Sigma as “a disciplined method of using extremely rigorous data gathering and statistical analysis to pinpoint sources of errors and ways of eliminating them”. Well-known statistician and quality consultant Ron Snee (2000) has indicated that “Six Sigma should be a strategic approach that works across all processes, products, company functions and industries” and Bajaria (2000) reinforces this idea - a “nuts and bolts” point-counterpoint discussion of each of 14 key Six Sigma ideas.

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXER

                                                           ABSTRACT:

OFDM otherwise called orthogonal frequency division multiplexing is a modulation and a multiple access technique that can be applied to mobile communications. OFDM or Multitone modulation as it is sometimes called is the basis for several commercial wireless applications. In OFDM the segments are according to frequency there by dividing the spectrum into a number of equally spaced tones, which are orthogonal with each other and carries a portion of user information on each tone. As the mobile cellular wireless system operates under harsh and challenging channel conditions, the wireless channel is distinct and much more unpredictable than the wired because of the factors such as multipath fading, shadow fading, Doppler spread and time dispersion or delay spread. OFDM over comes the ISI (intersymbol interference) in a multipath environment.

In order to combat these effects the modern wireless systems employ a variety of signal processing techniques, which include the factors such as equalization, error correction coding, spread spectrum, interleaving and diversity. The sinusoidal waveforms making up the OFDM tones have the very special property of being the only eigen functions of a linear channel. With this property and the incorporation of small amount of guard time, called the cyclic prefix to each symbol enables the orthogonality between tones to be preserved in the presence of multipath. The cyclic prefix allows the tones to be realigned at the receiver thus regaining orthogonality and is used to absorb transients

from previous bursts caused by multipath. Thus OFDM eliminates the effect of multipath, ISI (intersymbol interference), ICI (intercarrier interference) in Mobile channels.

 

          INTRODUCTION:

OFDM represents a different system-design approach. It can be thought of as a combination of modulation and multiple-access schemes that segments a communications channel in such a way that many users can share it. Whereas TDMA segments are according to time and CDMA segments are according to spreading codes, OFDM segments are according to frequency. It is a technique that divides the spectrum into a number of equally spaced tones and carries a portion of a user's information on each tone. A tone can be thought of as a frequency, much in the same way that each key on a piano represents a unique frequency. OFDM can be viewed as a form of frequency division multiplexing (FDM), however, OFDM has an important special property that each tone is orthogonal with every other tone. FDM typically requires there to be frequency guard bands between the frequencies so that they do not interfere with each other. OFDM allows the spectrum of each tone to overlap, and because they are orthogonal, they do not interfere with each other.

            DEFINITION:

Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in

each band. Each subcarrier is orthogonal (independent of each other) with every other subcarrier, differentiating OFDM from the commonly used frequency division multiplexing (FDM).

           OVERVIEW OF OFDM:

This paper describes OFDM and its application to mobile communications. OFDM is a modulation and multiple-access technique that has been explored for more than 20 years. Only recently has it been finding its way into commercial communications systems, as Moore's Law has driven down the cost of the signal processing that is needed to implement OFDM–based systems.

OFDM, or multitone modulation as it is sometimes called, is presently used in a number of commercial wired and wireless applications. On the wired side, it is used for a variant of digital subscriber line (DSL). For wireless, OFDM is the basis for several television and radio broadcast applications, including the European digital broadcast television standard, as well as digital radio in North America. OFDM is also used in several fixed wireless systems and wireless local-area network (LAN) products. A system based on OFDM has been developed to deliver mobile broadband data service at data rates comparable to those of wired services, such as DSL and cable modems.

OFDM enables the creation of a very flexible system architecture that can be used efficiently for a wide range of services, including voice and data. For any mobile system to create a rich user experience, it must provide ubiquitous, fast, and user-friendly connectivity. OFDM has several unique properties that make it especially well suited to handle the challenging environmental conditions experienced by mobile wireless data applications.

       OFDM FOR MOBILE COMMUNICATIONS:

OFDM represents a different system-design approach. It can be thought of as a combination of modulation and multiple-access schemes that segments a communications channel in such a way that many users can share it. Whereas TDMA segments are according to time and CDMA segments are according to spreading codes, OFDM segments are according to frequency. It is a technique that divides the spectrum into a number of equally spaced tones and carries a portion of a user's information on each tone. A tone can be thought of as a frequency, much in the same way that each key on a piano represents a unique frequency. OFDM can be viewed as a form of frequency division multiplexing (FDM), however, OFDM has an important special property that each tone is orthogonal with every other tone. FDM typically requires there to be frequency guard bands between the frequencies so that they do not interfere with each other. OFDM allows the spectrum of each tone to overlap as shown in the fig1, and because they are orthogonal, they do not interfere with each other. By allowing the tones to overlap, the overall amount of spectrum required is reduced.

 

     OVERVIEW OF THE WIRELESS ENVIRONMENT:

Mobile cellular wireless systems operate under harsh and challenging channel conditions. The wireless channel is distinct and much more unpredictable than the wireline channel because of factors such as multipath and shadow fading, Doppler spread, and time dispersion or delay spread.

Multipath is a phenomenon that occurs as a transmitted signal is reflected by objects in the environment between the base station and a user. These objects can be buildings, trees, hills, or even trucks and cars. The reflected signals arrive at the receiver with random phase offsets, because each reflection generally follows a different path to reach the user's receiver. The degree of cancellation, or fading, will depend on the delay spread of the reflected signals, as embodied by their relative phases, and their relative power.

                                                                   

 

       Fig 1. Time Delayed multipath signals

    Courtesy: Internet

Time dispersion represents distortion to the signal and is manifested by the spreading in time of the modulation symbols. This occurs when the channel is band-limited, or, in other words, when the coherence bandwidth of the channel is smaller than the modulation bandwidth. Time dispersion leads to intersymbolinterference, or ISI, where the energy from one symbol spills over into another symbol, and, as a result, the BER is increased. It also leads to fading.

 

                                                               

Fig2. Demonstration of multipath reflections

Courtesy: Internet

In many instances, the fading due to multipath will be frequency selective, randomly affecting only a portion of the overall channel bandwidth at any given time. Frequency selective fading occurs when the channel introduces time dispersion and when the delay spread exceeds the symbol period. When there is no dispersion and the delay spread is less than the symbol period, the fading will be flat, thereby affecting all frequencies in the signal equally. Flat fading can lead to deep fades of more than 30 decibels (dB).

                                                                 

 Fig3. Demonstration of time varying fading

Courtesy: Internet

Doppler spread describes the random changes in the channel introduced as a result of a user's mobility and the relative motion of objects in the channel. Doppler has the effect of shifting, or spreading, the frequency components of a signal. The coherence time of the channel is the inverse of the Doppler spread and is a measure of the speed at which the channel characteristics change. This in effect determines the rate at which fading occurs. When the rate of change of the channel is higher than the modulated symbol rate, fast fading occurs. Slow fading, on the other hand, occurs when the channel changes are slower than the symbol rate.

The statistics describing the fading signal amplitude are frequently characterized as either Rayleigh or Ricean. Rayleigh fading occurs when there is no line of sight (LOS) component present in the received signal. If there is a LOS component present, the fading follows a Ricean distribution. There is frequently no direct LOS path to a mobile, because the very nature of mobile communications means that mobiles can be in a building or behind one or other obstructions. This leads to Rayleigh fading but also results in a shadow loss. These conditions, along with the inherent variation in signal strength caused by changes in the distance between a mobile and cell site, result in a large dynamic range of signals, which can easily be as much as 70 dB.

    OVERVIEW OF TRADITIONAL MOBILE WIRELESS SYSTEMS:

All modern mobile wireless systems employ a variety of techniques to combat the aforementioned effects. Some techniques are more effective than others, with the effectiveness depending on the air-interface and the system-architecture approach taken to satisfy the requirements of the services being offered. As mobile systems evolved from analog to digital, more sophisticated signal-processing techniques have been employed to overcome the wireless environment. These techniques include equalization, channel or error-correction coding, spread spectrum, interleaving, and diversity.

Diversity has long been used to help mitigate the multipath-induced fading that results from users' mobility. The simplest diversity technique, spatial diversity, involves the use of two or more receive antennae at a base station that are separated by some distance, say on the order of five to 10 wavelengths. The signal from the mobile will

generally follow separate paths to each antenna. This relatively low-cost approach yields significant performance improvement by taking advantage of the statistical likelihood that the paths are not highly correlated with each other. When one antenna is in a fade, the other one will generally not be.

Spread spectrum systems employ a form of diversity called frequency diversity. Here the signal is spread over a much larger bandwidth than is needed for transmission and is typically greater than the coherence bandwidth of the channel. A wideband signal is more resistant to the effect of frequency selective fading than is a narrowband signal, because only a relatively small portion of the overall bandwidth use is likely to experience a fade at any given time. There are two forms of spread spectrum, code division multiple access (CDMA) and frequency hopping (FH).

          OPERATION OF OFDM:

The sinusoidal waveforms making up the tones in OFDM have the very special property of being the only Eigen-functions of a linear channel. This special property prevents adjacent tones in OFDM systems from interfering with one another, in much the same manner that the human ear can clearly distinguish between each of the tones created by the adjacent keys of a piano. This property, and the incorporation of a small amount of guard time to each symbol, enables the orthogonality between tones to be preserved in the presence of multipath. This is what enables OFDM to avoid the multiple-access interference that is present in CDMA systems.

The frequency domain representation of a number of tones, shown in Figure 1 highlights the orthogonal nature of the tones used in the OFDM system. Notice that the peak of each tone corresponds to a zero level, or null, of every other tone. The result of this is that there is no interference between tones. When the receiver samples at the center frequency of each tone, the only energy present is that of the desired signal, plus whatever other noise happens to be in the channel.

                                                                    

Fig4. Two-dimensional illustration of ofdm channel resource

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To maintain orthogonality between tones, it is necessary to ensure that the symbol time contains one or multiple cycles of each sinusoidal tone waveform. This is normally the case, because the system numerology is constructed such that tone frequencies are integer multiples of the symbol period, as is subsequently highlighted, where the tone spacing is 1/T. Viewed as sinusoids. The below figure shows three tones over a single symbol period, where each tone has an integer number of cycles during the symbol.

                                          .

                                      

      Fig5. Time- and frequency-domain representation

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Fig6. Integer number of sinusoid periods

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In absolute terms, to generate a pure sinusoidal tone requires the signal start at time minus infinity. This is important, because tones are the only waveform than can ensure orthogonality. Fortunately, the channel response can be treated as finite, because multipath components decay over time and the channel is effectively band-limited. By adding a guard time, called a cyclic prefix, the channel can be made to behave as if the transmitted waveforms were from time minus infinite, and thus ensure orthogonality, which essentially prevents one subcarrier from interfering with another (called intercarrier interference, or ICI).

Multipath causes tones and delayed replicas of tones to arrive at the receiver with some delay spread. This leads to misalignment between sinusoids, which need to be aligned as in fig shown below to be orthogonal. The cyclic prefix allows the tones to be realigned at the receiver, thus regaining orthogonality.

                                                             

                                                               

Fig7. Cyclic extension of sinusoid

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The cyclic prefix is sized appropriately to serve as a guard time to eliminate ISI. This is accomplished because the amount of time dispersion from the channel is smaller than the duration of the cyclic prefix. A fundamental trade-off is that the cyclic prefix must be long enough to account for the anticipated multipath delay spread experienced by the system. The amount of overhead increases, as the cyclic prefix gets longer. The sizing of the cyclic prefix forces a tradeoff between the amount of delay spread that is acceptable and the amount of Doppler shift that is acceptable.

            CONCLUSION:

This tutorial highlights the unique design challenges faced by mobile data systems that result from the vagaries of the harsh wireless channel, the wide and varied service profiles that are enabled by data communications, and the performance of wireline-based protocols, such as TCP/IP, with the realities of wireless links. OFDM has been shown to address these challenges and to be a key enabler of a system design that can provide high-performance mobile data communications.