Thursday, December 20, 2007
Thursday, October 11, 2007
Warren Buffett Quotes
Warren Buffett Quotes
A public-opinion poll is no substitute for thought.
Chains of habit are too light to be felt until they are too heavy to be broken.
I always knew I was going to be rich. I don't think I ever doubted it for a minute.
I am quite serious when I say that I do not believe there are, on the whole earth besides, so many intensified bores as in these United States. No man can form an adequate idea of the real meaning of the word, without coming here.
I buy expensive suits. They just look cheap on me.
I don't look to jump over 7-foot bars: I look around for 1-foot bars that I can step over.
I never attempt to make money on the stock market. I buy on the assumption that they could close the market the next day and not reopen it for five years.
If a business does well, the stock eventually follows.
If past history was all there was to the game, the richest people would be librarians.
In the business world, the rearview mirror is always clearer than the windshield.
It takes 20 years to build a reputation and five minutes to ruin it. If you think about that, you'll do things differently.
It's better to hang out with people better than you. Pick out associates whose behavior is better than yours and you'll drift in that direction.
It's far better to buy a wonderful company at a fair price than a fair company at a wonderful price.
Let blockheads read what blockheads wrote.
Look at market fluctuations as your friend rather than your enemy; profit from folly rather than participate in it.
Of the billionaires I have known, money just brings out the basic traits in them. If they were jerks before they had money, they are simply jerks with a billion dollars.
Only buy something that you'd be perfectly happy to hold if the market shut down for 10 years.
Only when the tide goes out do you discover who's been swimming naked.
Our favorite holding period is forever.
Our favourite holding period is forever.
Price is what you pay. Value is what you get.
Risk comes from not knowing what you're doing.
Risk is a part of God's game, alike for men and nations.
Rule No.1: Never lose money. Rule No.2: Never forget rule No.1.
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.
The business schools reward difficult complex behavior more than simple behavior, but simple behavior is more effective.
The first rule is not to lose. The second rule is not to forget the first rule.
The investor of today does not profit from yesterday's growth.
The only time to buy these is on a day with no "y" in it.
The smarter the journalists are, the better off society is. For to a degree, people read the press to inform themselves-and the better the teacher, the better the student body.
There seems to be some perverse human characteristic that likes to make easy things difficult.
Time is the friend of the wonderful company, the enemy of the mediocre.
Value is what you get.
We believe that according the name 'investors' to institutions that trade actively is like calling someone who repeatedly engages in one-night stands a 'romantic.'
We enjoy the process far more than the proceeds.
We simply attempt to be fearful when others are greedy and to be greedy only when others are fearful.
When a management team with a reputation for brilliance tackles a business with a reputation for bad economics, it is the reputation of the business that remains intact.
When a management with a reputation for brilliance tackles a business with a reputation for bad economics, it is the reputation of the business that remains intact.
Why not invest your assets in the companies you really like? As Mae West said, "Too much of a good thing can be wonderful".
Wide diversification is only required when investors do not understand what they are doing.
You do things when the opportunities come along. I've had periods in my life when I've had a bundle of ideas come along, and I've had long dry spells. If I get an idea next week, I'll do something. If not, I won't do a damn thing.
You only have to do a very few things right in your life so long as you don't do too many things wrong.
Your premium brand had better be delivering something special, or it's not going to get the business.
Thursday, September 27, 2007
10 Essential Health Tips
1. Move More
Make it a daily challenge to find ways to move your body. Climb stairs if given a choice between that and escalators or elevators. Walk your dog; chase your kids; toss balls with friends, mow the lawn. Anything that moves your limbs is not only a fitness tool, it's a stress buster. Think 'move' in small increments of time. It doesn't have to be an hour in the gym or a 45-minute aerobic dance class or tai chi or kickboxing. But that's great when you're up to it.
2. Cut Fat
Avoid the obvious such as fried foods, burgers and other fatty meats (i.e. pork, bacon, ham, salami, ribs and sausage). Dairy products such as cheese, cottage cheese, milk and cream should be eaten in low fat versions. Nuts and sandwich meats, mayonnaise, margarine, butter and sauces should be eaten in limited amounts. Most are available in lower fat versions such as substitute butter, fat free cheeses and mayonnaise.
3. Quit Smoking
The jury is definitely in on this verdict. Ever since 1960 when the Surgeon General announced that smoking was harmful to your health, Americans have been reducing their use of tobacco products that kill. Just recently, we've seen a surge in smoking in adolescents and teens. Could it be the Hollywood influence? It seems the stars in every movie of late smoke cigarettes. Beware. Warn your children of the false romance or 'tough guy' stance of Hollywood smokers.
4. Reduce Stress
Easier said than done, stress busters come in many forms. Some techniques recommended by experts are to think positive thoughts. Spend 30 minutes a day doing something you like. (i.e.,Soak in a hot tub; walk on the beach or in a park; read a good book; visit a friend; play with your dog; listen to soothing music; watch a funny movie. Get a massage, a facial or a haircut. Meditate. Count to ten before losing your temper or getting aggravated. Avoid difficult people when possible.
5. Protect Yourself from Pollution
If you can't live in a smog-free environment, at least avoid smoke-filled rooms, high traffic areas, breathing in highway fumes and exercising near busy thoroughfares. Exercise outside when the smog rating is low. Exercise indoors in air conditioning when air quality is good. Plant lots of shrubbery in your yard. It's a good pollution and dirt from the street deterrent.
6. Wear Your Seat Belt
Statistics show that seat belts add to longevity and help alleviate potential injuries in car crashes.
7. Floss Your Teeth
Recent studies make a direct connection between longevity and teeth flossing. Nobody knows exactly why. Perhaps it's because people who floss tend to be more health conscious than people who don't?
8. Avoid Excessive Drinking
While recent studies show a glass of wine or one drink a day (two for men) can help protect against heart disease, more than that can cause other health problems such as liver and kidney disease and cancer.
9. Keep a Positive Mental Outlook
There's a definitive connection between living well and healthfully and having a cheerful outlook on life.
10. Choose Your Parents Well
The link between genetics and health is a powerful one. But just because one or both of your parents died young in ill health doesn't mean you cannot counteract the genetic pool handed you.
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Tuesday, September 25, 2007
Warren Buffet's 10 Golden Rules for Investing
Warren Buffet's 10 Golden Rules for Investing
1 Never invest in a business you can not understand
2 Risk can be reduced by concentrating on few holdings
3 Stop predict direction of the stock market, economy,intrest rate etc
4 Buy companies with strong histories of profitibalities
5Be fearful when others are greedy and greedy only when others are fearful
6 Unless you can watch ur stock holding decline by 50% without becoming panic,you should not be in stock market
7 Do not take yearly results too serious,instead focus on five year average
8 Focus on return on equity ; not on EPS
9 Calcualte " owners earning" to get true of value of the stock
10 AlwaysInvest for a long term.Think about "Does business have favourable long term prospects?"
Monday, September 17, 2007
SEO Tips - External Linking
Posted: 16 Sep 2007 09:27 AM CDT
Even though you can't have an impact on every external link pointing to your blog, you still have a chance of getting some external links to your blog which you CAN control - control the linking (anchor) text, that is.
How to get desired anchor text for your external links:
1. Exchanging links with other bloggers - allows you to ask for desired anchor text
2. Submitting your blog to web directories where you have editorial control and you can specify how your link will look like
3. Getting listed on partner blogs and sites
4. Optimizing your post title - if you use a great title your post can be picked up by someone where you usually get the credit for it + the post title as the anchor text
5. Buying links on other blogs which gives you complete editorial rights and you can specify the desired anchor text.
6. Running a contest on your blog will bring you external links with the anchor text you specify in the contest rules.
After you have done that, your external links depend on the webmasters that post them. The best influence you can have on that is to optimize your post titles in the best possible way.
One last thing, external linking very much depends on your content. With high quality content you will have no problems at all in getting external links.
Friday, September 14, 2007
7 Steps to Becoming a Happier Person
Choosing To Be Happy
Strategies for Happiness: 7 Steps to Becoming a Happier Person
Reviewed by Cynthia Dennison Haines, MD
A popular greeting card attributes this quote to Henry David Thoreau: "Happiness is like a butterfly: the more you chase it, the more it will elude you, but if you turn your attention to other things, it will come and sit softly on your shoulder."
With all due respect to the author of Walden, that just isn't so, according to a growing number of psychologists. You can choose to be happy, they say. You can chase down that elusive butterfly and get it to sit on your shoulder. How? In part, by simply making the effort to monitor the workings of your mind.
Research has shown that your talent for happiness is, to a large degree, determined by your genes. Psychology professor David T. Lykken, author of Happiness: Its Nature and Nurture, says that "trying to be happier is like trying to be taller." We each have a "happiness set point," he argues, and move away from it only slightly.
And yet, psychologists who study happiness -- including Lykken -- believe we can pursue happiness. We can do this by thwarting negative emotions such as pessimism, resentment, and anger. And we can foster positive emotions, such as empathy, serenity, and especially gratitude.
Happiness Strategy # 1: Don't Worry, Choose Happy
The first step, however, is to make a conscious choice to boost your happiness. In his book, The Conquest of Happiness, published in 1930, the philosopher Bertrand Russell had this to say: "Happiness is not, except in very rare cases, something that drops into the mouth, like a ripe fruit. … Happiness must be, for most men and women, an achievement rather than a gift of the gods, and in this achievement, effort, both inward and outward, must play a great part."
Today, psychologists who study happiness heartily agree. The intention to be happy is the first of The 9 Choices of Happy People listed by authors Rick Foster and Greg Hicks in their book of the same name.
"Intention is the active desire and commitment to be happy," they write. "It's the decision to consciously choose attitudes and behaviors that lead to happiness over unhappiness."
Tom G. Stevens, PhD, titled his book with the bold assertion, You Can Choose to Be Happy. "Choose to make happiness a top goal," Stevens tells WebMD. "Choose to take advantage of opportunities to learn how to be happy. For example, reprogram your beliefs and values. Learn good self-management skills, good interpersonal skills, and good career-related skills. Choose to be in environments and around people that increase your probability of happiness. The persons who become the happiest and grow the most are those who also make truth and their own personal growth primary values."
In short, we may be born with a happiness "set point," as Lykken calls it, but we are not stuck there. Happiness also depends on how we manage our emotions and our relationships with others.
Jon Haidt, author of The Happiness Hypothesis, teaches positive psychology. He actually assigns his students to make themselves happier during the semester.
"They have to say exactly what technique they will use," says Haidt, a professor at the University of Virginia, in Charlottesville. "They may choose to be more forgiving or more grateful. They may learn to identify negative thoughts so they can challenge them. For example, when someone crosses you, in your mind you build a case against that person, but that's very damaging to relationships. So they may learn to shut up their inner lawyer and stop building these cases against people."
Once you've decided to be happier, you can choose strategies for achieving happiness. Psychologists who study happiness tend to agree on ones like these.
Happiness Strategy #2: Cultivate Gratitude
In his book, Authentic Happiness, University of Pennsylvania psychologist Martin Seligman encourages readers to perform a daily "gratitude exercise." It involves listing a few things that make them grateful. This shifts people away from bitterness and despair, he says, and promotes happiness.
Happiness Strategy #3: Foster Forgiveness
Holding a grudge and nursing grievances can affect physical as well as mental health, according to a rapidly growing body of research. One way to curtail these kinds of feelings is to foster forgiveness. This reduces the power of bad events to create bitterness and resentment, say Michael McCullough and Robert Emmons, happiness researchers who edited The Psychology of Happiness.
In his book, Five Steps to Forgiveness, clinical psychologist Everett Worthington Jr. offers a 5-step process he calls REACH. First, recall the hurt. Then empathize and try to understand the act from the perpetrator's point of view. Be altruistic by recalling a time in your life when you were forgiven. Commit to putting your forgiveness into words. You can do this either in a letter to the person you're forgiving or in your journal. Finally, try to hold on to the forgiveness. Don't dwell on your anger, hurt, and desire for vengeance.
The alternative to forgiveness is mulling over a transgression. This is a form of chronic stress, says Worthington.
"Rumination is the mental health bad boy," Worthington tells WebMD. "It's associated with almost everything bad in the mental health field -- obsessive-compulsive disorder, depression, anxiety -- probably hives, too."
Happiness Strategy #4: Counteract Negative Thoughts and Feelings
As Jon Haidt puts it, improve your mental hygiene. In The Happiness Hypothesis, Haidt compares the mind to a man riding an elephant. The elephant represents the powerful thoughts and feelings -- mostly unconscious -- that drive your behavior. The man, although much weaker, can exert control over the elephant, just as you can exert control over negative thoughts and feelings.
"The key is a commitment to doing the things necessary to retrain the elephant," Haidt says. "And the evidence suggests there's a lot you can do. It just takes work."
For example, you can practice meditation, rhythmic breathing, yoga, or relaxation techniques to quell anxiety and promote serenity. You can learn to recognize and challenge thoughts you have about being inadequate and helpless.
"If you learn techniques for identifying negative thoughts, then it's easier to challenge them," Haidt said. "Sometimes just reading David Burns' book, Feeling Good, can have a positive effect."
Happiness Strategy #5: Remember, Money Can't Buy Happiness
Research shows that once income climbs above the poverty level, more money brings very little extra happiness. Yet, "we keep assuming that because things aren't bringing us happiness, they're the wrong things, rather than recognizing that the pursuit itself is futile," writes Daniel Gilbert in his book, Stumbling on Happiness. "Regardless of what we achieve in the pursuit of stuff, it's never going to bring about an enduring state of happiness."
Happiness Strategy #6: Foster Friendship
There are few better antidotes to unhappiness than close friendships with people who care about you, says David G. Myers, author of The Pursuit of Happiness. One Australian study found that people over 70 who had the strongest network of friends lived much longer.
"Sadly, our increasingly individualistic society suffers from impoverished social connections, which some psychologists believe is a cause of today's epidemic levels of depression," Myers writes. "The social ties that bind also provide support in difficult times."
Happiness Strategy #7: Engage in Meaningful Activities
People are seldom happier, says psychologist Mihaly Csikszentmihalyi, than when they're in the "flow." This is a state in which your mind becomes thoroughly absorbed in a meaningful task that challenges your abilities. Yet, he has found that the most common leisure time activity -- watching TV -- produces some of the lowest levels of happiness.
To get more out of life, we need to put more into it, says Csikszentmihalyi. "Active leisure that helps a person grow does not come easily," he writes in Finding Flow. "Each of the flow-producing activities requires an initial investment of attention before it begins to be enjoyable."
So it turns out that happiness can be a matter of choice -- not just luck. Some people are lucky enough to possess genes that foster happiness. However, certain thought patterns and interpersonal skills definitely help people become an "epicure of experience," says David Lykken, whose name, in Norwegian, means "the happiness."
http://www.webmd.com/balance/guide/choosing-to-be-happy?page=1
Sunday, September 9, 2007
Balance Work and Life
I come across very good articles.
on
Work Less, Be Happier
Family First
http://finance.yahoo.com/expert/article/careerist/43903
http://blog.penelopetrunk.com/2006/08/27/your-family-would-be-better-off-with-a-housewife-so-would-mine/
http://www.washingtonpost.com/wp-dyn/content/article/2007/07/11/AR2007071102345.html?hpid=topnews
Friday, September 7, 2007
69 Wisdoms and Axioms for modern life
69 Wisdoms and Axioms for modern life
2) A conclusion is the place where you got tired of thinking.
3) Experience is something that you don't get until just after you need it.
4) For every action, there is an equal and opposite criticism.
5) He who hesitates is probably right.
6) No one is listening until you make a mistake.
7) Success always occurs in private, and failure in full view.
8) To steal ideas from one person is plagiarism; to steal from many is research.
9) Two wrongs are only the beginning.
10) Monday is an awful way to spend one-seventh of your life.
11) The sooner you fall behind, the more time you'll have for catching up.
12) A clear consciense is usually the sign of a bad memory.
13) It is much easier to apologize than to ask permission.
14) There are two rules for ultimate success in life. Never tell everything you know.
15) Never put off until tomorrow what you can avoid doing altogether.
16) The trouble with being in the rat race is that even if you win, you're still a rat.
17) Everyone has a right to be stupid. Some just abuse the privilege.
18) On the keyboard of life, always keep one finger on the escape key.
19) It's frustrating when you know all the answers, but nobody bothers to ask you the questions.
21) If you can remain calm, you just don't have all the facts.
22) Never do card tricks for the group you play poker with.
23) The colder the X-ray table, the more of your body is required on it.
24) The hardness of the butter is proportional to the softness of the bread.
25) The severity of the itch is proportional to the reach.
26) To succeed in politics, it is often necessary to rise above your principles.
27) You never really learn to swear until you learn to drive.
28) The problem with the gene pool is that there is no lifeguard.
29) If you must choose between two evils, pick the one you've never tried before.
30) Change is inevitable except from vending machines.
31) Don't sweat petty things or pet sweaty things.
32) A fool and his money are soon partying.
33) Light travels faster than sound. This is why some people appear bright until you hear them speak.
34) Money can't buy love But it CAN rent a very close imitation.
35) Plan to be spontaneous tomorrow.
36) Always try to be modest ... And be damn proud of it!
37) If you think nobody cares about you, try missing a couple of loan repayments.
38) How many of you believe in telekinesis? Raise my hands.
39) Everybody repeat after me "We are all individuals."
40) Chastity is curable, if detected early.
41) Love may be blind, but marriage is a real eye-opener.
42) Hell hath no fury like the lawyer of a woman scorned.
43) Bills travel through the post at twice the speed of cheques.
44) Hard work pays off in the future, Laziness pays off now.
45) Eagles may soar, but weasels aren't sucked into jet engines.
46) Borrow money from pessimists - they don't expect it back.
47) Half the people you know are below average.
48) 99 percent of lawyers give the rest a bad name.
49) 427.23 percent of all statistics are made up on the spot.
50) A conscience is what hurts when all your other parts feel so good.
51) Everyone has a photographic memory. Some don't have film.
52) He who laughs last, thinks slowest.
53) A day without sunshine is like, well, night.
54) On the other hand, you have different fingers.
55) Back up my hard drive? How do I put it in reverse?
56) I just got lost in thought. It was unfamiliar territory.
57) Seen it all, done it all, can't remember most of it.
58) Those who live by the sword get shot by those who don't.
59) I feel like I'm diagonally parked in a parallel universe.
60) He's not dead; he's electroencephalographically challenged.
61) You have the right to remain silent. Anything you say will be misquoted, then used against you.
62) I wonder how much deeper would the ocean be without sponges.
63) Despite the cost of living, have you noticed how it remains so popular?
64) Nothing is foolproof to a sufficiently talented fool.
65) Always remember that you are unique, just like everyone else.
66) War doesn't determine who's right, just who's left.
67) Madness takes its toll. Please have exact change.
68) I almost had a psychic girlfriend but she left me before we met.
69) Do unto others, then run .............
Found Good Site on Internet Marketing
Dave Wong - Motivation, NLP, Study Skills, Internet Marketing Coach
www.success-route.com
Description:The route to success is a road hard to thread and not many willing or dare to walk on it. It is filled with countless hardships and challenges. We are here to be your guide towards enroute success, clearing hardships and challenges together with you, making your enroute towards success smoother and easier to thread on. Dave Wong.
Google Optimization - Key Factors
Google Optimization is based on three things:
1. Google Page Rank
2. Google Sand Box
3. Inbound Links
You need to focus on those three factors to make the most from your Goo gle Optimization. What Google values the most are those high-quality inbound links . You can get them by writing articles, buying links and link exchanges. Try to get as much as you can of those one-way text links and by getting that alone, your Page Rank should increase by itself.
On the other hand, you should try to avoid getting caught in the Google Sand Box. The effect of the Google Sand Box is on the new domain names by holding them in a filter. Getting out of the Sand Box includes adding relevant content to your blog, linking to your external pages and using competitive keywords.
Goo gle Op timiz ation - Key Fact ors
- make sure your new pages link to your old ones
- add a sitemap to your blog
- use title optimization
- use relevant keywords in your posts
- submit your site to web-directories
- use robot.txt files to prevent non-relevant content being scanned
- submit your site to Google
- write content for your readers - not for search engines
Wednesday, August 29, 2007
Differences Between GSM & CDM
GSM and CDMA have been the two leading commercial wireless technologies that are being used all over the world. This paper presents to the readers the key differences between the two technologies 1. The various topics in which this paper presents the difference are:
- Radio Spectrum Usage
- Network architecture differences
- Radio channel differences
- Call Processing
- Evolution to 3G
- Network capacity differences
- Deployment
Introduction
This section presents the basic wireless network architecture and lays the foundation for the readers to understand the later sections of this paper.
The Mobile Station
The Base Transceiver Station
The Base Station Controller
The Mobile Switching Center
The Location Registers
Historical View of GSM and CDMA
GSM
| 1982 | Groupe Special Mobile established within CEPT |
| 1984 | Several proposals for GSM multiple access : wideband TDMA, narrowband TDMA, DS- CDMA, hybrid CDMA/FDMA, narrowband FDMA |
| 1986 | Eight prototype systems tested in CNET laboratories in France Permanent nucleus is set up |
| 1987 | Basic transmission principles selected : 8-slot TDMA, 200-kHz carrier spacing, frequency hopping |
| 1987 | MoU signed |
| 1988 | GSM becomes an ETSI technical committee |
| 1990 | GSM phase 1 specifications frozen (drafted 1987 – 1990) GSM1800 standardisation begins |
| 1991 | GSM1800 specifications are frozen |
| 1992 | GSM900 commercial operation starts |
| 1992 | GSM phase 2+ development starts |
| 1995 | GSM submitted as a PCS technology candidate to the United States |
| 1995 | PCS1900 standard adopted in the United States |
| 1996 | Enhanced full rate (EFR) speech codec standard ready |
| 1996 | 14.4-Kbps standard ready GSM1900 commercial operation starts |
| 1997 | HSCSD standard ready GSM cordless system (home base station) standardisation started EDGE standardisation started |
| 1998 | GPRS standard ready WCDMA selected as the third generation air interface |
Classification of CDMA
Frequency
Direct sequence
Frequency hopping
Time
Table 2 – CDMA Era
| John Pierce : time hopping spread spectrum | |
| 1949 | Claude Shannon and Robert Pierce : basic ideas of CDMA |
| 1950 | De Rosa-Rogoff : direct sequence spread spectrum |
| 1956 | Price and Green : antimultipath "RAKE" patent |
| 1961 | Magnuski : near-far problem |
| 1970s | Several developments for military field and navigation systems |
Narrowband CDMA Era
| Cooper and Nettleton : cellular application of spread spectrum | |
| 1980s | Investigation of narrowband CDMA techniques for cellular applications |
| 1986 | Formulation of optimum multiuser detection by Verdu |
| 1993 | IS-95 standard |
| Europe : FRAMES FMA2 Japan : Core-A USA : cdma2000 Korea : TTA I, TTA II | |
| 2000s | Commercialization of wideband CDMA systems |
Table 3 – Second Generation Digital Systems
| | GSM | IS-136 | IS-95 | PDC |
| Multiple access | TDMA | TDMA | CDMA | TDMA |
| Modulation | GMSKa | ð/4-DQPSKb Coherent ð/4- DQPSK Coherent 8-PSK | QPSK/0-QPSKc | ð/4-DQPSK |
| Carrier spacing | 200 kHz | 30 kHz | 1.25 MHz | 25 kHz |
| Carrier bit rate | 270.833 Kbps | 48.6 Kbps (ð/4-PSK and ð/4-DQPSK) 72.9 Kbps (8-PSK) | 1.2288 Mchip/sd | 42 Kbps |
| Frame length | 4.615 ms | 40 ms | 20 ms | 20 ms |
| Slots per frame | 8/16 | 6 | 1 | 3/6 |
| Frequency band (uplink/ downlink) (MHz) | 880-915 / 935-960 1720-1785 / 1805-1880 1930-1990 / 1850-1910 | 824-849 / 869-894 1930-1990 / 1850-1910 | 824-849/869-894 1930-1990 / 1850-1910 | 810-826 / 940-956 1429-1453/ 1477-1501 |
| Speech codec | RPE-LTPe 13 Kbps Half rate 6.5 Kbps Enhanced full rate (EFR) 12.2 kbps | VSELPf 8 Kbps IS-641-A: 7.4 Kbps (ACELP)g US1: 12.2 Kbps (ACELP) | QCELP 8 Kbps CELP 8 Kbps CELP 13 Kbps | VCELP 6.7 Kbps |
| Maximum possible data rate | HSCSD:115.2 Kbps GPRS : 115.2 – 182.4 Kbps (depending on the coding) | IS-136+: 43.2 Kbps | IS95A:14.4 Kbps IS95B:115.2 Kbps | 28.8 Kbps |
| Frequency hopping | Yes | No | N/A | No |
| Handover | Hard | Hard | Soft | Hard |
Comparison of Technologies
Frequency Division Multiple Access (FDMA):
Time Division to Multiple Access (TDMA):
Code Division Multiple Access ( CDMA):
Network Architecture
Mobile Station:
Cell Design
Base Station Sub-System (BSS):
Radio Interface Differences
- To transport user information, both speech and data – bi-directional.
- To exchange signaling information between the mobile station and the network.
Uplink and Downlink differences:
Long Code
Logical Channel differences
- Channel, which is used by the mobile to acquire the system. This is called the Pilot channel in CDMA whereas it is called the FCCH in GSM .
- A channel used by the mobile to synchronize to the network. This is called Synch channel in CDMA and in GSM it is called SCH.
- Channel to transmit the system wide information and also page the mobile for the termination calls. This in GSM is achieved by two channels called BCCH and PCH, where as in CDMA a single Paging channel does this.
- Traffic channels.
Call Processing
Evolution to 3G
Here is a brief summary changes for the evolution of each network.
GSM /GPRS to UMTS:
IS 95 to CDMA 2000:
Conclusion:
Reference:
- http://www.arcx.com/sites/index.htm
- GSM Wireless Networks – Nortel Networks Training Division
- IS –95 Overview – Award Solutions
- www.gmsworld.com
- The GSM systems for Mobile Communications – Michel Mouly
- Introduction to 3G Mobile Communications – Juha
I spend quite a bit of time reading the messages that flow through the various PCS newsgroups and forums on the Internet, and if one thing is abundantly clear, it is that people don't seem to know the true differences between CDMA and TDMA. And who could blame them? There is so much hype surrounding these two competing technologies that it is difficult for a regular PCS subscriber to know who is telling the truth.
I personally am NOT an RF engineer, nor do I work for any of the cellular or PCS companies. It is however my hobby to keep up with the latest developments in mobile communication (as this web site amply demonstrates). I would like to clear the air by interjecting my own spin on this debate. I hope that by the time you finish reading this editorial you will have a better understanding of the true strengths and weaknesses of both technologies.
The Basics
Let's begin by learning what these two acronyms stand for. TDMA stands for "Time Division Multiple Access", while CDMA stands for "Code Division Multiple Access". Three of the four words in each acronym are identical, since each technology essentially achieves the same goal, but by using different methods. Each strives to better utilize the radio spectrum by allowing multiple users to share the same physical channel. You heard that right. More than one person can carry on a conversation on the same frequency without causing interference. This is the magic of digital technology.
Where the two competing technologies differ is in the manner in which users share the common resource. TDMA does it by chopping up the channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in a round-robin fashion. In reality, only one person is actually using the channel at any given moment, but he or she only uses it for short bursts. He then gives up the channel momentarily to allow the other users to have their turn. This is very similar to how a computer with just one processor can seem to run multiple applications simultaneously.
CDMA on the hand really does let everyone transmit at the same time. Conventional wisdom would lead you to believe that this is simply not possible. Using conventional modulation techniques, it most certainly is impossible. What makes CDMA work is a special type of digital modulation called "Spread Spectrum". This form of modulation takes the user's stream of bits and splatters them across a very wide channel in a pseudo-random fashion. The "pseudo" part is very important here, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order.
If you are still having trouble understanding the differences though, perhaps this analogy will help you. This my own version of an excellent analogy provided by Qualcomm:
Imagine a room full of people, all trying to carry on one-on-one conversations. In TDMA each couple takes turns talking. They keep their turns short by saying only one sentence at a time. As there is never more than one person speaking in the room at any given moment, no one has to worry about being heard over the background din. In CDMA each couple talks at the same time, but they all use a different language. Because none of the listeners understand any language other than that of the individual to whom they are listening, the background din doesn't cause any real problem.
Voice Encoding
At this point many people confuse two distinctly different issues involved in the transmission of digital audio. The first is the WAY in which the stream of bits is delivered from one end to the other. This part of the "air interface" is what makes one technology different from another. The second is the compression algorithm used to squeeze the audio into as small a stream of bits as possible.
This latter component is known at the "Voice Coder", or Vocoder for short. Another term commonly used is CODEC, which is a similar word to modem. It combines the terms "COder" and "DECoder". Although each technology has chosen their own unique CODECs, there is no rule saying that one transmission method needs to use a specific CODEC. People often lump a technology's transmission method with its CODEC as though they were single entities. We will discuss CODECs in greater detail later on in this article.
Voice encoding schemes differ slightly in their approach to the problem. Because of this, certain types of human voice work better with some CODECs than they do with others. The point to remember is that all PCS CODECs are compromises of some sort. Since human voices have such a fantastic range of pitch and tonal depth, one cannot expect any single compromise to handle each one equally well. This inability to cope with all types of voice at the same level does lead some people to choose one technology over another.
All of the PCS technologies try to minimize battery consumption during calls by keeping the transmission of unnecessary data to a minimum. The phone decides whether or not you are presently speaking, or if the sound it hears is just background noise. If the phone determines that there is no intelligent data to transmit it blanks the audio and it reduces the transmitter duty cycle (in the case of TDMA) or the number of transmitted bits (in the case of CDMA). When the audio is blanked your caller would suddenly find themselves listening to "dead air", and this may cause them to think the call has dropped.
To avoid this psychological problem many service providers insert what is known as "Comfort Noise" during the blanked periods. Comfort Noise is synthesized white noise that tries to mimic the volume and structure of the real background noise. This fake background noise assures the caller that the connection is alive and well.
CDMA
Now that we have a rudimentary understanding of the two technologies, let's try and examine what advantages they provide. We'll begin with CDMA, since this newer technology has created the greatest "buzz" in the mobile communications industry.
One of the terms you'll hear in conjunction with CDMA is "Soft Handoff". A handoff occurs in any cellular system when your call switches from one cell site to another as you travel. In all other technologies this handoff occurs when the network informs your phone of the new channel to which it must switch. The phone then stops receiving and transmitting on the old channel, and it commences transmitting and receiving on the new channel. It goes without saying that this is known as a "Hard Handoff".
In CDMA however, every site are on the SAME frequency. In order to begin listening to a new site the phone only needs to change the pseudo-random sequence it uses to decode the desired data from the jumble of bits sent for everyone else. While a call is in progress the network chooses two or more alternate sites that it feels are handoff candidates. It simultaneously broadcasts a copy of your call on each of these sites. Your phone can then pick and choose between the different sources for your call, and move between them whenever it feels like it. It can even combine the data received from two or more different sites to ease the transition from one to the other.
This arrangement therefore puts the phone in almost complete control of the handoff process. Such an arrangement should ensure that there is always a new site primed and ready to take over the call at a moment's notice. In theory, this should put an end to dropped calls and audio interruptions during the handoff process. In practice it works quite well, but dropped calls are still a fact of life in a mobile environment. However, CDMA rarely drops a call due to a failed handoff.
A big problem facing CDMA systems is channel pollution. This occurs when signals from too many base stations are present at the subscriber's phone, but none are dominant. When this situation occurs the audio quality degrades rapidly, even when the signal seem otherwise very strong. Pollution occurs frequently in densely populated urban environments where service providers must build many sites in close proximity. Channel pollution can also result from massive multipath problems caused by many tall buildings. Taming pollution is a tuning and system design issue. It is up to the service provider to reduce this phenomenon as much as possible.
In defense of CDMA however, I should point out that the new EVRC CODEC is far more robust than either of the earlier CODECs. Because of its increased robustness it provides much more consistent audio in the face of high frame error rates. EVRC is an 8 kilobit CODEC that provides audio quality that is almost as good to the older 13 kilobit CODEC. Since CDMA consumes only as much of the "ether" as a user talks, switching everyone to an 8 kilobit CODEC was an inevitable move.
Supporters often cite capacity as one CDMA's biggest assets. Virtually no one disagrees that CDMA has a very high "spectral efficiency". It can accommodate more users per MHz of bandwidth than any other technology. What experts do not agree upon is by how much. Unlike other technologies, in which the capacity is fixed and easily computed, CDMA has what is known as "Soft Capacity". You can always add just one more caller to a CDMA channel, but once you get past a certain point you begin to pollute the channel such that it becomes difficult to retrieve an error-free data stream for any of the participants.
The ultimate capacity of a system is therefore dependent upon where you draw the line. How much degradation is a carrier willing to subject their subscribers to before they admit that they have run out of useable capacity? Even if someone does set a standard error rate at which these calculations are made, it does not mean that you personally will find the service particularly acceptable at that error rate.
Let's move away from CDMA now and have a look at TDMA. Before we can go any further though, I should note that there are actually three different flavors of TDMA in the PCS market. Each of these technologies implements TDMA in a slightly different way. The most complex implementation is, without a doubt, GSM. It overlays the basic TDMA principles with many innovations that reduce the potential problems inherent in the system.
To reduce the effects of co-channel interference, multipath, and fading, the GSM network can use something known as Frequency Hopping. This means that your call literally jumps from one channel to another at fairly short intervals. By doing this the likelihood of a given RF problem is randomized, and the effects are far less noticeable to the end user. Frequency Hopping is always available, but not mandated. This means that your GSM provider may or may not use it.
iDEN is a proprietary Motorola technology that no other company seems to participate in. Only Motorola makes iDEN phones, and only Motorola makes iDEN infrastructure equipment. Perhaps the company guards its technology on purpose. iDEN was initially deployed as an alternative to standard packet radio systems commonly used by public safety and business users. However, it also provided phone interconnect services that are extinguishable from phone services offered by the other PCS systems, as well as packet data services for web browsing and hooking up your laptop to the Internet.
Each of these TDMA technologies uses a different CODEC. GSM sports a CODEC called EFR (short for Enhanced Full Rate). This CODEC is arguable the best sounding one available in the PCS world. IS-136 used to sound horrible, but in the fall of 1997 they replaced their old CODEC with a new one. This new CODEC sounds much better than the old, but it doesn't quite match the GSM and CDMA entries.
TDMA systems still rely on the switch to determine when to perform a handoff. Unlike the old analog system however, the switch does not do this in a vacuum. The TDMA handset constantly monitors the signals coming from other sites, and it reports this information to the switch without the caller being aware of it. The switch then uses this information to make better handoff choices at more appropriate times.
Spectral Efficiency
Channel capacity in a TDMA system is fixed and indisputable. Each channel carries a finite number of "slots", and you can never accommodate a new caller once each of those slots is filled. Spectral efficiency varies from one technology to another, but computing a precise number is still a contentious issue. For example, GSM provides 8 slots in a channel 200 kHz wide, while iDEN provides 3 slots in a channel only 25 kHz wide. GSM therefore consumes 25 kHz per user, while IS-136 consumes only 8.333 kHz per user. When Direct Connect is used on iDEN, 6 users can be stuffed into a single channel, thus only 4.166 kHz is consumer per user. There is also a new 6:1 interconnect CODEC coming for iDEN which will allow 6 phone users per channel.
One would be sorely tempted to proclaim that iDEN has 3 to 6 times the capacity of GSM. In a one-cell system this is certainly true, but once we start deploying multiple cells and channel reuse the situation becomes more complex. Due to GSM's better error management and frequency hopping the interference of a co-channel site is greatly reduced. This allows frequencies to be reused more frequently without a degradation in the overall quality of the service.
Capacity is measured in "calls per cell per MHz". An GSM system using N=4 reuse (this means you have 4 different sets of frequencies to spread out around town) the figure is 5.0 We get an efficiency value of 6.6 fo
