Mobile Phones
  1. AMPS Analog FM Modulation
  2. TDMA Time Division Multiplexing
  3. GSM Global System for Mobiles
  4. CDMA Code Division Multiplexing
  5. Choosing a Cellular Provide and Cell Phone

 

 

Cellular telephone service is based on a simple idea, which turned two-way radio into a circuit switched telephone network comparable in functionality to its landline counterpart. Before cellular, many companies offered two-way radio services which bridged radio users to the public telephone network. This approach worked fine, except that one base station would cover a large territory. Thus, subscriber capacity was limited, and there was essentially no security on the wireless network.

 

Cellular telephony modified this approach by dividing a coverage area into many smaller zones (cells), which are served by a low power two-way radio base station. Telephone handsets (original phones were more like suitcase sets) would continuously search for the closest base station, and could hop from base station to base station if the user was on the move. This strategy of replacing one high power base station with many low power base stations distributed throughout the coverage area made it possible to handle far more subscribers, and reduced the power requirements for wireless telephones. Reducing the amount of power needed to carry a conversation through the airwaves, made it possible for manufacturers to produce much smaller telephones, which is why we now have handsets, which fit into a shirt pocket. While solid state circuitry advances have played an important role in reducing the size and weight of portable phones, the reduction of power requirements eliminated the need for bulky, heavy batteries to power handsets.

 

The first cellular telephone network put to widespread use in the United States was an analog system called AMPS. It is still the most widely used cellular infrastructure in the United States and Canada, although digital networks are beginning to make inroads. AMPS is an analog, frequency modulation scheme (the same scheme used in FM radio). At the time cellular was introduced in the United States, FM modulation/demodulation circuitry was a commodity product, and so this was the most cost effective way to introduce cellular service. Digital cellular did not become cost effective until the 1990s when DSP based platforms came down in price enough to make it practical to create an all-digital telephone handset for under $1000.

 

Analog cellular has some major limitations; among the most important are power consumption and sound quality. An AMPS telephone is essentially a miniature FM radio station that broadcasts at a power ranging from 0.1 Watt to 3 Watts. This might not sound like much, but this type of output can quickly drain a battery. So, AMPS telephones are usually limited to an hour or two of talk time, and 8 to 20 hours of standby time. Equally as important, the analog modulation scheme is vulnerable to interference and cross-talk, which is why analog cell phone calls are often filled with static and cross-talk from other subscribers.

 

Digital cellular service, which was introduced in the US and Europe in the 1990s, provides improved sound quality, enhanced services such as Caller ID, voice message notification, and better security. However, both the US and European versions of digital cellular now in use, TDMA and GSM respectively, are really first generation services. They were designed to make use of the components, which were widely and cheaply available at their time of deployment, and have some limitations of their own.

 

Enter CDMA, a second-generation digital cellular service which, despite the rhetoric you may have heard to the contrary, offers substantial improvements over TDMA and GSM cellular networks.

 

Unfortunately, none of these standards are compatible, and nobody manufactures a cellular phone, which supports all of them. This article will help you select the system which will work best for you.

 

1.      AMPS - Analog FM Modulation

 

AMPS is the FM modulation scheme described earlier in this article. Because it is analog, it requires more power and is susceptible to interference.

 

Despite its shortcomings, AMPS coverage is available throughout most of North America. This makes it a least common denominator service, which can be used when you are outside of your home coverage area. Unless digital coverage is unavailable in your home coverage area, you should not subscribe to analog-only service. All digital cellular phones manufactured today are dual mode phones. They will use the digital network if it is available. If not, they will revert to the analog network, which, in North America, is AMPS.

 

2.      TDMA - Time Division Multiplexing

 

TDMA is the most common type of digital cellular service available in the United States. Carriers such as Cellular One began introducing TDMA service in major cities several years ago, and now have fairly extensive digital networks in place. TDMA stands for time division multiplexing algorithm. TDMA works by performing the following steps:

1. Digitizing your voice into a stream of 8 bit samples

2. Compressing this data using algorithms similar to those used in Internet telephony applications

3. Introducing error correction data, which can be used to repair corrupted data on the receiving end

4. Breaking the data stream into frames which are interwoven with data from other users

 

The reverse process is performed on the opposite end of the call. This process is very similar to time division multiplexing performed on T1 and E1 digital telephone circuits. The main difference between the cellular and landline networks is the cellular networks employ compression and error correction algorithms designed to optimize performance over an unreliable virtual connection.

 

TDMA, while it is digital, is not the panacea it may sound like. It does substantially reduce the power requirements for telephone handsets, and that has a positive effect on battery life. However, the sound quality on TDMA networks is not what you will get on a landline. The reason for this has to do with the way the data is transmitted.

 

TDMA relies on a single virtual pipe, or band of frequencies, to carry data from handset to base station. Because data is sent on a relatively narrow band of frequencies, the transmission is vulnerable to interference from a variety of sources. The problem with TDMA is that interference from another transmission source, reflection from buildings, etc, will disrupt the transmission for an extended period of time (several hundred milliseconds or longer). There is no ideal solution to dealing with these dropouts. You can either introduce a long delay to permit the retransmission of corrupted data (which is like talking over a multi-hop satellite call), or simply leave the errors uncorrected.

 

If you are in a good reception area, TDMA works great. However, if you are in a marginal reception area, you may actually find that analog service sounds better. The reason is that the static encountered in analog calls is annoying, but usually does not completely garble the underlying signal. Just as you can make out a song on a noisy FM radio station, you can still understand what someone is saying on a noisy analog circuit. With TDMA, however, marginal reception can often render a conversation completely unintelligible. Errors in an analog transmission manifest themselves as static. In a digital transmission, a few lost bits can completely corrupt a frame, and so the equipment on the receiving end produces unintelligible garbage.

 

I would go with GSM or CDMA over TDMA any time if the others are available in your area and offer good service plans.

3.      GSM - Global System for Mobiles

 

GSM is similar to TDMA in its overall operation. It is the overseas answer to digital cellular, and has been widely adopted outside the United States (900 Mhz is the international standard) and now finally in the last 3 years 200-2002 is also becoming a major player in the USA on the 1900 Mhz standard.

 

One of the principle benefits of GSM is its international roaming capability. A GSM user in the UK, for example, can travel to South Africa, turn on his phone, and receive his calls as if he were still in his home coverage area. Of course, there is no reason you can't do this with TDMA except that TDMA hasn't been widely adopted outside the US and so there aren't many other systems to roam to.

 

In general, the behavior of GSM handsets is similar to that of TDMA handsets. If you are in a good coverage area, they work well. If you wander into a marginal coverage area, everything goes to hell in a handbasket.

 

For a listing of all the new GSM phones available internationally click here. If you frequently travel internationally the best type of phone to have is a Tri or Dual Band GSM Phone this gives you the ability to operate on GSM networks in Europe, the Middle East, Africa, Asia, the Americas or Australia (essentially as good as a satellite telephone without the cost).  The main US GSM companies are T-Mobile (formerly Voicestream) and Cingular and now finally AT&T has been porting over to GSM in 2002 giving you a huge player in the field.  Remember, however, that the beauty of GSM is that you have a smart chip that can be moved from phone to phone letting you get away with a lot of stuff.  For example, if you buy a world cell phone in the UK and bring it back with you to the USA or Canada and put in a GSM chip from a local providor like Cingular, T-Mobile or Fido it will work great.  In Canada for GSM, Microcell or Fido was the original and only GSM service provider but as in the USA AT&T in the Form of Rogers AT&T has entered the GSM Market now as well.  In the USA GSM is also provided by other smaller local providers listed here.

 

 

4.      CDMA - Code Division Multiplexing

 

CDMA is the next generation version of digital cellular, and it is very impressive. CDMA is a spread spectrum system, which works quite differently from older digital systems. The best way to explain this is through analogy. CDMA takes your digitized voice and breaks it into packets. Rather than transmitting all of this data on one frequency, a CDMA phone transmits the data on many frequencies simultaneously. Older generation phones transmit data on a relatively narrow band of frequencies.

 

The benefit of the CDMA approach is that radio frequency interference usually affects transmissions at specific frequencies. So, if you break a message down and send pieces of it over a wide band of frequencies, interference may corrupt part of the message, but is very unlikely to corrupt all of it. This means that even under bad conditions, most of the message is received intact on the other end.

 

To further improve reliability, CDMA phones introduce some redundancy into packets prior to transmission.

 

This forward error correction technique allows the receiver to reconstruct lost or damaged packets in many cases, and thus further minimizes the impact of interference in marginal reception areas.

 

In terms of sound quality, CDMA beats the other standards hands down. All you need to do is walk or drive around a city while on your phone, and you will notice a substantial improvement. The sound is generally clearer, with far fewer drop outs. Most importantly, the sound quality degrades gracefully when you get into a bad reception area. Rather than dropping out or becoming completely garbled, the audio takes on a sort of warbling quality. Unless the reception becomes very poor, you can still make out what the other person is saying and vice versa. With TDMA and GSM, the audio will abruptly cut out or become garbled through the course of a conversation.

 

CDMA also boasts improved security because the data sent to/from the phone is divided over many frequencies, and also encrypted. This makes it much more difficult to pinch a phone's serial number out of the air and clone it.

 

The biggest CDMA provider in the USA is Sprint PCS

 

6.      Choosing A Cellular Provider and Cell Phone

Choosing a cellular provider is mostly a matter of where you live, availability of digital services, and your roaming patterns.   Often in the USA your choice of telephone is restricted by your choice of cellular provider, unless you have a very strong phone preference and do things the other way round.   The best internet services to help you make your pick are Point.com   or  Wireless Advisor.

 

Generally speaking, go with a provider in your city who offers either GSM or CDMA based service and has the best coverage and reputation and best plan to suit your needs.  While roaming is important, most cellular users spend most of their time in their home coverage area. With a CDMA carrier, you will get highly reliable service while you are in your home area, and can use AMPS when you are outside your home coverage area.  If a CDMA or GSM operator is not available, then your next best bet is a TDMA.  If you roam internationally, GSM is worth a serious look because you will be able to use your phone internationally. With TDMA and CDMA networks, you have to rent another phone to use when you travel overseas. Do not sign up with a carrier who only offers analog service.

 

 

AMPS

TDMA

GSM

CDMA

Sound Quality - Good Coverage Areas

Good

Excellent

Excellent

Excellent

Sound Quality - Marginal Reception Areas

Fair

Varies Widely

Varies Widely

Good

Sound Quality Degrades Gracefully In Marginal Areas

Sort of, you can usually hear over static

No, audio becomes garbled or drops out

No, audio becomes garbled or drops out

Yes

Availability

Throughout North America

Most North American metropolitan areas

Getting better and better

Getting better and better

Regional Roaming (US, Canada, Mexico)

Yes

Throughout North America, with rollover to AMPS

Throughout North America, with rollover to AMPS

Throughout North America with rollover to AMPS

International Roaming

No

Yes, with rental of GSM phone some carriers offer international roaming

Yes, worldwide roaming to dozens of GSM countries

Yes, with rental of GSM phone some carriers offer international roaming

Enhanced Services (i.e. Caller ID, paging, message notification)

No

Yes

Yes

Yes

Data Capability

No, except using cellular modem, doesn't work well

Yes

Yes

Yes