Mobile Telephony: What Makes it Mobile?

This chapter is from the book

Essential Guide to RF and Wireless, The, 2nd Edition

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This chapter is from the book

This chapter is from the book 

Essential Guide to RF and Wireless, The, 2nd Edition

Learn More Buy

Cellular Evolution

Different Generations

Mobile telephony has evolved over the years reflecting the improvements in technology, specifically microelectronics and digital signal processing. The first generation of mobile telephony, dubbed 1G, modulated an analog signal onto an RF carrier and utilized something called circuit switched technology. Circuit switched technology establishes a persistent connection between the two communicators. (It is the same technology that is used in plain old telephone service [POTS].) Circuit switched technology makes for a good quality of service but an inefficient use of the equipment. For example, when two people establish a phone connection but nobody says anything for a prolonged period of time (think husband and wife fighting), the telephone line is still tied up. No one else can use it.

Analog modulation was used in 1G because it was easy to implement, given the state of the art at the time. Also, the supply of bandwidth exceeded the demand for bandwidth (back in the days of $500 cellular phones) and so efficient use of spectrum was not a priority. As mentioned previously, the only available air interface for analog is FDMA, in which the entire allotted bandwidth is divided into smaller frequency bands. When all 416 slots are in use, the next one in gets a busy signal. The 1G mobile phone service in the U.S. is called Advanced Mobile Phone Service or AMPS.

The best feature of first-generation cellular service was that every service provider throughout the United States used the same modulation and air interface. This uniformity led to the concept of roaming: using a cellular phone outside its home area. In effect, one cellular phone could be used everywhere. Unfortunately, analog technology quickly ran out of capacity. The identifying characteristics of all the mobile phone generations are summarized in Table 7–3.

Table 7–3 Characteristics of the Various Cellular Generations

The growing demand for cellular telephony, combined with advances in digital technology, led to the second generation of cellular telephony (2G). In 2G, a digital signal is modulated onto an RF carrier, but circuit switched technology is still employed. Even though the channels are still tied up, digital modulation allows for the use of novel new air interfaces (like TDMA and CDMA) and power management. In addition to longer talk time and accommodating more users, 2G offers other first-time mobile features such as conference calling and voice mail.

In 2G systems, FDMA is still used to divide the total bandwidth into smaller frequency chunks, but it is also combined with other air interfaces: either TDMA or CDMA. Second-generation data rates approach 14 Kbps (kilobits per second). In upgrading to the new 2G systems, service providers tried to use as much of their existing hardware and software as possible. Unfortunately, little could be used. Much of the RF and signal processing equipment needed to be upgraded. In the case of the RF hardware, it was mostly the result of the new digital signals requiring much higher linearity performance. And since all of the upgrades could not be done instantaneously, the result was a geographical area with part digital and part analog coverage. This led to the creation of the dual mode phone. These (expensive) dual mode phones communicate with both analog and digital signals (and switch between the two). They typically attempt to communicate digitally first, and, if that does not work, they switch to analog. Of course, to take advantage of the new digital technology, somebody (you) has to go out and buy a new mobile phone.

By far the biggest problem with upgrading to digital technology stems from the fact that there is more than one scheme from which to choose. Many of the service providers chose different technologies for different reasons. You may have already guessed the problem. The roaming feature that was so universal in analog cellular systems is not quite so universal in the 2G systems.

Another aspect of 2G (both in the U.S. and Europe) is the allocation of another frequency band. While 1G only offered service in the 900 MHz band (U.S), 2G offers service in both the 900 MHz and 1900 MHz bands (U.S.). This 2G 1900 MHz service is referred to as PCS (DCS in Europe), or more specifically wideband PCS. Wideband PCS is nothing more than 2G cellular technology at a slightly higher frequency. (Don't you believe any of that marketing stuff.)

Just when everyone was getting comfortable with 2G features and performance, along comes this thing called the wireless Internet. This led to what has become known as 2.5G. While 2.5G does utilize some more advanced modulation techniques (compared to 2G), the real change is in the use of packet switching technology, which is the same one used to access the Internet (using IP or Internet Protocol). Unlike circuit switching with its persistent connection, packet switching assigns a channel (i.e., frequency band) only as long as the user needs it and no longer. When the user is done transmitting or receiving information, they relinquish the channel so that someone else might use it. When the user needs a channel again, they get another one, but not necessarily the one they just gave up.

Packet switching is a much more efficient use of bandwidth than circuit switching. And besides, all data transferred to and from the Internet is done in (non-continuous) packets, so it is a perfect fit. What about telephone conversations? As things turn out, transferring voice information in packets is okay too (remember TDMA?).

The inherent efficiency of packet switching leads to more users (per bandwidth) and higher data rates, up to 144 Kbps for 2.5G. And while other features are available with 2.5G, the best new feature is the ability to surf the Internet from a mobile unit. In most cases, changes to the infrastructure for 2.5G are just bolted on to existing 2G networks. As a result, these 2.5G systems are not pure packet switched networks. In reality, data is being sent as packets over circuit switched networks. Pure packet switched wireless networks will not be available until the so-called third-generation networks or 3G.

The vision for 3G began in 1992. The vision was of a single global standard (at a single global frequency), which included using digital, packet-based networks to deliver bandwidth on demand and variable data rates up to 2 Mbps. With these lofty goals, it is envisioned that multimedia content could be delivered to the mobile unit (although who is going to watch a movie on a cellular phone is beyond me). This vision for 3G requires new packet switched networks and new frequency allocations.

Alas, the vision is not meant to be, at least not in its entirety. First, the United States refuses to allocate the agreeded upon spectrum (2000 MHz) for 3G services. Second, because incumbent service providers want to take advantage of their existing infrastructure and incumbent equipment providers want to take advantage of their existing product lines, there are at least three different paths to 3G utopia being pursued. All this will end up doing is making the semiconductor companies, which manufacture dual band and tri-mode devices, rich.

Paths of Migration to 3G

For many reasons, the world of cellular telephony evolved along three different paths in three different places: Europe, the United States, and Japan. All three started with their own analog standard, which at the time was fine since no one expected their cellular phone to work in some other country. But from those first-generation analog systems, as the world turned digital, things became really complicated.

Somebody got the idea that it would be really nice if there were a single digital technology deployed worldwide that allowed the use of a single mobile phone anywhere in the world (which has service). In steps the International Telecommunications Union or ITU. The ITU is the FCC of the world, with responsibility for allocating "international" frequencies. The ITU, along with all the member nations, started a program called IMT-2000 (International Mobile Telephone). The goal of the program is to develop a single, digital standard that will work all over the world. The IMT-2000 frequency allocation is situated between 1885 and 2200 MHz. Unfortunately, the fracturing that resulted from the switch to digital technology in 2G remains an obstacle in the way to 3G.

Europe

Europe started with several first-generation analog systems like TACS in England and NMT in the Scandinavian countries. These systems did not even operate at the same frequencies, so there was no use trying to get them to talk to each other.

Fortunately, when it came to upgrading to second-generation digital technology, Europe got it right and decided on a single technology for every country. That technology is GSM (Group Special Mobile), which uses a type of TDMA air interface and operates in just two bands: 800 MHz and 1800 MHz.

Not wanting to wait for a true 3G system to take hold (it could take years), Europe has more or less fashioned a path to 3G that consists of several intermediate steps. The first of these intermediate steps (2.5G) is something called General Packet Radio Services or GPRS. In oversimplified terms, GRPS just overlays packet switching on the existing GSM system. Recall that packet switching is more efficient than circuit switching and therefore allows more simultaneous users. Additionally, packet switching is more similar to the way data is transferred to and from the Internet, which makes this technology better for doing that.

The next intermediate step is called Enhanced Data GSM Environment or EDGE. (It can also stand for Enhanced Data Rates for Global Evolution.) Among other things, EDGE incorporates a modulation improvement to GRPS. Where GPRS uses GMSK modulation, EDGE uses 8 PSK modulation that delivers three times the data rate, which is a good thing when you are trying to watch a Lethal Weapon rerun on your mobile phone.

The "official" 3G system for Europe is Universal Mobile Telecommunications System or UMTS. The good news about UMTS is that it is a true packet switched network (none of this overlay stuff). The bad news is that it is at a different frequency (2000 MHz) than either 1G or 2G, which means big bucks to upgrade the system. Strangely, UMTS is not based on a TDMA air interface (like GSM), but rather on a CDMA air interface called Wideband CDMA or WCDMA.

WCDMA uses the same spread spectrum technology used by the North American CDMA, but has a 5 MHz bandwidth as opposed to a 1.25 MHz bandwidth, ergo the wideband. All of the 3G migration paths are summarized in Figure 7–16.

Figure 7–16 Migration paths to 3G.

The United States

Unlike Europe, the U.S. started with a single analog cellular standard (AMPS) in one frequency band (900 MHz). Ironically though, when it came to upgrading to 2G, the United States took a completely different tack and splintered into three different technological approaches. The three systems are IS-95 (also called cdma-One and based on a CDMA air interface), IS-54 (based on a GSM TDMA air interface), and IS-136 (also called D-AMPS and based on a different TDMA air interface). These three approaches are spread across two different frequency bands: 900 MHz and 1900 MHz.

As much as I would like to tell you that 2.5G or 3G will reunite the U.S into one uniform cellular technology, it will not. There will be two distinct migration paths to 3G in the United States. And to make matters worse, they will not be compatible with those in Europe.

The TDMA path will unite in 2.5G under the EDGE technology, which is the same one being used in Europe. So these 2.5G system will interoperate with those in Europe, albeit at slightly different frequencies. Unfortunately, as it migrates to 3G, it will not go to UMTS and it will not be in the 2 GHz band.

The CDMA path will go through a series of CDMA upgrades. The 2.5G version is called cdma2000 1x while the 3G version is called cdma2000 3x. Each of these represents an increase or improvement in signal processing, bandwidth, and/or modulation. If you guessed that this version of CDMA is incompatible with the WCDMA used in Europe, you are right. What a mess!

Japan

For the first two generations, Japan pretty much worked in isolation with their 1G analog system (JTACS) and their 2G digital system (PDC based on a TDMA air interface). Because of all this uniformity, it was possible for Japan to essentially jump over 2.5G and go right to a 3G system. As a result, Japan is the first country to deploy a system with 3G capabilities. The system is based on a WCDMA air interface similar to that in UMTS.

Ultimately, the goal for the ITU is to somehow harmonize one or more of these 3G systems to try and achieve their original vision: a single universal standard. Good luck.