WHAT’S INSIDE A CELL PHONE?
A cell phone can legitimately be called the world’s most complicated two-way radio. Sure, it is just a transceiver, but it uses practically every radio trick in the book to ensure that you can make your calls reliably anytime with mini- mal hassle. It is a two-way radio that is used just like any other phone you are familiar with. In other words, you do not have to say “over,” “come back,” or any other phrase when you are finished talking and want to listen, as with some two-way radios (CB, family radio, ham radio, aircraft, marine, etc.).
Simultaneous send and receive is referred to as full duplex. All telephones are full duplex. It is easy to do in the wired telephone system but complex and costly to do by radio. Yet, full duplex is what makes cell phones so comfort- able to use. There is none of this “roger and over” stuff. What this means is that your transmitter and receiver are working at the same time. There is more about how duplexing works later.
Figure 8.4 shows a general block diagram of what’s in a cell phone. The receiver and transmitter making up the transceiver are obvious and they share the single antenna. The transceiver is usually a single chip. What is not so obvious is what we call the baseband part of the cell phone. Baseband refers to the voice and data to be transmitted or received. In most modern digital cell phones, this is a very complex integrated circuit that handles the translation of voice between analog and digital, modulation and demodulation, and voice compression. All these functions are done digitally by a digital signal processor (DSP) or special DSP circuits in combination with a powerful embedded processor. This processor or a separate processor handles all the transparent house- keeping jobs of managing the keyboard and the LCD screen as well as the basic control features of the cell system, which also include automatic operating frequency selection and automatic power control under the direction of the MTSO via the cell site. The DSP also implements the messaging functions.
What Type of Cell Phone Do You Have?
Shame on you if you don’t know. Well, not really. Actually, it is tough to know just what type of cell phone you have. You can’t tell by looking, even inside, since even if you open up your cell phone, God forbid, all you see is a group of integrated circuits, and you can’t tell one from another. Furthermore, your cell phone carrier doesn’t tell you and usually your user’s manual never really says. You can probably ask by calling your carrier, but even then you may not get an answer. I guess the bottom line here is, do you really care? If it does what you want, what difference does it make what technology you are using? On the other hand, if you are an electronics geek, you want to know. I do.
So even though no one knows what type of phone you are using, it does impact the overall system and what phone features and capabilities you have. Multiple types of cell phone technologies are in use concurrently, and that is what makes the cell phone system so bloody complex and expensive. Most of the original carriers have to continue to support the older original technologies as well as implement new ones to stay competitive with other carriers offering their hot new data technologies and features such as email, instant messaging, games, Internet access, mobile TV, and the like.
The first characteristic of a cell phone is the frequencies it uses. Most cell phones operate in the 800- to 900-MHz band where there are hundreds of channels for cell phone calls. Another range of frequencies is the so-called personal communications system (PCS) band from 1850 to 1990 MHz (1.85 to GHz), which has hundreds more channels. Some of the newer phones use the advanced wireless services (AWS), 1700- and 2100-MHz spectrum assignments. Because of the higher frequency, PCS and AWS systems have a shorter range, and thus use smaller area cells, which means many more cells sites to cover a given area. Some cell phones actually operate on two or more bands.
Next, a cell phone is known by the technology and access methods it uses. The early original analog phones used first-generation (1G) technology, or FM, but have now been phased out so that only second-generation (2G) digital phones are in use. We are now in the third generation (3G) of cell phone technology and 4G is almost with us. A summary of these technologies follows.
GSM—The Global System for Mobile Communications (originally called Groupe Spécial Mobile) is the 2G digital system developed in Europe. Now all of Europe, most of the United States, and the rest of the world use this excellent but seriously complex technology. It uses time-division multiple access (TDMA), a digital system using digital modulation and a method of multiplying the number of channels in a given amount of frequency spectrum. Your voice is converted to binary 1’s and 0’s before being transmitted. This allows the carrier to put eight subscribers in one 200-kHz-wide frequency channel, thereby greatly multiplying their subscriber capacity and income. GSM uses a form of FSK called Gaussian minimum shift keying (GMSK). Today, U.S. carriers ATT Wireless and T-Mobile use GSM. By going to
GSM, these carriers can take advantage of several enhancements that facilitate high-speed data transmissions such as the General Packet Radio System (GPRS) and Enhanced Data Rate for GSM Evolution (EDGE).
GPRS—This is an extension of GSM that gives users an always-on packet data transmission capability that they can use for email, Internet access, messaging, or games. GPRS steals one or more of the eight TDMA channels to transmit or receive data instead of digital voice. You can achieve a data rate of about 20 to 60 kbps. GPRS is known as a two-and-a-half generation (2.5G) technology, and is generally available in most larger cities. EDGE—This 2.5G system is a software upgrade from GPRS/GSM systems that gives you even higher packet data rates. It uses a multilevel modulation called 8PSK that triples the speed of data transmission. Practical speeds up to 180 kbps are possible.
CDMA—Code division multiple access is another 2G digital cell phone technology developed by San Diego–based Qualcomm Inc. The basic technology is known as spread spectrum where many signals are trans- mitted simultaneously over a very wide 1.25-MHz frequency band with- out interfering with one another. The original standard is called IS-95, and CDMA signals coexist in the same spectrum with TDMA systems in many areas.
cdma2000—This is the 2.5G version of CDMA. It adds high-speed packet data transmission called 1xRTT with a data rate up to 144 kbps. A more recent upgrade called 1xRTT EV-DO has even higher data rates up to 2 Mbps, making Internet access a cinch. Many cdma2000 systems are in operation around the United States by Verizon and Sprint and in Korea. WCDMA—This is wideband CDMA, which is a third-generation (3G) technology. The access technology is CDMA, but it uses wider 5-MHz bands to provide greater user capacity and very high potential data rates from 384 kbps to 2 Mbps. WCDMA is the upgrade from GSM/EDGE. Carriers using GSM upgraded their systems to WCDMA for greater data speeds and services at greater subscriber capacity. Base stations support both GSM and WCDMA.
HSDPA/HSUPA—High-speed downlink packet access (HSDPA) and high- speed uplink packet access (HSUPA) are upgrades from the WCDMA sys- tem. Both use QAM to boost data speeds in the same 5-MHz channels. Data rates as high as 14 Mbps down-link and 5 Mbps up-link are possible. Most cellular companies have now implemented this technology. More advanced versions called HSPA or HSPA+ can give even higher data rates. cdma2000 Rev A and Rev B—These are more advanced CDMA versions that are used to boost data speeds in cdma2000 handsets. Rev A boosts download data rates to 3.1 Mbps using QPSK and 16-QAM in the standard 1.25-MHz channel. Rev B is not widely implemented, but can use three 1.25-MHz channels and boost download speeds to 14.7 Mbps.
Long-Term Evolution—Long-Term Evolution (LTE) is the next or fourth- generation (4G) cell phone technology. It is just now being implemented. It is appearing in Europe first and the United States later. It uses an entirely different radio technology called orthogonal frequency division multiplexing (OFDM) to give even higher data rates up to 100 Mbps. More on that later.
How can you tell which of the above phone technologies you have? You cannot tell by the manufacturer, such as RIM BlackBerry, Samsung, LG, Motorola, Nokia, Sony Ericsson, and so on, because they all make multiple types. However, a few clues follow.
Carriers AT&T and T-Mobile provide GSM/WCDMA phones. Most also have some form of HSPA. These carriers will eventually morph their systems to LTE. Carriers Sprint and Verizon use cdma2000 phones. Most phones today are also what we call multimode phones in that they incorporate two or more different technologies. The most common combination is GSM/WCDMA/HSPA.