Cellular Telephone Systems
A cellular radio system provides standard telephone service by two-way radio at remote locations. Cellular radios or telephones were originally installed in cars or trucks, but today most are handheld models. Cellular telephones permit users to link up with the standard telephone system, which permits calls to any part of the world.
The Bell Telephone Company division of AT&T developed the cellular radio system during the 1970s and fully implemented it during the early 1980s. Today, cellular radio telephone service is available worldwide. The original U.S. cell phone system, known as the advanced mobile phone system, or AMPS, was based on analog FM radio technologies. AMPS has gradually been phased out and replaced by second-generation (2G), third-generation (3G), and fourth-generation (4G) digital cell phone systems. This section provides an overview of this awesome worldwide network.
The basic concept behind the cellular radio system is that rather than serving a given geographic area with a single transmitter and receiver, the system divides the service area into many smaller areas known as cells, as shown in Fig. 20-1. The typical cell covers only several square miles and contains its own receiver and low-power transmitter. The coverage of a cell depends upon the density (number) of users in a given area.
See Fig. 20-2. For a heavily populated city, many small cells are used to ensure service. In less populated rural areas, fewer cells are used. Short cell antenna towers limit the cell coverage area. Higher towers give broader coverage. The cell site is designed to reliably serve only persons and vehicles in its small cell area.
Each cell is connected by telephone lines or a microwave radio relay link to a master control center known as the mobile telephone switching office (MTSO). The MTSO controls all the cells and provides the interface between each cell and the main telephone office. As the person with the cell phone passes through a cell, it is served by the cell transceiver. The telephone call is routed through the MTSO and to the standard telephone system. As the person moves, the system automatically switches from one cell to the next. The receiver in each cell station continuously monitors the signal strength of the mobile unit. When the signal strength drops below the desired level, it automatically seeks a cell where the signal from the mobile unit is stronger. The computer at the MTSO causes the transmission from the person to be switched from the weaker cell to the stronger cell. This is called a handoff. All this takes place in a very short time and is completely unnoticeable to the user. The result is that optimum transmission and reception are obtained.
Cellular radio systems operate in the UHF and microwave bands as assigned by the Federal Communications Commission (FCC). The original frequency assignments were in the 800- to 900-MHz range previously occupied by the mostly unused UHF TV channels 68 through 83. Fig. 20-3 shows the most widely used bands. The frequencies between 824 and 849 MHz are reserved for the uplink transmissions from the cell phone to the base station. These are also called the reverse channels. The frequencies between 869 and 894 MHz are the downlink bands from base station to cell phone. Both of these25-MHz segments of the spectrum were originally divided into 832 channels 30 kHz wide.
While these are still used, the different cell phone technologies use different amounts of bandwidth, such as 30 kHz, 200 kHz, and 1.25 MHz, so this spectrum gets used in different ways by different cell phone companies in different locations.
Another commonly used block of spectrum is shown in Fig. 20-4(a). Again, the use of this spectrum varies depending upon the cell phone carrier and the geographic area. A more recently allocated block of spectrum is shown in Fig. 20-4(b). These two blocks of 60 MHz are referred to as the personal communications systems (PCS) channels.
While the range at these higher microwave frequencies is somewhat less than that achievable in the UHF bands, this block of frequencies provides greater system capacity, meaning more subscribers. Also the antennas are smaller at these frequencies.
One of the major issues in the cell phone business lies in obtaining more spectrum for more subscribers. More subscribers mean greater income. Yet, spectrum is scarce and very expensive. Now spectrum is available in the 700- to 800-MHz range. More recently, there is available space in the 1700- to 1750-MHz range. Some spectrum is also available in the 1900- to 2300-MHz and 2500- to 2700-MHz range for newer 4G systems. A new band near 3650 MHz is also available. Also keep in mind that different countries use different spectrum blocks. For example, in Europe the most commonly used bands are 900 and 1800 MHz.
Multiple access refers to how the subscribers are allocated to the assigned frequency spectrum. Access methods are the ways in which many users share a limited amount of spectrum. These are similar to multiplexing methods you learned about in previous chapters. The techniques include frequency reuse, frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), and spatial-division multiple access (SDMA).
In frequency reuse, individual frequency bands are shared by multiple base stations and users. This is possible by ensuring that one subscriber or base station does not interfere with any others. This is achieved by controlling such factors as transmission power, base station spacing, and antenna height and radiation patterns. With low-power and lower-height antennas, the range of a signal is restricted to only a mile or so. Furthermore, most base stations use sectorized antennas with 1200radiation patterns that transmit and receive over only a portion of the area they cover. See Fig. 20-5. In any given city, the same frequencies are used over and over simply by keeping cell site base stations isolated from one another.
Frequency-Division Multiple Access
FDMA systems are like frequency- division multiplexing in that they allow many users to share a block of spectrum by simply dividing it up into many smaller channels. See Fig. 20-6. Each channel of a band is given an assigned number or is designated by the center frequency of the channel. One subscriber is assigned to each channel. Typical channel widths are 30 kHz, 200 kHz, 1.25 MHz, and 5 MHz. There are usually two similar bands, one for uplink and the other for downlink.
Time-Division Multiple Access
TDMA relies on digital signals and operates on a single channel. Multiple users use different time slots. Because the audio signal is sampled at a rapid rate, the data words can be interleaved into different time slots, as Fig. 20-7 shows. Of the two common TDMA systems in use, one allows three users per frequency channel and the other allows eight users per channel.
Code-Division Multiple Access
CDMA is just another name for spread spectrum. A high percentage of cell phone systems use direct sequence spread spectrum (DSSS). Here the digital audio signals are encoded in a circuit called a vocoder to produce a 13-kbps serial digital compressed voice signal. It is then combined with a higher-frequency chipping signal. One system uses a 1.288-Mbps chipping signal to encode the audio, spreading the signal over a 1.25-MHz channel. See Fig. 20-8. With unique coding, up to 64 subscribers can share a 1.25-MHz channel. A similar technique is used with the wideband CDMA system of third-generation cellphones. A 3.84-Mbps chipping rate is used in a 5-MHz channel to accommodate multiple users.
Orthogonal Frequency Division Multiplexing Access (OFDMA)
OFDMA is the access method used with OFDM. OFDM uses hundreds, even thousands, of subcarriers in a wideband channel. This large number of subcarriers can be subdivided into smaller groups, and each group can be assigned to an individual user. In this way, many users can use the wideband channel assigned to the OFDM signal.
Fig. 20-9 illustrates the principle. Groups of subcarriers are formed to create a subchannel assigned to one user. In some systems, the subcarriers do not have to be contiguous, but instead might be spread around inside the total OFDM signal bandwidth.
Spatial-Division Multiple Access
This form of access is actually an extension of frequency reuse. It uses highly directional antennas to pinpoint users and reject others on the same frequency. In Fig. 20-10, very narrow antenna beams at the cell site base station are able to lock in on one subscriber but block another while both subscribers are using the same frequency. Modern antenna technology using adaptive phased arrays is making this possible. Such antennas allow cell phone carriers to expand the number of subscribers by more aggressive frequency reuse because fi ner discrimination can be achieved with the antennas. SDMA is also widely used in wireless local-area networks (WLANs) and other broadband wireless applications.
Duplexing refers to the ways in which two-way radio or telephone conversations are handled. Many two-way radio applications still use half duplex where one party talks at a time. The communicating individuals take turns speaking and listening. Telephone communications has always been full duplex, where both parties can simultaneously send and receive. All cell phone systems are full duplex.
To achieve full duplex operation, however, special arrangements must be made. The most common arrangement is called frequency-division duplexing (FDD). In FDD, separate frequency channels are assigned for the transmit and receive functions. The transmit and receive channels are spaced so that they do not interfere with one another inside the cell phone or base station circuits. The uplink and downlink channels in Figs. 20-3 and 20-4 are an example.
Another arrangement is time-division duplexing (TDD). This is less common but is used in a few systems. The system assigns the transmit and receive data to different time slots, both on the same frequency. For example, the transmitted and received data is alternated in sequential time slots. While the transmitted and received signals do indeed occur at different times, the speed of the signals is fast enough that a human feels as though they are occurring at the same time. The primary benefi t of TDD is that only one channel is needed. With FDD two separated channels are required. A TDD signal uses half the spectrum of FDD.
Still, FDD is far more widely used than TDD.
A Cellular Industry Overview
The cellular telephone industry is one of the largest industries in the world. Also known as the wireless or mobile business, it generates billions of dollars in revenue each year for the telephone companies, the cell phone manufacturers, and other allied businesses. And as you probably know, the cell phone has become the world’s most popular consumer product. Over one billion new cells phones are sold each year. In developed countries, most consumers already have a cell phone of some type. China is the largest user of cell phones; the United States in second place in terms of total number of users.
The cell phone is so pervasive that it has greatly affected how we live and work and how we do business. This section takes a brief look at how the cellular industry functions and some of the trends and critical issues associated with it.
The Cellular Carriers
Central to the functioning of the mobile wireless business are the cellular carriers, also known as the network operators. In the United States, there are four major carriers: AT&T, Verizon, Sprint, and T-Mobile. There are also a number of smaller carriers, but the four mentioned represent over 90 percent of the business. These are the organizations that build out the cellular network infrastructure and provide the mobile services. They are also the ones who offer the cell phones to the consumer. Even though the carriers do not design or build the cell phones themselves, they are very much a part of the design process.
The other major players are the cell phone companies themselves. The leaders are Samsung and Apple; other players include LG, HTC, Nokia/Microsoft, Google/Motorola,and newer players such as Huawei and Lenovo. These companies design and build cell phones to the specifications provided by the carriers.
The reason the carriers have so much influence in the phones themselves relates to their frequency-spectrum holdings. Each carrier has access to its own specific range of frequency bands, and phones must be made to match their holdings. This means that for the most part a cell phone purchased from AT&T will not work on the Verizon network and so on. Another factor is that some carriers use different second-and third-generation technologies that are not compatible from carrier to carrier.
Other players in the industry include the software companies that provide the operating systems for the cell phones. The leaders are Google with its Android operating system, Apple with its iOS software, and Microsoft. There are also many companies offering additional software in the form of applications (apps) that give the cell phone far more capability than was ever imagined at first. You can actually look at the cell phone as a general-purpose high-power microcomputer with a radio built in. Most of the software apps do not use the radio communications capability. The semiconductor companies are also major players. They develop and provide the chips to implement the cell phones and the base station equipment.
The Technology Generations
The cellular industry is driven not only by customer demand for service but also by the available technology. In this regard, the cellular industry is one of the fastest changes in existence. New products and services are offered as rapidly as they are conceived and developed. Changes in the technological standards also force rapid changes. The technology itself is divided into and defined by generations.
The first-generation cell phones used analog technology with frequency modulation. This was rapidly abandoned as it was determined that carriers could not provide enough services to meet the demand. As a result, the first generation was quickly replaced with a newer, second-generation (2G) digital cell phone technology.
Multiple 2G standards were developed worldwide, yet only two major technologies and standards have survived to this day. These include GSM and the original CDMA. Although 2G technology is still in use today around the world, it is slowly fading away and many carriers have stated their desire to end second-generation services in the near future. This allows them to repurpose their precious spectrum holding for greater subscriber capacity and higher data rates.
A key factor was the development of data services, which happened during the second generation. Cell phones originally were designed primarily as voice telephones, but it was quickly discovered that it was possible to use them for data purposes. For 2G phones, data rates were slow, thereby limiting the functions to simple applications such as texting and e-mail. Once greater data capabilities were discovered, the demand for higher speeds and more exotic applications developed. This led to the creation of thirdgeneration cell phones.
Third-generation (3G) cell phones continued to use standard digital voice techniques but also developed high-speed data capability. New modulation and access methods were created and standards were ratified. Third-generation phones rapidly became popular, and over a period of several years carriers adopted the new technology and built out their networks. New cell phones were developed to take advantage of the applications potential.
During this time, several competing 3G technologies were created. These incompatible technologies were adopted by different carriers, making cell phones and the networks incompatible. This ultimately led to the desire for a complete industry standard, which in turn led to the development of fourth-generation phones and systems. The fourth generation (4G) has brought about the creation of a single standard or family of standards that all carriers could adopt. This 4G technology is known as Long Term Evolution (LTE), and it is slowly being adopted in one form or another by all U.S. and worldwide carriers. While most of the major carriers have already converted to the4G LTE technology, the process is still rolling out across the United States and the progress is widely varied worldwide.
The 4G systems and phones have led to much higher data rates and amazing new cell phone capabilities, particularly that of being able to receive and generate video. Technology advances have given us not only the high-speed data capability necessary for video but also large, color touch screens, making the cell phone a more popular consumer product than ever.
While most carriers are still implementing 4G LTE systems in the States and in other countries, work is already under way to define the next, fifth generation. The main purpose of 5G is to make the cellular and data services available over a wider range and to provide even higher data speeds.
Trends and Critical Issues
The basic trend in the cellular industry is continued growth and technological development. In the developing countries, market saturation has already been achieved, meaning that most individuals already have cell phones and services. This has the effect of reducing the growth rate, but it still provides an opportunity for consumers to upgrade to improved phones and services. In addition, substantial growth is still available in other parts of the world, especially undeveloped countries where telephone service is not typically available. In many parts of the world, cellular telephone service is the only telephone service available.
Another major trend is the decline in the number of wired telephone customers. Many consumers have already abandoned their basic wired telephones in favor of using only cell phones. Today over 50 percent of consumers now use cellular telephones as their primary communications service and that trend appears to continue.
Another trend is that carriers are continuing to build out their networks and improve data rates in an effort to reach more consumers and capture more business. Keeping in step with that, the cell phone manufacturers continue to provide even more sophisticated and capable cell phones that consumers desire.
As for critical issues, the primary one is frequency spectrum availability. Only certain parts of the frequency spectrum (roughly 600 MHz to 4 GHz) are useful for cellular service, and this frequency spectrum is already heavily occupied. The ability of a carrier to continue to offer expanded service capacity as well as higher speeds is determined strictly by the amount of spectrum available. High capacity and high speed require wider bandwidth. However, there is only a certain amount of spectrum available, and most of it is already owned and occupied. Today carriers who want to expand their operations must buy spectrum from others or purchase it through the regulatory agency, the Federal Communications Commission (FCC), in their regular auctions. Spectrum costs billions of dollars, and only the largest of carriers can afford it.
To solve this problem, the FCC has been attempting to free up spectrum from other spectrum holders, such as the government, the military, and the TV broadcasters. The TV broadcasters gave up spectrum during the changeover to digital TV in 2009. The 700- to 800-MHz band is now available and is already being used by some of the cellular carriers. Forthcoming is another segment of spectrum in the 600- to 700-MHz range, which will be auctioned off to the highest bidders. Eventually, higher frequencies are going to have to be used to achieve improved service and data rates. The 5G standards are already looking to the millimeter wave bands (28 to 70 GHz) as a potential solution to the need for increased spectrum holdings.
Another issue is how long older technology should continue to be supported. With today’s high-speed LTE 4G capability, most voice calls are still handled by the carriers’ 2G and 3G switched-circuit networks. Soon these will be phased out in favor of internet protocol (IP) voice services such as voice over LTE (VoLTE). This has yet to be fully implemented by the carriers. And few cell phones support it. Yet it is the voice method of the future. In the meantime, all cell phones, despite the fact they may be 4G LTE capable, must continue to carry 2G and/or 3G capability to provide voice service.
2G and 3G Digital Cell Phone Systems
The original cellular technology AMPS used FM analog communications. However, today all new cell phones and systems use digital methods. These all-digital systems were developed primarily to expand the capacity of the cell phone systems already in place. The rapid growth of the number of wireless subscribers forced the carriers to seek new and more efficient methods of increasing the number of users a system could handle. The main problem was that the carriers were restricted by the Federal Communications Commission to specific segments of the frequency spectrum. No additional space was available for expansion. Digital techniques provide several ways to multiplex many users into the same spectrum space.
The use of digital techniques brought several additional benefits. Digital communication systems are inherently more robust than analog systems in that they are more reliable in a noisy environment. Furthermore, digital circuits can be made smaller and more power-efficient, and therefore handsets can be more compact and can operate for longer times on a single battery charge. In addition, digital cell phones greatly facilitate the transmission of data as well as voice, so that data services such as e-mail and Internet access are possible with a cell phone. Digital methods also offer high-speed data capability making video, gaming, and social media applications possible.
The first digital cell phones are referred to as second-generation (2G) phones. Today, third- generation (3G) and fourth-generation (4G) cell phones and systems are in use. But 2G phones are still in use in the United States and around the world. The 2G phones will eventually be phased out. This section covers the 2G systems still in use and describes the various 3G systems that have mostly replaced 2G technology. 4G phone are discussed in a separate section.
2G Cell Phone Systems
Three basic second-generation (2G) digital cell phone systems were developed and deployed. Two use time-division multiplexing, and the third uses spread spectrum (SS) or CDMA. The TDM systems are the Global System for Mobile Communications (GSM) and the IS-136 standard for time-division multiple access. The SS system is code-division multiple access. The IS-136 system was phased out early and replaced with GSM. Both GSM and CDMA are still widely deployed throughout the world.
To use digital data transmission techniques first requires that the voice be digitized. The circuit that does this is a vocoder, a special type of analog-to-digital (A/D) converter, and digital-to-analog (D/A) converter. With voice frequencies as high as 4 kHz, the minimum Nyquist sampling rate is two times the highest frequency, or 8 kHz. This means that the A/D converter in a vocoder should sample the voice signal every 125 μs and generate a proportional binary word. Assuming that it is an 8-bit value, during the 125-μs period, the 8 bits is transmitted serially. This translates to a serial data rate of 125/8 = 15.625 μs/bit, or 1/15.625 X 10-6 = 64 kbps. This is how the T1 telephone system described.
This serial data signal, representing the voice, is now used to modulate the carrier and the composite signal transmitted over the assigned channel. Recall that the bandwidth required to transmit a digital signal depends primarily upon the data rate. The higher the data rate, the wider the bandwidth required. As a rule of thumb, the bandwidth is roughly equal to the data rate. For example, a 64-kbps signal would require about 64 kHz of bandwidth. That represents 1 bit/Hz. Different modulation methods result in different degrees of data rate per bandwidth. Some are more spectrally efficient than others. A 1-bit/Hz rating is essentially wasteful of precious spectrum space. If the 30-kHz AMPS channels are to be used to transmit 64-kbps voice, a more efficient modulation scheme is needed, or some other technique is required.
The main function of a vocoder is data compression. Data compression techniques are used to process the digitized voice signal in such a way as to reduce the number of bits needed to represent the voice reliably. This in turn allows the speed of data transmission to be reduced to a level compatible with that of the available channel bandwidth. In modern cell phones a variety of vocoding data compression schemes are used. An A/D converter is followed by a digital signal processing (DSP) chip that does the compression in accordance with some algorithm. The vocoder then generates a serial digital voice signal at a rate of 7.4 to 13 kbps. This permits three to eight voice signals to occupy the same channel by using TDM. At the receiver, the demodulated digital data is sent to the vocoder, where a DSP chip takes the serial bits and converts them back to binary words representing the voice. A D/A converter then recreates the voice. All 2G and 3G phones contain a vocoder.
The most widely used 2G digital system is GSM. GSM originally stood for “Group Special Mobile” but has become known as the “Global System for Mobile Communications.” It was developed in Europe under the auspices of the European Telecommunications Standardization Institute (ETSI) to replace the many incompatible analog systems used in different European countries. The GSM was designed to permit widespread roaming from country to country throughout Europe. GSM is implemented primarily in the 900-MHz band in Europe but is also used in the 1800-MHz (1.8-GHz) range in Europe, where it is referred to as the digital cellular system (DCS), or DCS-1800. GSM is also widely implemented in the United States in both the 800- and 1900-MHz personal communication system band.
GSM uses TDMA. The vocoder uses a compression scheme called regular pulse excitation-linear prediction coding (RPE-LPC) or residual excited linear predictive (RELP) coding that produces a 13-kbps voice bit stream. It allows eight telephone calls to be transmitted concurrently in a single 200-kHz-wide channel. The modulation method, known as Gaussian minimum shift keying (GMSK), is similar to frequency-shift keying (FSK) but has improved spectral properties that allow higher speeds to be transmitted in a narrower channel. A Gaussian response filter shapes the serial digital bit stream before modulation to narrow the signal bandwidth. The basic GSM data rate is 270 kbps in the 200-kHz channel, giving 270 kbps/200 kHz = 1.35 bits/Hz. Considerable error detection and correction coding is used to improve the reliability in the presence of noise, multipath fading, interference, and Doppler shifts. The basic GSM TDMA frame is shown in Fig. 20-11. Each frame is 4.615 ms long, and each voice slot is 0.577 ms long. GSM also uses a frequency-hopping scheme to minimize interchannel interference. The hop rate is 217 hops per second, or about 1200 bits per hop. FDD is used for full-duplex operation.
Two key additions to GSM are general packet radio service (GPRS) and enhanced data rate for GSM evolution (EDGE). These are packet-based data services designed to permit Internet access, e-mail, and other forms of digital data transmission. These technologies are described later under the section 2.5G Cell Phone Systems.
IS-95 CDMA. This TIA cell phone standard is called code-division multiple access (CDMA). Also known as cdmaOne, it uses a spread spectrum. This system was invented by Qualcomm, a company that makes the chipsets used in CDMA cell phones. The company also holds most of the patents in this field. CDMA uses a direct sequence spread spectrum (DSSS) with a 1.2288-MHz chipping rate that spreads the signal over a 1.25-MHz channel. As many as 64 users can use this band simultaneously with little or no interference or degradation of service, although in practice typically only 10 to 40 subscribers occupy a channel at one time. This CDMA system uses FDD for duplexing.
As in other cell phone systems, CDMA takes the voice signal and digitizes it in a vocoder. The output is a 13-kbps serial voice signal that is further processed before it is used to modulate the carrier. The digitized voice is fed to an exclusive-OR (XOR) gate where it is mixed with a 64-bit pseudorandom code occurring at the chip rate of 1.2288 Mbps. This signal is then used to modulate the carrier with QPSK. The carrier may be in the regular 800- to 900-MHz band or in the PCS-1900 band. The resulting signal occupies a huge bandwidth spread over a wide spectrum. It may also coexist with up to 64 other CDMA signals that use the same carrier but have different pseudorandom codes. These special codes are known as Walsh codes and are chosen so that they are easily recognized and recovered at the receiver by using the correlation technique described earlier.
A key part of a CDMA system is APC. All cell phones have APC, but for CDMA it is especially important. For the receivers to recover a CDMA signal, all incoming signal levels must be at the same power level. This ensures that the receiver does not confuse a higher-power signal with a lower-power signal during the de correlation detection process. The base stations increase the power level of weak distant signals and decrease the power level of signals near the cell site.
Digital Cell Phone Architecture
Fig. 20-12 is a block diagram of a 2G cell phone. The RF section contains the transmitter and receiver circuits including mixers, local oscillators, or frequency synthesizers for channel selection, the receiver low-noise amplifier (LNA), and the transmitter power amplifier (PA). The baseband section contains the vocoder with its A/D and D/A converters plus a DSP chip that handles many processing functions typically performed by analog circuits in older systems. For example, today most baseband and intermediate-frequency filtering is done digitally, as are modulation, demodulation, and mixing.
An embedded controller handles all the digital control and signaling, handoffs, and connection and identification operations that take place transparent to the subscriber. It also takes care of running the display and keyboard and all user functions such as number storage, auto-dialing, and caller ID. Because of the complexity of the baseband and control functions, this embedded controller is usually a very fast (more than 100-MHz) 32-bit microprocessor with considerable RAM, ROM, and flash memory. A separate DSP chip handles the signal processing duties.
In addition to adopting digital techniques in 2G and later phones, designers have worked hard to eliminate costly components such as filters and to create circuitry that conserves power and thereby provides longer battery life. This has led to some interesting architectures, especially in the receiver section. Today, the dominant architectures are direct-conversion and very low-IF designs.
The direct-conversion or zero-IF design sets the LO frequency to the incoming signal frequency so that the translation is made directly to the baseband signal. See Fig. 20-13. Because direct conversion works only with double-sideband (DSB) suppressed AM signals, changes have been made to accommodate FSK, BPSK, QPSK, and other forms of digital modulation. Specifically, the incoming signal is applied to two mixers simultaneously. One mixer receives the LO signal directly (sine), and the other receives a signal shifted 90° (cosine). This results in down conversion to baseband as well as the generation of in-phase (I) and quadrature (Q) signals that preserve the frequency and phase information in the signal necessary for demodulation.
Direct conversion is popular because it eliminates the need for an expensive and physically large selective SAW IF filter. It also eliminates the imaging problem so common in superheterodyne designs, especially in the crowded multiband cellular spectrum. With direct conversion, baseband filtering can be accomplished by using simple low-pass RC filters and/or DSP filters. The I and Q signals are digitized, and a DSP chip performs additional filtering, demodulation, and voice decoding. Modern IC designs have essentially eliminated the LO leakage and dc offset problems ordinarily associated with direct-conversion designs.
Low IF. Another popular alternative is low-IF architecture. When an IF is used near the baseband frequencies, filtering is simple and very effective. Using an IF near 100 kHz permits simple RC filters to be used, eliminating the larger and more expensive SAW filters. The low IF design is no longer widely used.
Another popular alternative is low-IF architecture. When an IF is used near the baseband frequencies, filtering is simple and very effective. Using an IF near 100 kHz permits simple RC filters to be used, eliminating the larger and more expensive SAW filters. The low IF design is no longer widely used.
2.5G Cell Phone Systems
The designation 2.5G refers to a generation of cell phones between the original second-generation (2G) digital phones and the newer third-generation (3G) phones. The 2.5G phones bring data transmission capability to 2G phones in addition to normal voice service. A 2.5G phone permits subscribers to exchange e-mails and text messages and access the Internet by cell phone. Because of the small screen size and a small or very restricted keyboard, data transmission capability is limited but available to those who need it.
Currently, three technologies are used in 2.5G systems: GPRS, EDGE, and CDMA2000. Although GPRS and EDGE systems have been implemented, they are generally considered to be temporary solutions to the need for data transmission capability in cell phones. True high-speed packet data capability is available with 3G phones. The CDMA2000 technology is an extension and improvement of the IS-95A/B standard.
One popular 2.5G technology is the general packet radio service (GPRS). This system is designed to work with GSM phones. It uses one or more of the eight TDMA time slots in a GSM phone system to transmit data rather than digitized voice. Depending on how many of the eight-time slots are used, the data rate can vary from about 20 kbps up to a maximum of 160 kbps. A typical rate is about 40 kbps, which is more than enough for e-mail and short message service (SMS) but poor for Internet access.
Each GSM frame has eight-time slots for data. Refer to Fig. 20-11. The overall bit rate is 270 kbps. In voice operation, each slot contains the compressed or vocoded voice signal. In GPRS, other types of data can be transmitted. The data rate that can be achieved is a function of the type of coding used (FEC) and the number of time slots allotted to the data. The GPRS standard, which was created by the European Telecommunications Standards Institute (ETSI), is now maintained by the 3rd Generation Partnership Project (3GPP).
It defines four levels of data coding referred to a CS-1 through CS-4. The most robust coding scheme CS-1 produces fewer errors, but the maximum data speed per slot is 8 kbps. The least robust coding method is CS-4, but it produces a data rate to 20 kbps. To achieve maximum data rate, you could use all eight slots for a rate of 8 X 20 kbps = 160 kbps.
However, this is never done. Instead, GPRS defines 12 classes that give different levels of data speed. The selection of the desired class is made by the cell phone carrier who sets just how much of the network capacity is devoted to voice and to data.
Class 12 gives downlink and uplink data rates of 80 kbps maximum. The carrier usually adjusts the class to match its own mix of voice and data users and often charges the data user on a per-kbps basis. Keep in mind, too, that the GPRS method involves an automatic rate adjustment algorithm that adjusts the class and data rate to the robustness of the wireless channel. Over shorter distances with less noise and interference, the system can achieve the maximum data rate. Over longer distances with added noise, the system adjusts itself to a lower data rate to ensure accurate transmission of the data.
Virtually all modern GSM phones come with GPRS, but the user must sign up for services (instant messaging, e-mail, etc.) related to this capability.
A faster 2.5G technology is enhanced data rate for GSM evolution (EDGE). It is based upon the GPRS system but uses 8-PSK modulation instead of GMSK to achieve even higher data rates up to 384 kbps.
EDGE is sometimes referred to as enhanced GPRS (EGPRS). It is usually implemented as a software upgrade to the base stations but also requires a linear power amplifier. Both hardware and software changes are needed in a GPRS handset. EDGE uses the GPRS class concept whereby the data rate is a function of the encoding and the number of time slots used. By using 3π/8-8PSK modulation, 3 bits is coded per symbol change, thereby tripling the gross data rate. The theoretical maximum data rate is 473.6 kbps with all eight slots used. A more typical implementation is the use of four slots for a data rate of 236.8 kbps. Again, a data rate algorithm automatically backs off on the rate as channel conditions degrade due to noise or increased distance. Typical everyday rates are usually over 100 kbps but less than 200 kbps.
One of the key changes required when EDGE is implemented is the need for linear power amplifiers both at the base station and in the handset. GMSK as used in GSM and GPRS is a type of FM with a constant envelope (amplitude) carrier that changes in frequency with the modulation. FM permits more efficient class C, D, E, and F amplifiers to be used. These amplifiers clip or distort the amplitude of a signal but with FM that does not interfere with the modulation. When 3π/8-8PSK is used, the envelope does change as the signal switches from one phase to another. Therefore, to retain the information content, the amplitude of the signal must be preserved through amplification.
A class AB linear power amplifier must be used. Some base stations already use such amplifiers and so may simply adjust them to maximum linear operation rather than maximum efficiency.
In the handsets, efficient class C or E/F power amplifiers in the transmitter must be replaced with a class AB linear amplifier. This is a significant change in a handset as the lower efficiency produces greater heat and shortens the battery life.
3G Cell Phone Systems
Third-generation (3G) cell phones are true packet data phones. They feature enhanced digital voice and high-speed data transmission capability. Third-generation phones were originally described by the term International Mobile Telecommunication 2000, or IMT-2000. 2000 refers to 2000 MHz, the approximate center of the frequency range defined for 3G (1800 to 2200 MHz). The goal of the International Telecommunications Union (ITU) was to define a worldwide standard for future cell phones to which all other systems could evolve, thereby providing full global roaming. An IMT-2000 phone can achieve a data rate of up to 2.048 Mbps in a fixed position, 384 kbps in a slow-moving pedestrian environment, and 144 kbps in a fast mobile environment. With such high-speed capability, a 3G phone can do lots more than just transmit high-quality digital voice.
Some potential 3G applications include fast e-mail and Internet access. With larger color screens and full keyboards, cell phones can act more as small computer terminals High speed also permits the transmission of video. Subscribers can watch a movie on their 3G phones, although the small screen limits viewing. In most models, a built-in image sensor and lenslet cell phones become picture phones and digital cameras.
The ITU did not specify a particular technology to implement 3G. However, it did recommend one worldwide version known as wideband CDMA (WCDMA). This system is also known as the Universal Mobile Telecommunications Service (UMTS). While the standard is still based in the ITU, it is developed, maintained, and promoted by the Third Generation Partnership Project (3GPP).
WCDMA is a direct sequence spread spectrum technology. In the most popular configuration, it is designed to use a 3.84-MHz chipping rate in 5-MHz-wide bands. Duplexing is FDD requiring the matching of 5-MHz channels. The modulation is QPSK. It can achieve a packet data rate up to 2 Mbps.
A key problem in implementing 3G is the need for huge portions of spectrum. New spectrum is scarce and expensive. In Europe, paired bands in the 1900- to 2200-MHz range are available. In the United States, the 806- to 890-MHz range can be used for 3G in some areas. Some spectrum in the 1710- to 1885-MHz range is also available to some carriers. Also, some segments of the 2500- to 2690-MHz band are available. The exact 3G spectrum varies widely depending on which part of the world you are in, making it extremely difficult to design a cell phone that is fully operable worldwide.
The UMTS 3G standard also defines a TDD version known as TD-SCDMA for time-division synchronous code-division multiple access. It is designed to use a 1.6-MHz-wide channel with a chipping rate of 1.28 MHz. Different time slots in the time-multiplexed data stream are assigned to uplink and downlink activity. The number of uplink and downlink channels may be dynamically assigned so that a carrier can adjust the system to the traffic load at any given time. The primary benefits of TD-SCDMA are that less spectrum is needed. Only a single 1.6-MHz channel is needed.
Furthermore, since duplexing is TDD, there is no need for paired spectrum as in WCDMA or GSM or any other FDD system. The downside is that the system is more complex because of the extreme need for accurate timing and synchronization required for proper operation. So far, the only nation to adopt TD-SCDMA as a standard is China.
High-speed packet access (HSPA) is an enhancement to WCDMA systems to make them faster. There is a high-speed downlink packet access (HSDPA) and a high-speed uplink access (HSUPA) version. They can be used separately or together. Together, they are referred to as HSPA. When the 3G WCDMA standard was first adopted, it was assumed that it would be put into use far faster than it has. During the past years, wireless technology has changed, making the original specifications somewhat behind the times. The maximum 2-Mbps data rate was assumed to be fast enough for any service. But today, the demand for faster data speeds is growing, especially because of the growing demand for mobile video services. Because of the need for faster systems, a new system compatible with WCDMA has been developed. Known as high-speed packet access (HSPA), it provides a packet data rate from the base station to the handset many times that of the 2-Mbps maximum rate of WCDMA.
HSDPA uses an adaptive coding and modulation scheme with QPSK and 16-QAM. Data is transmitted in 2-ms frames. There are 12 categories of HSDPA that define different coding and modulation schemes. The minimum is category 11,900 kbps using QPSK.
Category 6 gives 3.6 Mbps using 16-QAM. The maximum data rate is 14.4 Mbps using 16-QAM in category 10. The actual rate achieved is a function of the link quality. High noise and long-range give a lower rate. The rate adapts to the channel conditions automatically.
While most data needs will be served by a high-speed downlink capability, in some applications a fast uplink may be needed. This is accommodated by a companion standard known as high-speed uplink packet access (HSUPA). A fast handset to base station rate is more difficult to implement so uplink rates are naturally slower. HSUPA provides a maximum data rate of 5.76 Mbps. Again, the rate adapts to the channel conditions.
An enhanced version called HSPA+ is now also available from some carriers. It permits the use of 64QAM allowing many systems to deliver downlink rates of 21, 28 or 42 Mbps. Although not common there are more advanced versions that use multiple carriers and MIMO to deliver even higher speeds. For example two or four 5 MHz channels can be combined to double or quadruple data speeds to 84 or 168 Mbps. Adding 2 X2 or 4 X4 MIMO can deliver peak rates in the 336 to 672 Mbps. Today most HSPA systems are still in operation but have been replaced in usage by the newer faster LTE 4G systems.
This standard was developed by Qualcomm. It is an extension of the widely used IS-95 CDMA standard also known as cdmaOne. The earliest versions of this radio system were correctly designated as a 2.5G technology, but subsequent improved versions have clearly made it a 3G technology because of the high data rates it can achieve.
The basic CDMA2000 data transmission method is generally called 1XRTT (radio transmission technology). It uses the same 1.25-MHz-wide channels but also changes the modulation and coding formats to actually double the voice capacity over that in IS-95. The data capability is packet-based and permits a data rate of up to 144 kbps, which is comparable to EDGE. A version designated 3XRTT uses three 1.25-MHz channels for a total bandwidth of 3.75 MHz. By using a higher chip rate, a maximum date rate roughly three times the 1XRTT speed (432 kbps) is possible.
The more recent version is called 1XEV-DO or Evolution-Data Optimized. It has a higher data rate approaching 3.1 Mbps downlink and an uplink rate up to 1.8 Mbps. Another version known as 1XEV-DV for Evolution-Data/Voice has a maximum packet rate of 3.07 Mbps. Uplink speed is the same as that of 1XEV-DO. Many carriers still implement CDMA2000, but it is used by the older phones for voice and low-speed data like texting and e-mail. It has been superseded by the newer LTE 4G systems.
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