Olja Kavaja, ID# 0275726
Wen Feng Xu, ID# 0226293
EE 3414
Final Project Report
Due Date: 5/12/2003
The Cell Phone Mission
I. INTRODUCTION:
The basic concept of cellular phones began in 1947, when researchers looked at car phones and realized that by using small cells with frequency reuse they could increase the traffic capacity of mobile phones substantially. However at that time, the technology to do so did not exist. In 1968, AT&T and Bell Labs proposed a cellular system to the Federal Communications Commission (FCC) of many small, low powered broadcast towers. Each tower would cover a “cell” a few miles in radius and collectively covering a larger area.
Each tower would use only a few of the total frequencies allocated to the system. As the phones traveled across the area, calls would be passed from tower to tower. It is apparent that cells must somehow overlap, and when a user travels between cells, one cell must hand the call off to the other cell. The cells must also not interfere with each other. This is accomplished by giving each cell a slightly different chunk of the frequency spectrum (note that CDMA does not do this) and by measuring power levels. When the power level of the user begins to fade, the cell tower determines which cell is the closest cell. Upon finding this information, the current cell tower sends an over-the-air message to the new cell tower and to the cell phone. At this point, the new cell tower picks up the call and the old one drops the call as the cell phone switches frequencies. This type of handoff is called a "hard handoff" since the audio feed is lost for between 10 milliseconds and 100 milliseconds while the new tower picks up the signal. Often these "hard" handoffs fail when the new tower tries to pick the call up, leading to frequent dropped calls.
Overview:
Some of the different access systems that can be used in the cell phone industry are the Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Frequency Division Multiple Access (FDMA). The two major and competing systems today are TDMA and CDMA. We will thoroughly study, discuss and compare both systems and survey the commercial usage of them.
Also, we will discuss the third-generation networks that are the present step in the evolution of wireless voice and data services. With substantially enhanced capacity, quality, and data rates, 3G is positioned to deliver on the promise of anytime, anywhere, any way access for mobile users.We will also explore the future of wireless phone technologies like Wideband Code Division Multiple Access or also known as WCDMA.
II. CDMA
We'll begin with CDMA, since this new technology has created the greatest "buzz" in the mobile communications industry. Code division multiple access (CDMA) does let everyone transmit at the same time using conventional modulation techniques. 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. In CDMA all sites 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. 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.
III. FDMA:
Frequency division multiple access (FDMA) is the division of the frequency band allocated for wireless cellular telephone communication into 30 channels, each of which can carry a voice conversation or, with digital service, carry digital data. FDMA is a basic technology in the analog Advanced Mobile Phone Service (AMPS), the most widely-installed cellular phone system installed in North America. With FDMA, each channel can be assigned to only one user at a time.
Although technically simple to implement, FDMA is wasteful of bandwidth: the channel is assigned to a single conversation whether or not somebody is speaking. Moreover, it cannot handle alternate forms of data, only voice transmissions.
IV. TDMA:
Time division multiple access (TDMA) is digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel. TDMA is basically analog’s FDMA with a time-sharing component built into the system. The TDMA digital transmission scheme multiplexes three signals over a single channel.
The current TDMA standard for cellular divides a single channel into six time slots, with each signal using two slots, providing a 3 to 1 gain in capacity over advanced mobile-phone service (AMPS). Each caller is assigned a specific time slot for transmission. The TDMA system is designed for use in a range of environments and situations, from hand portable use in a downtown office to a mobile user traveling at high speed on the freeway. The system also supports a variety of services for the end user, such as voice, data, fax, short message services, and broadcast messages. TDMA offers a flexible air interface, providing high performance with respect to capacity, coverage, and unlimited support of mobility and capability to handle different types of user needs.
There are actually different types of technologies that use 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. GSM overlays the basic TDMA principles with many innovations that reduce the potential problems inherent in the system. Because of its adoption by the European standard GSM, the Japanese Digital Cellular (JDC), and North American Digital Cellular (NADC), TDMA and its variants are currently the technology of choice throughout the world.
S-136 is another form for TDMA and it is the only technology that integrates with existing analog systems and in S-136 you can go back from digital to analog and from analog to digital at any time.
V. TDMA vs. CDMA
Over the last few years, a debate has convulsed the wireless community over the respective merits of TDMA and CDMA. The proponents of CDMA have claimed bandwidth efficiency of up to 13 times that of TDMA and between 20 to 40 times that of analog transmission. Moreover, they note that its spread-spectrum technology is both more secure and offers higher transmission quality than TDMA because of its increased resistance to multi-path distortion.
The defenders of TDMA, on the other hand, point out that to date there has been no successful major trial of CDMA technology that support the capacity claims. Moreover, they point out that the theoretical improvements in bandwidth efficiency claimed for CDMA are now being approached by enhancements to TDMA technology. The evolution of TDMA will allow capacity increases of 20 to 40 fold over analog in the near future. This combined with the vastly more expensive technology needed for CDMA ($300,000 per base station compared with $80,000 for TDMA) calls into question what real savings CDMA technology can offer. So far, IS–136 TDMA is the proven leader as the most economical digital migration path for an existing AMPS network.
VI. GSM
GSM stands for "Global System for Mobile Communications." GSM is mostly a European system and is largely unused in the US. GSM is interesting in that it uses a modified and far more efficient version of TDMA. GSM keeps the idea of timeslots and frequency channels, but corrects several major shortcomings. Since the GSM timeslots are smaller than TDMA, they hold less data but allow for data rates starting at 300 bits per second. Thus, a call can use as many timeslots as necessary up to a limit of 13 kilobits per second. When a call is inactive or may be compressed more, fewer timeslots are used. To facilitate filling in gaps left by unused timeslots, calls do "frequency hopping" in GSM. This means that calls will jump between channels and timeslots to maximize the system’s usage. A control channel is used to communicate the frequency hopping and other information between the cell tower and the phone. To compare with the other systems, it should be noted that GSM requires 1 Watt of output power from the phone.
- GPRS
GPRS (General Packet Radio Service) is a step between GSM and 3G cellular networks. GPRS offers faster data transmission via a GSM network within a range 9.6Kbits to 115Kbits. This new technology makes it possible for users to make telephone calls and transmit data at the same time. (For example, if you have a mobile phone using GPRS, you will be able to simultaneously make calls and receive e-mail massages.) The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. GPRS enabled networks offer 'always-on', higher capacity, Internet-based content and packet-based data services. This enables services such as color Internet browsing, e-mail on the move, powerful visual communications, multimedia messages and location-based services. With GPRS, an IP data transmission protocol, which is characteristic of computer networks, is being introduced to GSM. IP is a data transmission protocol which is used in Internet, the largest computer network in the world today.
VIII. 3G
3G is a so-called "third-generation" broadband, packet-based transmission of text, digitized voice, video, and multimedia at data rates up to and possibly higher than 2 megabits per second (Mbps), offering a consistent set of services to mobile computer and phone users no matter where they are located in the world. Based on the GSM communication standard, 3G, endorsed by major standards bodies and manufacturers, was the planned standard for mobile users around the world by 2002. Once 3G is fully implemented, computer and phone users can be constantly attached to the Internet as they travel and, as they roaming service, have the same set of capabilities no matter where they travel to. Users will have access through a combination of terrestrial wireless and satellite transmissions. Until 3G is fully implemented, users can have multi-mode devices that switch to the currently available technology such as GPRS where 3G is not yet available.
Today's cellular telephone systems are mainly circuit-switched, with connections always dependent on circuit availability. Packet-switched connection, using the Internet Protocol (Internet Protocol), means that a virtual connection is always available to any other end point in the network. It will also make it possible to provide new services, such as alternative billing methods (pay-per-bit, pay-per-session, flat rate, asymmetric bandwidth, and others). The higher bandwidth of 3G also promises new services, such as video conferencing. 3G promises to realize the Virtual Home Environmentin which a roaming user can have the same services to which the user is accustomed when at home or in the office, through a combination of transparent terrestrial and satellite connections. 3G uses two different access systems WCDMA and CDMA2000 1X EV-DO or EV-DV. WCDMA was implemented in 2002 in the US, while CDMA2000 was implemented the same year in Europe. They use different chip rates and carrier spreading. But, both can reach almost the same bit rate of 2.07Mbps. This is why we are going to concentrate only on one of them, in this case WCDMA.
IX. WCDMA
Providing mobile users with data rates up to 2 Mbps, WCDMA is an ultra high-speed, ultra high-capacity radio technology that generates and carries a new range of rich, fast, colorful media that consumers will be able to access over their mobiles: color graphics, video, animations, digital audio, Internet and e-mail. Ericsson built the first commercial WCDMA networks in the world and leads the development of WCDMA at every level. In nearly all 3G agreements announced so far, operators have selected Ericsson as their WCDMA supplier of choice. Ericsson has captured a 40 percent market share in WCDMA and has been named supplier in 35 commercial WCDMA agreements, more than any other vendor in the world. Ericsson has delivered more than 30 initial WCDMA systems to operators worldwide. One of the main differences between WCDMA and regular CDMA is the higher bandwidth that in this case is used to not only support data and voice coding, but also video coding. Voice, images, data, and video are first converted to a narrowband digital radio signal. The signal is assigned a marker (spreading code) to distinguish it from the signal of other users. WCDMA uses variable rate techniques in digital processing and it can achieve multi-rate transmissions.
X. Low Earth Orbit Satellites
As cellular systems reach maturity, many are looking to the future. While the PCS frequencies (1800 MHz – 2000 MHz) are still being built out, the next generation of cellular communication is being launched. To provide true homogeneous worldwide wireless coverage with no gaps, LEO (low earth orbit) satellites are being used. These LEO satellites orbit the earth in high speed, low altitude orbits with an orbital time of 70-90 minutes and an altitude of 400 – 700 miles. LEO’s provide small coverage cells around the size WashingtonState. Since LEO’s are not geosynchronous, they are forced to fly complete orbits, and thus many must exist to guarantee every area is covered by at least one satellite at all times. Therefore, call handoff is executed when the satellite moves, not when the person moves as it is with ground-based cellular systems.
LEO is based on three systems: Iridium, Globalstar and Teledesic.
Iridium-- Motorola has just launched the last satellite in the Iridium system, a LEO system consisting of 66 satellites with 6 in-orbit spares. Iridium satellites have an orbital altitude of 485 miles and weigh 1,500 pounds each. Iridium will allow a person to place a call anywhere in the world using a device slightly larger than a small hand-held cell phone. Iridium will begin service this summer.
Globalstar-- Globalstar is a CDMA system headed up by Qualcomm. This system will have 48 satellites with 8 in-orbit spares. Each satellite will weigh 1000 pounds and will orbit at 880 miles. Again, this system will use phones that are just slightly larger than today’s conventional cell phones and allow calls to be placed anywhere in the world. Globalstar expects to have the system launched in 1998 and will begin to provide service in 1999.
Teledesic -- Teledesic is the most promising and furthest off of the LEO systems. Teledesic is a partnership of many large players in this industry, including Motorola, Microsoft, Boeing, Nextel, and Matra Marconi, with Motorola being the prime contractor. Teledesic will use 288 satellites flying at an altitude of less than 800 miles. This means that Teledesic will have five times the satellites as the other systems and will fly lower than the other systems. This translates into more bandwidth per region (smaller coverage area per satellite) and smaller cellular phone devices (lower power since satellites are closer). Teledesic also plans to provide high-speed data access, starting at 64 megabits per second. Teledesic has begun development of their satellites, and expects to start service in 2003.