INTRODUCTION After the widespread use of PCs in the 80s, the desktop computer was transformed from a stand-alone static device to a portable networked device. This is because of the emergence of laptops and the network evolution. Portables have had a huge success, showing that people wanted their information to be as portable as they were. At the same time the rapid growth of networking was showing that we want our information connected and not isolated. Wireless communications is trying to provide us today with the ability to access information on the move regardless of geographical location. In order to deliver the rich content of the Internet on a mobile device higher bandwidth channels must be used. The current second generation standards, like GSM (limited to 9600Kbps), do not provide adequate bandwidth to support high bandwidth services. Third generation technology is focusing on ways to upgrade the existing cellular network to be able to transmit in higher data rates. Data rates is said to be of the scale 384Kbps to 2Mbps. With these transmission speeds in mind rich content can be delivered to mobile devices, like multimedia content. At the moment these data rates are a bit optimistic and do not map very well to reality. Even if the underlying cellular network accomplishes such transmission speeds the mobile devices have to be able to accommodate enhanced applications. With major limiting factor the battery life and the size of the handheld devices, pumping-up current mobile phones to become small PCs can be a bit tricky. GENERATION OF MOBILE TECHNOLOGIES Cellular standards fall into three categories; first generation analog cellular systems, and second and third generation digital cellular systems. An interim technology, usually known as second generation-plus supports high speed data communication over today’s digital cellular systems. Table 1 shows main feature of these technologies. A brief discussion and of these technologies and a comparison of data services follows: Main Features of Cellular Technologies 1st 2nd Generation 2nd + 3rd Generation Generation Generation Analog trans. Digital trans Digital trans Digital trans Mainly speech Mainly speech Mainly speech Speech and video Voice band data Digital data Increasing Mainly digital data digital data Circuit switched Circuit switched Increasingly Mainly packet switched packet switched Local systems Global roaming Global roaming Global roaming First Generation Technologies First-generation mobile communications systems, sometimes referred as 1G, were basic analog radio systems that established the first cellular radio infrastructure. The biggest problem with this system for cellular providers is the lack of capacity to handle the sheer number of users that demand voice service. The analog cellular networks use circuit switched connections for data transport; however, the radio link performance for data is considered marginal due to the limitations imposed by the analog nature of the technology. Radio channel dynamics such as dropouts, signal fades, and multi-paths, which can be tolerated during a voice connection, can be disastrous to a mobile data subscriber. Subscriber data rates of 2400 bits/s or less can be sustained using standard modems with some adaptation for connection to the cellular network. In general, the analog cellular infrastructure systems are not an efficient means of sending data due to limited available capacities, limitations of data recovery, low security, and the high cost of use for many applications. Some of the widely used standards include the following: Advanced Mobile Phone System (AMPS): The AMPS was the first standardized cellular service in the world and was released for commercial use in 1983 in USA. The system uses 800 MHz to 900 MHz frequency band and the 30 KHz channel bandwidth. This is the most widely used analog cellular standard. Narrow-band Advanced Mobile System (N-AMPS): This system operates in 800 MHz range and provides three times greater capacity than AMPS by using 10 KHz channel bandwidths instead of the standard 30 KHz channel bandwidths used in the AMPS system. Nordic Mobile Telephone (NMT): This system was in use throughout the Nordic countries. The system has two variants based on the frequency of allocation. NMT450 operates on 450 MHz, while NMT900 operates on 900 MHz. Total Access Communications Systems (TACS): This system was based in the U.K and has several variants. The most popular are J-TACS (similar to AMPS) and E-TACS (Expanded TACS). Second Generation Technologies Second-generation mobile communications systems, sometimes referred as 2G, are currently predominant in the wireless communication industry. These use digital technology to provide many advantages for both the voice- and data- based mobile professional. These include increased system capacity, increased security against casual eavesdropping, superior cell hand-off, and better recovery of radio signal under different conditions. In addition to speech, these support services such as fax, short messaging, and roaming of mobile end- stations. Technical Summary of Second Generation Technologies ---- EUROPE - ------------ UNITED STATES ---- ---- -------- GSM TDMA CDMA Frequency band 890-960 MHz 824-894 MHz 824-894 MHz Allocated bandwidth 50 50 50 Access scheme TDMA TDMA CDMA Duplex method FDD FDD TDD Channel bandwidth 200 KHz 30 KHz 1250 KHz No. of voice/freq. channels 8 / 16 3/6 Total traffic channels 1000 / 2000 2496 / 4992 Channel bit rate 270.833 Kbps 48.6 Kbps Vendor dependent Voice coding 22.8 Kbps 8 / 4.5 Kbps 8 Data rate 9.6 Kbps 9.6 Kbps 14.4 Kbps The second-generation technologies use circuit switched connections for data transport and provide data transmission rate of 9.6 to 14.4 Kbps. These implement a high level of flow control and error correction and provide reliable data transfer. With second-generation systems, multiple users can share a single cellular channel, thus reducing congestion and providing access for more users. These use the multiple access methods and provide extensive coverage with a proven and reliable communications infrastructure. The existing standards in use worldwide include the following: GSM (Global System for Mobile Communications): This was the first European digital open standard and is in commercial use in 1992. It was developed to establish cellular compatibility throughout Europe. Its success has spread to all parts of the world and by the year 2000, there were over 250 million subscribers worldwide. It is based on a combination of TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) techniques and operates at 900 MHz and 1800 MHz frequency bands in many parts of the Europe and Asia, and uses 1900 MHz in North America. Today, it provides an error-free Internet access at 9600 bps to the subscribers. Some analysts suggest that due to a single dominant network standard, GSM, Europe is 18 months ahead of the US wireless market. TDMA (Time Division Multiple Access): TDMA refers to products developed using the IS-136 specification for advanced digital wireless services. It was the first U.S. digital standard and was started in 1993. It is a natural evolution of analog AMPS networks and, therefore, was previously known as D-AMPS (Digital AMPS). It is the most widely used wireless technology in the USA, and as of year-end 2000, there were about 61 million TDMA subscribers worldwide, with an estimated 31 million subscribers in the North America. TDMA technology provides a 3 to 1 gain in capacity over analog technology by dividing a single radio frequency channel into a series of timeslots. Each user is assigned a set of timeslots during which they are allowed to broadcast. This technique is better at handling heavy traffic than others, since there is a hard upper limit on the amount of bandwidth that a particular user will utilize, but this is its weakness as well. Cells that do not have a large number of users will have underutilized bandwidth. Similarly, there is a much harder limit on the total number of users that can be supported within a cell. CDMA (Code Division Multiple Access): This system, known as IS-95, was adopted by the Telecommunications Industry Association (TIA) in 1993. It uses the same frequency bands as AMPS and supports AMPS operation, employing spread-spectrum technology and a special coding scheme. In this technique the call is spread over a series of frequencies based on a sequence of jumps that are semi random in nature. The spread spectrum approach minimizes signal loss within any particular frequency band, as well as providing security for the communications. The handset and the base station agree on the sequence ahead of time, which gives the base station the capability to minimize collisions within a cell. It is characterized by high capacity and small cell radius. CDMA provides outstanding voice and call quality, fewer dropped calls, improved security and privacy, greater capacity, reduced background noise and interference, and possibility of simultaneous voice and data calls. Designed with about 4.4 trillion codes, CDMA virtually eliminates cloning and other types of fraud. Globally, commercial CDMA networks serve tens of millions of subscribers. Second –plus Generation Technologies The second-generation technologies provide data transfer rates only up to 14.4 Kbps. The high data speeds that are needed for video and graphic image transmission are not available on most of the today’s mobile phone systems. Such capabilities require a highly complex and robust technology platform that will not be available in most of the countries until few years from now. An interim step to the next generation technologies is second-plus generation or 2.5G technologies as shown in Figure 4. These technologies support data transfer rates of 57.6 Kbps and higher and offer subscribers access to the Internet at speeds that are comparable to a wire-line ISDN connection or even faster. These include HSCSD, GPRS and EDGE. An overview of these technologies is given next: Figure 4: Evolution of 2G Networks to 2.5G and 3G HSCSD (High Speed Circuit Switched Data): HSCSD is a circuit-switched mobile data standard that gives a single user simultaneous access to multiple channels, up to four, at the same time. In comparison, GSM supports only one user per channel per time slot. Assuming a standard data transmission rate of 14.4 Kbps, using four timeslots with HSCSD allows theoretical speeds of up to 57.6 Kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel. HSCSD does not disrupt voice service availability. GPRS (General Packet Radio Service): GPRS is a new packet-based bearer that is being introduced on many GSM and TDMA mobile networks from the year 2001 onwards. It is a non-voice value added service that allows a subscriber to send and receive data in an end-to-end packet transfer mode, without using any network resources in circuit-switched mode. It also permits the user to receive voice calls simultaneously when sending or receiving data calls. GPRS facilitates instant connections (no dial-up) whereby information can be sent or received immediately as the need arises. This is why GPRS users are sometimes referred to be as being “always connected”. A GPRS mobile device displays a mobile portal service all the time, but it is only activated, and the user is only charged, when information is being transmitted. The main feature of GPRS is that it reserves radio resources only when there is data to send and that these radio resources are shared by all Mobile Stations (MSs) in a cell. It handles data transfer rates from 14.4 Kbps, using just one TDMA slot, up to 115.2 Kbps, using all eight TDMA slots. This will allow it to handle all types of transmission from slow-speed short messages, to the higher speeds needed for browsing complex web pages with high graphics content. GPRS fully enables a true “Mobile Internet” scenario by allowing integration between the existing Internet and the GPRS network, via interfaces to TCP/IP. Its network can be viewed as a sub-network of the Internet with GPRS capable mobile phones being viewed as mobile hosts. This means that each GPRS terminal can potentially have its own IP address and will be addressable as such. Any service that is used over the fixed Internet today – web browsing, file transfer, chat, email, telnet – will also be available over mobile network via GPRS. In addition, higher data rates will allow users to take part in video conferencing and interact with multimedia websites and similar applications as well. Enhanced Data rates for Global Evolution (EDGE): EDGE is a radio based high-speed mobile data standard that was first proposed to the European Telecommunications Standards Institute (ETSI) in 1997 as an evolution of GSM. In fact, it was formerly called GSM384. It is the result of a joint effort between TDMA industry association and the GSM Alliance to develop a common set of third generation wireless standards which supports high-speed modulation. EDGE allows mobile operators to offer 3G services without having to purchase a 3G license. It allows data transmission speeds from 48 Kbps, using just one timeslot, up to 384 Kbps, using all eight timeslots. It supports 800/900/ 1800/1900 MHz frequency bands. Although it reuses the GSM carrier bandwidth and timeslot structure, it is by no means restricted to use within GSM cellular systems. In fact, by enhancing the capability of existing GSM or TDMA systems, it facilitates an evolution of existing cellular systems towards third-generation capabilities. Third Generation Technologies Two shortcomings of the second generation bearer networks are low bandwidth and limited network capacity which negatively impact the user experience and the reliability of the service. Third generation or 3G technology is a new technological evolution that will offer far more bandwidth and greater data and voice call capacity than today's digital mobile networks allow. It is a next giant step in mobile technology development with its goal being full interoperability and inter-working of mobile systems. The idea behind 3G is to unify the disparate standards that today's second generation wireless networks use. With 3G technology, portable bandwidth will rise to the level of wired broadband connections and the data transfer rates of up to 2 Mbps will be possible (128 Kbps in a car, 384 Kbps when a device is stationery or moving at pedestrian speed and 2 Mbps in fixed applications). When this speed is achieved, wireless technology will find a new audience that is interested in Internet browsing, wireless gaming, and listening to music. Current mobile networks are only designed for voice and text messaging, whereas 3G networks will allow faster and more complex data transmission such as streaming video and audio, video conferencing, satellite navigation and interactive application sharing. These networks will provide packet switched data access to the Internet with an end-to- end IP connection. This means that when the mobile phone is activated it is automatically connected to the Internet via a normal browser. Subscribers will then enjoy capabilities similar to today’s fixed-line Internet services with significant add-ons such as location-based and highly personalized services. Third generation technology allows handsets to be left permanently connected to the network and use capacity only when they receive or transmit packages. Subscribers can thus pay for the volume of data transmitted, not how long they talk. Although the technology behind 3G may seem complicated, the ways in which 3G will affect all of our lives are easy to imagine. Just imagine having a combined camera, computer, stereo, and radio included in your mobile phone. Rich-media information and entertainment will be at your fingertips whenever and wherever you want. Being able to do so much, the end user device is no longer just a mobile phone, and will be referred to as a terminal. Market Snapshot and Status of Deployment of Mobile Internet Technologies AMOUNT HOME PAID FOR MOBILE 1ST 3G INTERNET 2G 3G 2.5G MMS COUNTRY PENETRA- SERVICE PENETRA- LICENSES LICENSES LAUNCHED? AVAILABLE? TION 3G LAUNCH TION LICENSES 64.8% 50% 4 6 US$ 580 Yes Yes Q1 Australia (Jun (Dec Million (Q2 2000) (Aug 2002) 2003 2002) 2001) Canada 36.1% 49% 4 5 US$ 919 Yes Yes Q1 (Jun (Dec Million (Apr 2001) (Oct 2002) 2002 2002) 2001) France 61.9% 23.9% 3 4 US$ 2.23 Yes Yes(May Q1 (Jun (Oct Billion (Q4 2000) 2002) 2004 2002) 2002) 67.9% 31% 4 6 US$46.11 Yes Yes Q4 Germany (Jun (Q3 2001) Billion (Dec 2000) (Apr 2002) 2003 2002) Hong 86.8% 53% 6 4 US$ 524 Yes Yes Q1 Kong (Jun (Dec Thousand (Q4 2000) (Jul 2002) 2003 2002) 2001) Italy 91.5% 34% 4 5 US$10.04 Yes Yes Q4 (Jun (Dec B1illion (May 2001) (May 2002) 2002 2002) 2001) Japan 55.6% 44% 5 3 N/A Yes Yes Q4 (Jun (Dec (Q1 1999) (Q4 2000) 2001 2002) 2001) Malaysia 32.6% 5% 5 2 US$26.33 Yes No 2004 (Jun (2001) Million (Q2 2002) (Q2 2003) 2002) 67.5% 56% 3 3 US$173.4 Yes Yes Q1 Singapore (Jun (Mar Million (Q4 2000) (Sep 2002) 2004 2002) 2001) South 64.34% 58% 5 2 US$ 3.3 Yes Yes Oct Korea (Jun (Dec Billion 2000 2002) 2001) Taiwan 100% 50% 6 5 US$ 14 Yes Yes Q4 (Jun (Dec Million (Q2 2001) (Oct 2002) 2003 2002) 2001) U.K. 81.7% 38% 4 5 US$35.36 Yes Yes Q1 (Jun (Dec Billion (May 2001) (Jun 2002) 2003 2002) 2001) Standards: Standardization of third generation mobile communications began in the mid- 1990s under supervision of the International Telecommunications Union (ITU). The goal was full interoperability and inter-working of mobile systems capable of providing value-added services. In 1998, the ITU called for Radio Transmission Technology (RTT) proposals for IMT-2000 (International Mobile Telecommunications-2000), the formal name for the third generation standard. Under the brand IMT-2000, it approved three standards to achieve this: W- CDMA, CDMA2000 and TD-SCDMA. W-CDMA (Wideband Code Division Multiple Access) was backed by the European Telecommunications Standards Institute (ETSI) and the GSM operators in Europe and elsewhere; while the CDMA2000 was backed by the North American CDMA community, led by the CDMA Development Group (CDG). The third standard won the support in the other parts of the world. Earlier, in January 1998, the W-CDMA standard was also incorporated by ETSI in the specification of UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access; hence W-CDMA and UMTS are often used synonymously. IMT-2000 is to ensure that these technologies can work in different networks, primarily in IP networks, but for the sake of backwards compatibility, in the GSM and the American ANSI networks as well. Most major network operators in Europe and Asia are committed to the W-CDMA standard for 3G mobile communications. Nevertheless, other standards are being implemented in other parts of the world. In North America and Asia Pacific, the next generation wireless network is going to be mainly based on CDMA2000 and China, the world's largest market for mobile communication, will be using TD-SCDMA standard for 3G networks. Comparison of Data Services The demand for mobile data services is growing. Increased mobility has fueled an expanding market for both consumers and the enterprises. A comparison of various data services for cellular networks is shown in Table 4. For consumers, second-plus and third generation networks will bring access to the Internet, with near wire-line speed and quality. 2.5G services will mostly be text-based with still images and short audio clips. Services will include web browsing, financial transactions, image downloads, e-mail and instant messaging. As networks migrate to 3G, these same services will be enriched with multimedia content including full audio and video clips. For enterprises, second-plus generation networks will allow access to corporate intranet and e-mail, business applications and databases, and increasing mobile sales and field employees’ productivity. In the future, 3G capabilities will enable even greater benefits from wireless business applications through VoIP (Voice over IP), rapid file transfer and video-conferencing. Comparison of Data Services for 2G, 2.5G and 3G Networks ND ND RD SERVICES 2 GENERATION 2 + GENERATION 3 GENERATION Web browsing Short text screens 100KB web page takes 100KB web page takes approx. 30 sec to download approx. 2 sec to download File transfers No 500 KB document takes 500 KB document takes approx. 2mn to download approx 10 sec to download e-mail Short Message Service Text-based with small Full attachments (SMS) attachments Instant messaging SMS Text-based With audio/video clips VoIP (Voice over IP) No Limited Yes Streaming audio/video No Short clips Yes Access to corporate intranet Very limited Text-based Yes Access to corporate apps Very limited Text-based Yes CURRENT STATE OF THE ART MOBILE TECHNOLOGIES When we refer to state of the art technologies we are referring to the technologies that are currently operational and offer mobile services. The most widely adopted technology for mobile communications today is GSM. The first GSM standard was published in 1990 and commercial service started in 1991. The evolution of GSM will provide the future infrastructure for the third generation mobile systems. Global System for Mobile Communications (GSM) The Global System for Mobile Communication is an international standard created in 1987. Thirteen European countries signed a Memorandum of Understanding (MoU) and agreed upon the construction of a standardized telecommunications system operating on a 900MHz frequency band. GSM, a digital, multi-cellular mobile telephone network system, was developed from this standard. This ISDN-like technology features a D channel for call set up and B channel for transferring data. It converts the voice into a coded digital signal, which is transmitted and then decoded in the receiving handset. This way, atmospheric noise has less of an effect on voice quality and the encoded messages provide security. Originally, GSM was intended to be established as a Pan-European system, but today has expanded into many other parts of the world. GSM is established in nearly 100 countries worldwide, and growing. Although this technology has not been widely adopted in North America, GSM phone systems are popular throughout Europe, Australia, Hong Kong, Singapore, South Africa and the UAE. New GSM networks are being created monthly, and it is predicted that by the end of the decade, it will have a subscriber base of more than 100 million. This has opened the opportunity of constructing a coherent telecommunications system which would network the world--without the constraint of borders. Other GSM networks operate at 1800MHz (DCS 1800) and use smaller cells designed for high-density areas like cities, and have been implemented throughout, with additional DCS-1800 networks planned. Future North American digital services for PCS systems will use modified DCS-1800 equipment operating at 1900MHz so as to be compatible with European systems. The uniqueness of GSM technology lies in the fact that users must insert Subscriber Identity Module Cards (SIM-Cards) in their handsets. SIM Cards are small chips delivered by the GSM service provider. These SIM cards contain crucial information such as a phone number and all billing coordinates and can store telephone numbers. This enables users to switch GSM handsets by simply pulling out their SIM Card and inserting it into another device. As a result, users can keep the same number although they've switched handsets. Users can also travel to certain areas of the U.S., or countries where the GSM network uses a different frequency, hire a mobile phone, plug in their SIM Card and receive all their phone calls and SMS messages using their own usual number. The GSM phone system used with a notebook PC and PC Card provides a comprehensive plug-and-play solution for communications on the go. Data and fax capabilities at 9600bps, along with special features like inter- national roaming and Short Message Service (SMS) enable mobile users to connect easily and reliably when traveling from country to country. Keep in mind that data capabilities are not automatic--GSM service providers must implement this functionality for mobile users to take advantage of it. Data services can be Mobile Originated or Mobile Terminated. Mobile Originated (MO) implies that users can send data from a remote location using the GSM network. Mobile Terminated (MT) means users can receive data, faxes or SMS messages on their notebook using the GSM network. Figure: GSM data over the PSTN/ISDN network The GSM system is being compromised by the following subsystems: Mobile station (MS) Base station subsystem (BSS) Base Transceiver (BTS) Base Station Controller (BSC) Mobile services Switching Centre (MSC) Home Location register (HLR) Visitors Location register (VLR) Equipment Identity Register (EIR) The ITU is responsible for allocating the radio spectrum used by the MS and the BTS. GSM is using TDMA (Fig2.2) where each signal uses the same frequency, but is given a time slot in this frequency. Features of GSM Roaming When accessing foreign GSM networks, bilateral roaming agreements between local service providers regulate reciprocal access to each others network. Mobile users do not have to worry about the various foreign dialing tones, access codes, country codes and incompatible plugs. Once users turn on their phones and log onto the network, the system finds them. Users are able to make voice calls, collect E-mail and receive database updates just as if they were at home. This feature will be supported in all future developments of GSM technology. Short Message Service. The GSM SMS was previously used to indicate that voice mail was waiting, but it can also be used to indicate E-mail and faxes. Short messages can contain 160 characters. With appropriate software, SMS messages can be viewed, logged, sent and received on your portable computer like it is done today with E-mail. This service functions similarly to two-way paging, but has quickly evolved into an electronic messaging system. Specialized communications software like "Wireless Office" from Trio, enables full integration of SMS on the notebook so that messages may be typed directly from the portable PC. Otherwise, messages must be typed on the user's handset. SMS can provide an applications gateway that facilitates message transfers between corporate information platforms, or LANs, and the network operator's SMS center. GSM Modem Connection GSM data transfers currently operate at maximum data rates of 9600bps. Data communications can be performed in either transparent or non-trans-parent modes depending on the application and needs of the user. Using a 9600bps link means that data is being converted from the PC's digital signal to analog, then back to digital. Since a GSM mobile phone and a PC are both digital equipment, a modem in the true sense of the word is not necessary. What is needed is a Terminal Adapter Equipment (TAE) to translate between different digital formats of the GSM phone and the PC, and to simulate an ordinary modem to the PC by sending signals such as a dial tone and busy tone. By using a data compression protocol like MNP 5 or V.42bis, large amounts of data can be transferred in shorter times. The increase is in throughput, not in actual rate. The extent of compression varies due to file type and can only be used in data transmissions, not fax. Transparent and Non-transparent Mode GSM data transfers are performed in two ways--transparent and non-trans- parent. In transparent mode, error correction is not integrated in the GSM system and is handled by the two communicating modems using standard protocols such as MNP 2. This is fine as long as the radio connection is stable. Should there be unstable conditions, like a weak signal, non-trans-parent mode is preferred. Non-transparent mode means that the GSM system handles error correction by using the Radio Link Protocol (RLP), which gives a more reliable transmission. In non-transparent mode, data compression can also be added which increases data transmission speeds to above 9600bps. CDMA CDMA is a modulation and multiple access technique that is based on spread spectrum technology. It is designed to combat the problem of capacity shortage in mobile telecommunication world. With CDMA, multiple user can share the same frequency band without interfering each other if the transmit power of mobile station (MSs) is carefully controlled. The same frequency can be used in all neighboring cells (almost 1/1 frequency reuse). On the other hand there is no hard limit on the number of active users that can be serviced by a base station (BS). When the number of active users exceeds the design value, more traffic channels can be provided by degrading quality of service. This is usually referred to as soft capacity. Therefore, the capacity of CDMA systems is potentially unlimited. With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo- Random Code Sequences." All users share the same range of radio spectrum. THE GOAL OF SPREAD SPECTRUM is a substantial increase in bandwidth of an information-bearing signal, far beyond that needed for basic communication. The bandwidth increase, while not necessary for communication, can mitigate the harmful effects of interference, either deliberate, like a military jammer, or inadvertent, like co-channel users. The interference mitigation is a well-known property of all spread spectrum systems. However the cooperative use of these techniques in a commercial, non-military, environment, to optimize spectral efficiency was a major conceptual advance. SPREAD SPECTRUM systems generally fall into one of two categories: frequency hopping (FH) or direct sequence (DS). In both cases synchronization of transmitter and receiver is required. Both forms can be regarded as using a pseudo-random carrier, but they create that carrier in different ways. FREQUENCY HOPPING is typically accomplished by rapid switching of fast- settling frequency synthesizers in a pseudo-random pattern. The references can be consulted for further discussions of FH, which is not a part of commercial CDMA. CDMA uses a form of direct sequence. Direct sequence is, in essence, multiplication of a more conventional communication waveform by a pseudonoise (PN) ±1 binary sequence in the transmitter. We are taking some liberties with the details. In reality spreading takes place prior to any modulation, entirely in the binary domain, and the transmitted signals are carefully band limited. A second multiplication by a replica of the same ±1 sequence in the receiver recovers the original signal. The noise and interference, being uncorrelated with the PN sequence, become noise-like and increase in bandwidth when they reach the detector. The signal to-noise ratio can be enhanced by narrowband filtering that rejects most of the interference power. It is often said, with some poetic license, that the SNR is enhanced by the so-called processing gain W/R, where W is the spread bandwidth and R is the data rate. This is a partial truth. A careful analysis is needed to accurately determine the performance. In IS-95A CDMA W/R = 10 log(1.2288 MHz/9600Hz) = 21 dB for the 9600 bps rate set. Features of CDMA Noise suppression- Based on spectrum spreading , the CDMA receiver can operate in very low SNR (Signal-to-Noise Ratio) environment (say less than 0db), due to its high processing gain (21db). Soft capacity - This means high capacity gain i.e. the capacity of the cdma channel is much more than analog and digital channel. We can say that much data can be transmitted at a high speed. Soft hand off - When a subscriber moves from one cell to another a connection is established between the subscriber and the new BTS automatically this technique is called make before break and so it is called soft handoff. When there is a break, when a subscriber moves from one cell to another and then a connection is established it is called break before make. High capacity- CDMA system provides an increase in capacity in order of 4 to 6 fold over digital TDMA (e.g., GSM, IS-54) and 10 fold over analog FM/FDMA (AMPS). A number of CDMA features contribute to the increased capacity. High capacity is due to the single frequency channel, with one to one frequency reuse. The total bandwidth in which the CDMA signal propagates is 800MHz-1900MHz which is of very high bandwidth so the capacity is also high. as many as 95 million subscribers worldwide can use this technology at the same time. General Packet Radio Service (GPRS) As the name implies GPRS is based on packet switching technology making this technology a better candidate for Internet related applications that can run on mobile phones. Again GPRS implementation requires little changes on the existing GSM networks. As shown in Figure, the only addition to the GSM network is a packet radio network, and a couple of software upgrades to some existing node Figure: IEEE Spectrum October 10 Network Features Of GPRS Packet Switching GPRS involves overlaying a packet based air interface on the existing circuit switched GSM network. This gives the user an option to use a packet-based data service. To supplement a circuit switched network architecture with packet switching is quite a major upgrade. However, as we shall see later, the GPRS standard is delivered in a very elegant manner- with network operators needing only to add a couple of new infrastructure nodes and making a software upgrade to some existing network elements. With GPRS, the information is split into separate but related "packets" before being transmitted and reassembled at the receiving end. Packet switching is similar to a jigsaw puzzle- the image that the puzzle represents is divided into pieces at the manufacturing factory and put into a plastic bag. During transportation of the now boxed jigsaw from the factory to the end user, the pieces get jumbled up. When the recipient empties the bag with all the pieces, they are reassembled to form the original image. All the pieces are all related and fit together, but the way they are transported and assembled varies. The Internet itself is another example of a packet data network, the most famous of many such network types. Spectrum Efficiency Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues. GPRS should improve the peak time capacity of a GSM network since it simultaneously: allocates scarce radio resources more efficiently by supporting virtual connectivity imigrates traffic that was previously sent using Circuit Switched Data to GPRS instead, and reduces SMS Center and signalling channel loading by migrating some traffic that previously was sent using SMS to GPRS instead using the GPRS/ SMS interconnect that is supported by the GPRS standards. Internet Aware For the first time, GPRS fully enables Mobile Internet functionality by allowing interworking between the existing Internet and the new GPRS network. Any service that is used over the fixed Internet today- File Transfer Protocol (FTP), web browsing, chat, email, telnet- will be as available over the mobile network because of GPRS. In fact, many network operators are considering the opportunity to use GPRS to help become wireless Internet Service Providers in their own right. The World Wide Web is becoming the primary communications interface- people access the Internet for entertainment and information collection, the intranet for accessing company information and connecting with colleagues and the extranet for accessing customers and suppliers. These are all derivatives of the World Wide Web aimed at connecting different communities of interest. There is a trend away from storing information locally in specific software packages on PCs to remotely on the Internet. When you want to check your schedule or contacts, instead of using something like "Act!", you go onto the Internet site such as a portal. Hence, web browsing is a very important application for GPRS. Because it uses the same protocols, the GPRS network can be viewed as a sub- network of the Internet with GPRS capable mobile phones being viewed as mobile hosts. This means that each GPRS terminal can potentially have its own IP address and will be addressable as such. Supports TDMA And GSM It should be noted right that the General Packet Radio Service is not only a service designed to be deployed on mobile networks that are based on the GSM digital mobile phone standard. The IS-136 Time Division Multiple Access (TDMA) standard, popular in North and South America, will also support GPRS. This follows an agreement to follow the same evolution path towards third generation mobile phone networks concluded in early 1999 by the industry associations that support these two network types. Enhanced Data rates for Global Evolution (EDGE) EDGE is a radio based high-speed mobile data standard. EDGE was first proposed to ETSI in the beginning of 1997, as a means of evolving GSM. EDGE reuses the GSM carrier band- width and time-slot structure and provides an efficient way of increasing bit rates, there- by facilitating the evolution of existing cellular systems toward third-generation capabilities. The future of the current GPRS technology is EDGE, which was initially trumpeted as being three times faster than GPRS, giving a theoretical top speed of 384 Kbit/s. But GPRS only manages 50 Kbit/s, so realistically Edge will probably be closer to 150 Kbit/s. GPRS is based on a modulation technique known as Gaussian minimum-shift keying (GMSK). EDGE is based on a new modulation scheme that allows a much higher bit rate across the air interface- this is called eight-phase-shift keying (8 PSK) modulation. Since 8 PSK will also be used for UMTS, network operators will need to incorporate it at some stage to make the transition to third generation mobile phone systems. EDGE can be thought as the next upgrade step to GSM operators that are using GPRS. One of the great strengths of Ericsson´s EDGE (Enhanced Data rates for Global Evolution) solution is that it leverages the fusion of all the best aspects of the two leading standards: TDMA and GSM. For the first time, it will converge TDMA and GSM standards to evolve a common third generation network. A network that ultimately will provide seamless roaming for voice and data between TDMA and GSM via the same terminal. Implementation of EDGE by network operators has been designed to be simple. Only one EDGE transceiver unit will need to be added to each cell. The new EDGE capable transceiver can also handle standard GSM traffic and will automatically switch to EDGE mode when needed. EDGE capable terminals will also be needed since the existing mobile phone or terminals do not support the new modulation techniques and will need to be upgraded to use EDGE network functionality. EDGE provides the most cost-effective means to provide IP-based multimedia services and applications within existing spectrum. The advantages of EDGE include rapid availability, the reuse of existing GSM and TDMA infrastructure, and support for gradual introduction. In addition, it allows the full advantages of GPRS to be explored, with fast connection set-up, higher bandwidth, and data rates as high as 384 Kbps. FUTURE TECHNOLOGIES It looks like the road to third generation mobile systems has already begun. For some time there was a great confusion on the standards that are going to support enhanced GSM services. It is not clear yet as to what will be the final solution but there are some indications of the most powerful candidates. The ITU has put together a project that will bring 3G technologies together and create a set of de facto standards. This project is the IMT-2000. International Mobile Telecommunications-2000 (IMT-2000) are third generation mobile systems which are scheduled to start service around the year 2000 subject to market considerations. The picture shows the how IMT-2000 will accomplish such a goal. CDMA2000 cdma2000 is a 3G Radio Transmission Technology (RTT) proposal submitted by TIA. TR45.5 subcommittee of TIA developed the cdma2000 proposal. 2G CDMA standards i.e. IS-95 family of standards is also developed and maintained by the same group. The latest version of IS-95 is IS-95B. IS-95 based systems have been developed and deployed in the USA and around the world except for the European countries. cdma2000 is designed to be backward compatible with IS-95B. This ensures reuse of most of IS-95 and related standards, graceful evolution to third generation systems from 2G systems and protection of investments of 2G systems operators. cdma2000 builds on top of the IS-95 channel structure. cdma2000 provides two deployment options. One of them is Multi-Carrier configuration where available bandwidth is divided into N 1.2288 MHz carriers. The user data is demultiplexed and spread on to N separate channels. This allows IS-95 and cdma2000 systems to co-exist in overlay/underlay configuration. Multi-Carrier option is supported only in the forward direction. The other deployment configuration is Direct Spreading (DS) option where the user data is spread over the entire bandwidth. There are several new features in cdma2000 compared to IS-95B. cdma2000 enhances supplemental channel to support high data rate requirements (up to 2 Mbps) of IMT-2000. It introduces dedicated and common control channels to provide efficient packet data service. Variable length walsh codes (from 4 bits to 1024 bits in length) are used for spreading on supplemental channel to support various information rates. In addition to fast reverse link closed loop power control, cdma2000 provides fast forward link closed loop power control. Dedicated and common auxiliary pilot channels are introduced to take advantage of smart antennas. Smart antennas have the ability to provide coverage directed at specific geographical areas or mobile stations. W-CDMA W-CDMA is an RTT proposal submitted by ETSI. It is developed by SMG2 working group of ETSI. It is a radio interface of choice for Universal Mobile Telecommunication System (UMTS). UMTS is a forum under which ETSI, and European companies are developing specifications for 3G systems. UMTS is the successor to the 2G GSM systems. SMG2 is also responsible for development of Global System for Mobile communication (GSM) radio interface for 2G systems. GSM systems are deployed in Europe and around the world with a small presence in the USA. The 2G GSM radio interface is based on TDMA technology. W-CDMA represents a significant change in 3G systems. W-CDMA is not designed to be backward compatible with GSM air interface. However, handoffs between UMTS and GSM systems are supported. There are several new features in W-CDMA. W-CDMA supports high data rate requirements of IMT-2000. Orthogonal Variable Spreading Factor (OVSF) codes are supported to provide variable data rates. W-CDMA supports asynchronous mode of operation where reception and transmission timings of different cell sites are not synchronized. It supports fast cell acquisition when a mobile initially powers up. It also supports forward and reverse fast closed loop power control operation. COMPARISON BETWEEN CDMA2000 AND W-CDMA In this section, we will analyze the similarities and differences between the two RTT proposals. Our analysis is presented based on the following categories: Categories for analysis of details CATEGORY DETAILS Bandwidth and Data Rates Channel bandwidths, Data rates, Spread rates Deployment configuration Deployment configuration in reverse and forward direction Pilot channel and system acquisition Code based and Time multiplexed Pilot channel, continuous vs. discontinuous pilots, common, dedicated and auxiliary pilots, system acquisition differences Synchronous/Asynchrounous mode of Inter-cell synchrounization operations requirements, impact on handoffs, timing issues Channel and frame structures Logical and physical channels, frame sizes and content structures Cell and users identifications codes PN, walsh, OVSF, cell specific scrambling, preamble and gold codes Power control and handoffs Open loop, closed loop power control schemes, hard, soft and softer handoff comparison, inter-technology and inter- generation(2G and 3G)handoffs Data services support Packet and circuit data, multimedia support, MAC control states, LAC layer support, Qos mechanism Smart antenna applications Beam forming, adaptive steering application, dedicated and auxiliary pilots Other areas Diversity mechanisms-Orthogonal tansmit and multi-carrier transmit diversities Bandwidth and Data rates The bandwidths and spreading rates for cdma2000 are given in table 2. The supported bandwidths are 1.25 MHz, 3.75 MHz, 7.5 MHz, 11.25 MHz and 15 MHz. cdma2000 uses a base spreading rate of of 1.2288 MHz. The spreading rates are 1.2288 * N Mcps. The value on N can be 1, 3, 7, 9 and 11. The spreading rates are defined as multiple of 1.2288 Mcps to ensure backward compatibility with IS-95B. Bandwidth Spreading Rates (Mhz) (Mcps) 1.25 1.2288 3.75 3.6864 7.5 7.3278 11.5 11.0952 15 14.0456 The bandwidths and spreading rates for W-CDMA are given in table. The supported bandwidths are 5, 10 and 20 MHz. W-CDMA uses a base spreading rate of 4.096 MHz. The spreading rates are in multiples of 4.096 Mcps. Unlike cdma2000, W-CDMA is not burdened by requirements of backward compatibility with a 2G CDMA technology. Bandwidth Spreading Rates (Mhz) (Mcps) 5 4.096 10 8.192 15 16.384 Deployment Configurations Cdma2000 RTT supports two different deployment configurations; one configuration is a Multi-Carrier (MC) based approach and the other one is the Direct Spread (DS) approach. In the forward link, i.e. in the direction of transmission from the base station to mobile stations, both the MC and DS configurations are defined and supported by cdma2000. However, in the reverse direction i.e. transmission from the mobile stations to the base station only DS is defined and supported. In Multi-Carrier (MC) configurations, the available bandwidth is divided into multiple carriers of 1.25 MHz each to carry data over the air. Each carrier supports a chip rate of 1.2288 Mcps to transport data over the air. MC configuration has been designed in the cdma2000 to provide a graceful evolution from IS-95B 2G systems to cdma2000 3G systems. In that case, backward compatibility with IS-95B systems is required. For example, Multi-carrier systems may be used to deploy as an overlay configuration. In this configuration, we can set up systems to provide IS-95B and cdma2000 services simultaneously to mobile stations. This means that we can deploy cdma2000 services without needing a clear spectrum for deployment. Both IS-95B and cdma2000 subscribers are supported over the same frequency channel. Moreover, an overlay configuration allows IS-95B system may share its pilot channel with a cdma2000 system. It is also possible to share paging channels and provide service in a cooperative manner between the IS-95B and cdma2000 systems. cdma2000 supports Direct Spread (DS) configuration in both the forward and reverse links. The chip rates supported are at 1.2288N where N = 1,3,6,9,12. In DS configuration, transmission is done using a single carrier with appropriate bandwidth to carry the spread signal at the rate of 1.2288N Mcps. Note that for both the MC and DS configurations, a guard band of 1.25/2MHz (0.625MHz) is provided on both sides. In the forward link, cdma2000 DS configuration may be used in environments where there are no constraints of backward compatibility and clear spectrum for the bandwidth desired is available. W-CDMA supports only Direct Spread (DS) configuration in both the forward and reverse links. Again, this is because W-CDMA does not have any backward compatibility constraints. Pilot channels Pilot channels, in general, are used to provide a reference signal to receivers. Receivers derive timing and phase information from pilot signal, which helps in coherent demodulation of user signals. cdma2000 employs a common pilot channel in forward direction that is shared by all mobiles as shown in the figure below. . This cdma2000 pilot channel is a code-multiplexed channel using Walsh codes for orthogonal spreading. Common pilot channel provides all the mobiles with a reference signal for channel estimation and a timing reference. The pilot channel is also used for system acquisition. In the reverse direction cdma2000, uses time multiplexed pilot signals which are dedicated to each user. In WCDMA, pilots are time multiplexed signals that are part of traffic and overhead channels in both directions. Therefore, W-CDMA pilots are dedicated channels. As shown in the figure below, each traffic frame contains certain number of pilot bits. Unlike cdma2000, Pilots are discontinuous waveform signals in WCDMA. W-CDMA uses pilot channel for channel estimation and coherent demodulation in both directions. Since, pilots are dedicated, they cannot be used for system acquisition. The system acquisition in W-CDMA is performed using synchronization channel. System acquisition System acquisition in cdma2000 is performed using pilot and sync channel. All cdma2000 base stations transmit the pilot contents over the air using QPSK modulation. First, QPSK modulation is applied to the pilot sequence of 0’s. The QPSK components, the In-phase and Quadrature components, are separately spread using Walsh code 0. The resulting sequence (which is again a sequence of 0’s) is then complex PN spread using two PN sequences. The two PN sequences are the same ones used in IS-95B and have a period of 26.66 msec i.e. 32768 or 2 15 chips. Mobile stations acquire a base station by searching for forward common pilot channels. One way to search is to locally guess or generate the short PN sequence and correlate it with received signal. Mobile chooses the signal with the highest correlation value, i.e. the base station with the strongest signal. When a match is found, mobile continues the PN sequence generation and continually verifies it. When mobile concludes that it has acquired a base station, it only means that the mobile station has found a pilot of a base station. No other information could be deduced at this point. Sync channel demodulation follows system acquisition. Sync channel uses Walsh code 32 for orthogonal spreading. Sync channel parameters message contains information about system time, PN offset used and the long code state value to be used. Mobile station sets the long code state to the received long code state value. Mobile station then aligns its time with the base station. Mobile is now ready to demodulate the paging channel. System acquisition in WCDMA is performed using synchronization channel and primary common control physical channel which carries Broadcast Control Channel (BCCH). The BCCH carries system and base station related configuration information. WCDMA mobile stations acquire the system using the following procedure: 1. Mobile station searches for the base station to which it has the least path loss. This is done by looking for the primary SCH code which is the same for all base stations. Mobile station has now acquired slot synchronization 2. Next the mobile station looks for the secondary synchronization channel sequence used by monitoring the secondary SCH and matching it to the 32 possible sequences. This identifies which of 32 scrambling code groups are used 3. Mobile station does symbol-by-symbol correlation to determine the exact scrambling code used by the cell. Now the mobile station has acquired the system and is ready to receive system information on the primary common control channel. Synchronous vs. Asynchronous cells cdma2000 employs synchronous mode of operation . The synchronous operation means the transmission and reception timings of cell sites are synchronized. All cdma2000 cell sites use single common timing source such as Global Positioning System (GPS). All cell sites transmit a Pseudo Random (PN) sequence of length 2 15 -1 chips with a period of 26.67 milliseconds. The transmission of this sequence in different cell sites is offset in multiples of 64 chips. These timing offsets, known as pilot PN offsets, distinguish one cell site from another. Tight synchronization between different cell sites assists in soft handoff. The mobile can measure the timing and signal strength differences between its current reference cell and candidate cells in terms of 64 chips. This measurement is reported to the network to assist in making decisions on whether to add cells into soft handoff. However, dependence on common source for synchronization can result in a single point of failure. The loss of that source may make entire system unavailable for service. W-CDMA employs asynchronous mode of operation as. In other words, transmission and reception timings of different cell sites need not be synchronized. Each cell site transmits cell-specific scrambling code sequence. This is different from cdma2000 where same scrambling code is transmitted in all cells with different time offsets. Scrambling code uniquely identifies a cell site. The scrambling codes are 40,960 chips in length. They are transmitted with a period of 10msec. The asynchronous mode of operation avoids having a single point of failure. Channel Structure The cdma2000 channel structure is derived from IS-95B. Additional channels are provided to support high rate data and multimedia services.. cdma2000 channels can be divided into common and dedicated channels. Common channels are point to multipoint channels between base station and multiple mobile stations used in shared access mode. The pilot, sync, paging and access channels are common channels derived from IS-95B. The functionality of pilot and sync channels is described in system acquisition section. The paging channel is used in forward direction (from base station to mobile station) to carry system wide and mobile specific information when mobile is not in a call. For example, system wide information such as number of paging channels is sent on paging channel. Mobile specific messages such as the paging message required to initiate a mobile terminated call is also sent on paging channel. Access channels correspond to paging channels in the reverse direction. Access channels are uncoordinated, shared channels used by the mobile stations to communicate with the base station while not on a call. Access channels are typically used to originate call requests or respond to page messages received from base stations. In addition to these channels, cdma2000 defines common control channel. Common control channel has been introduced for data services. It carries short user bursts, MAC signaling for packet and other data services. Dedicated channels are point to point channels between base station and a single mobile station carry innformation dedicated to single user. cdma2000 defines fundamental and supplemental channels to carry information related to user services. The fundamental and supplemental channels are derived from IS- 95B. However, different coding and modulation techniques are used in cdma2000. The fundamental channel is transmitted at variable rate as in IS-95B and requires blind detection at the receiver. Therefore it suited for voice services. It is also used for low rate data services. The data rates on fundamental channel is limited to 14.4 kbps. Fundamental channel supports rate set 1 (9.6, 4.8, 2.4 and 1.2 kbps) and rate set 2 (14.4, 7.2, 3.6 and 1.8 kbps). Different rates are used based on voice activity. Fundamental channel supports 5 and 20 msec frame sizes. The fundamental channels use fixed walsh code length. Supplemental channels in both directions are similar to IS-95B supplemental channels. Supplemental channels support data rates from 1.2kbps to more than 2Mbps. cdma2000 specifies supplemental channel as a way of meeting IMT- 2000 high rate data rate services requirements. Supplemental channel can be operated in two modes. In the first mode, supplemental channel supports Rate Set 1 and Rate Set 2. in variable rate mode. It can carry voice and low rate data in this mode. In the second mode, supplemental channel supports data rates higher than 14.4 kbps. The data rates can go upto 2 Mbps. However, in the second mode, supplemental channel does not operate in variable rate mode and does not require blind detection at the receiver. The data rate must be explicitly negotiated between base station and mobile station and any change must be renegotiated. In both modes, supplemental channels support only 20msec frames. The supplemental channel supports different walsh code lengths. This is required to support various rates as specified in IMT-2000. In addition to these channels, cdma2000 defines dedicated control channel. It carries upper layer signaling, MAC signaling and short user bursts. Dedicated control channel has several purposes. It can carry short user bursts for data services. It can be used in association with fundamental channel carrying signaling and power control information. Thus fundamental channel carries only voice information providing high quality voice services. Dedicated control channel can also be used in association with supplemental channel carrying power control and link continuity information. This is useful for high rate data services configuration and eliminates the need for fundamental channel. W-CDMA channel structure is very simple one. As with cdma2000, channels can be classified as common and dedicated channels. There are four common channels in W-CDMA. They are Broadcast control channel (BCCH), Paging Channel (PCH), Forward Access Channel (FACH) and reverse Random Access Channel (RACH). The BCCH is defined in forward direction. It carries system wide and cell specific information. After mobile powers up and synchronizes with the system, it tunes to BCCH to obtain system configuration. The paging channel is used to carry control information to a mobile station when the base station does not know the location of mobile in the cell. The forward access channel is used in forward direction. It carries control information and short user packets when base station knows the location of the mobile in the cell. It may be used for directed coverage by smart antenna beam-forming applications. The reverse random access channel is similar to access channel in cdma2000. As in cdma2000, it is accessed by multiple mobile stations in un-coordinated shared access mode. It is typically used to carry control information. All the common channels in W-CDMA use frame size of 10 msec. W-CDMA defines only one dedicated channel It is known as Dedicated Channel (DCH). W-CDMA specifies DCH as a way of meeting IMT-2000 high rate data rate requirements It supports data rate of upto 2 Mb/s. Unlike fundamental channel or supplemental channel, the DCH does not operate in variable rate mode and does not require any blind detection at the receiver. The data rates must be explicitly negotiated and any changes must be renegotiated. The DCH supports variable rate OVSF codes which are described in next section. This is required to support different data rates required by IMT-2000. Frame Sizes Layer 3 signaling and user traffic is carried in 20 msec frames in cdma2000 In addition, cdma2000 employs 5 msec frames to carry MAC signaling. Longer frame sizes improve time diversity as interleaving and sequence repetition is done over the entire frame span. This improves the error performance. Thus, longer 20 msec frames are used for voice and data services. Shorter frame length reduces the total delay which is required for voice and MAC signaling. One of the applications of MAC signaling is faster reassignment of traffic channels. This is typically used in packet data services. Packet data services are characterized by discontinuous traffic with short bursts of high activity interleaved with periods of idle time. Since idle times are usually high compared to burst times, radio channel usage can be inefficient. Traffic channels are deallocated during idle periods. When user has traffic to send, fast assignment of traffic channel is done using MAC signaling. . On the other hand, W-CDMA uses a frame size of 10 msec for Layer 3 signaling, user traffic and MAC signaling. The choice of one frame size allows for simplicity and ease of implementation. It is a compromise between better error performance requirements of voice and data services and low delay requirements of voice and MAC signaling. Channelization codes Channelization codes are required to distinguish channels ( discussed in previous section ) in both directions. cdma2000 uses Walsh codes to differentiate between channels in the forward link. Walsh codes are orthogonal codes. Channels are spread using appropriate length codes based on the data rate supported on the channel. All the forward link channels within a cell/sector are separated using Walsh codes. Walsh codes are unique not just within channels of same user, but across different users in the same cell. The network controls Walsh code allocation for forward link channels. On reverse link also, cdma2000 uses walsh codes to differentiate between channels. However, unlike in forward link, walsh codes only distinguish channels from same user. In other words, channels from different users may use the same walsh code. User separation on reverse link is achived by user specific PN codes as discussed in the next section. W-CDMA uses Orthogonal Variable Spreading Factor (OVSF) codes for channel separation in the forward link. OVSF codes are similar to Walsh codes and the length of code used for a channel is dependent upon the channel’s data rate. The network controls OVSF allocation for forward link channels. On reverse link also, WCDMA uses OVSF codes to differentiate between channels. Similar to cdma2000, OVSF codes only distinguish channels from same user. User separation on reverse link is achieved by user specific scrambling codes as discussed in the next section. Cell and User separation codes Cell separation codes are required to identify transmission from different cells in forward direction. In cdma2000, cell separation is performed by two PN sequences of length 2 15 –1 chips, one for I channel and another for Q channel. These sequences have a duration of 26.67 msec. Same sequences are used in all cells. However, transmission of these sequences in different cells is offset in multiples of 64 chips. These offsets are called PN offsets. Each cell uses a unique PN offset to distinguish its transmission from its neighboring cells. In W-CDMA, cell separation is achieved by gold sequences (called as scrambling codes) . Unlike cdma2000, each cell uses a different gold sequence to identify its transmission uniquely. These sequences have a length of 40,960 chips and a duration of 10 msec. User separation codes are required to identify transmission from different users in the reverse link. In cdma2000 long PN code of length 2 42 -1 are used to distinguish different users in the reverse direction. Again, the same long code is used by all the users in all cells. However, the transmission from different users is offset by different number of bits. This offset is achieved by using Electronic Serial Number (ESN) which is unique to each user. In WCDMA, either short or long scrambling codes are used to distinguish users in reverse direction. Short codes are very large kasami codes of length 256 while long codes are gold sequences of length 2 42 . Power Control Both cdma2000 and W-CDMA use reverse open loop power control and bi- directional closed loop power control. In open loop power control, the mobile estimates transmit power based on received power in forward direction. This is based on the assumption that the path loss is equal in both forward and reverse direction. Open loop power control is used when mobile is initially accessing the system. The closed loop power control is based on feed back scheme. The receiver instructs to the sender either to increase or decrease transmit power based on received power. The sender then adjusts the transmit power. The instructions are one bit up/down commands. These are called power control commands. Both, cdma2000 and W-CDMA employs closed loop power control in forward and reverse direction. However, the difference is in the rate of power control commands. In cdma2000, power control commands are sent at the rate of 800 bits/second whereas in W-CDMA power control commands are sent at the rate of 1,600 bits/second. Handoff cdma2000 supports both soft and softer handoffs. In soft handoff, mobile measures the pilot strength from candidate base stations. The measurements are reported to network through current source base station. Please note the cdma2000 uses synchronous cells in which case the transmissions from different base stations is time offset in multiples of 64 chips. Therefore, mobile can measure signal strength once every 64 chip time period or multiple of 64 chip time period. Thus, the measurement becomes simpler. The measurements are associated with timing offsets in multiples of 64 chips from the current source base station. The network can infer base station identity from the timing offsets. It uses measurements to add or delete base stations to soft handoff. Once base stations are added to soft handoff, they begin to transmit towards mobile. The transmissions from different base stations arrive at mobile stations at different time offsets which is again in multiple of 64 chips. The mobile coherently combines signals from different base stations. In the reverse direction the transmission from the mobile is received by all base stations involved in soft handoff. After demodulation, the base stations send frames to a centralized location in the network. Among these frames, the best frame is selected and used. W-CDMA also supports soft and softer handoff. The soft handoff mechanism has many similarities to soft handoff mechanisms in cdma2000. However, there are many differences. As in cdma2000, the mobile measures signal strength from different base stations. But W-CDMA uses asynchronous cells where transmissions from different base stations are not coordinated. The mobile must estimate the timing offset between current base station and the target base station. It reports the estimated timing offset for candidate base stations to network through the current base station. The network adds the target base station to the soft handoff and informs it off the timing offset. The target base station uses the timing offset to adjust the transmission on downlink. The adjustment is done in steps of 256 chips to maintain orthogonality. Thus, mobile receives transmissions from different base stations at approximately same time instant. This is different from cdma2000 where the transmissions from different base stations are time offset in multiples of 64 chips. The mobile uses maximal ratio combining to combine signals from different base stations. In reverse direction, the transmission from mobile station is received. In addition, both cdma2000 and W-CDMA support inter-frequency handoffs. cdma2000 also supports handoffs from cdma2000 system to IS-95B system and vice versa. The W-CDMA supports handoffs from W-CDMA system to GSM system and vice versa. Smart antenna applications Smart antenna applications provide for beam forming covering portions of cells/sectors or adaptive beams directed at a single mobile station. Such applications provide for enhanced coverage in hot spots such as sports arenas and high propagation loss areas or increase the data rate for high speed terminals. As discussed earlier, cdma2000 uses common pilot channel applied to entire cell. However, beam forming is applied to cover a smaller portion of a cell. Therefore, the receivers would require an additional dedicated pilot for reliable channel estimation for beam forming application. Channel estimations will not be accurate if the reference pilot traverses a different path compared to the data signal. Thus, cdma2000 supports additional auxiliary pilots dedicated to one or group of mobiles in an area. Auxiliary pilots use a different Walsh code for spreading to differentiate it from common pilot. In order to conserve number of walsh codes, cdma2000 uses expanded walsh codes on auxiliary pilot channels. In W-CDMA, pilots are time multiplexed and dedicated to each mobile station. Therefore, in order to support adaptive beam steering applications, no additional pilots are necessary. However, W-CDMA does not have a way to support common pilot for hot spot areas like cdma2000 9 Wireless vs. Fixed In recent years a wide variety of wireless equipment has become available commercially. At first big antennas and stations were needed to transmit and receive data signals. The wireless technology has advanced dramatically making small devices capable of transmitting data at high data rates with little power consumption. The size and the cost of the devices and stations have been reduced dramatically as well. Making the use of wireless products more widespread. Wireless communications can be used to provide broadband services between a communications provider equipment to the end user without having to install new cabling. Speeds up to 10Mbps can be accomplished in short range (up to 90ft) and up to 860 Kbps (max. distance 800ft). Cellular technology, which was originally developed to carry voice, has been adapted to carry data as well. For short-range connectivity and in-house applications the new gadget is called Bluetooth. Bluetooth is a very small chip that can be fitted in almost every handheld device providing with the ability to communicate with other devices with speeds up o 2 Mbps. The Ericcson R520 WAP (Wireless Application Protocol) phone will support both GPRS and HSCSD and it will have a Bluetooth chip as well. But others question why the world needs five different 3G standards. With 2.5G -- a necessary evil in the move to 3G -- there are going to be a huge number of standards that need to be supported. "Handsets are going to need to support multiple standards, so an average handset will have to support GSM, GPRS, Edge [a 2.5G standard] and HSCSD [the 2.5G standard endorsed by Orange]," says Russell Inman, 3G system design manager for infrastructure provider Crown Castle. "This plethora of standards places quite a demand on manufacturers," he says. However rocky the journey to 3G there is one thing all the experts agree on -- it is going to be huge. Conclusion The future mobile network seems to be worth waiting. The functionality that provides is amazing, bringing rich multimedia content into our hands and at the same time giving the flexibility to the user to go anywhere and have access to any information needed. There are however some potential problems. Providers in Europe have already invested millions just to buy 3G licenses and are expected to spend more than $300 billion to bring together the mobile phone and the Internet. There is a certain risk involved in such a move. The problem is not so great in Asia, Japan and South Korea where they have been given their licenses for free. Many of the expectations might not fulfill. The operators hope that 3G mobile phones will attract a large number of customers willing to pay for the extra capacity of 3G. Still there are a lot of details to be refined and a strong collaboration must exist among the various players of the mobile industry. It seems likely that third-generation phones be available sometime in 2002 to coincide with the completion of the UMTS networks. The wireless road ahead won't be an instant bonanza for just anyone, however. "There will be some spectacular failures, and some spectacular successes,"
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