The present report is a critical evaluation of the technologies that would allow for the migration from 3G to 4G Cellular Mobile Communication Systems. A brief description of the most important cellular technologies has been considered as an introduction to 4G concepts, characteristics and main objectives. Special attention will be given to the techniques that allow for 4G development. Cellular Mobile Communication Systems
For the past three decades, the development of technology has allowed people to communicate their ideas and share information by means of a mobile device. Since the early 1980 s, when the first commercial cellular system appeared, demands have changed and technology has evolved to meet the needs of communication. First generation mobile communication’s systems used analogue voice transmission; this was followed by the introduction of digital communications on 2G systems. The result was improved network capacity and quality and it created the possibility of text messaging and data transmission at low rates. Making data transmission and multimedia services available requires higher data rates than can be achieved by 2G due to its spectral inefficiency. To meet these demands, 3G was developed. Its advantages are that it offers a solution to 2G backward compatibility and fully compatible multimedia functions that improve system capacity and offer higher speed packet data rates. The reference data rates required for 3G systems are shown in Table 1. Table 1 . 3G Mobility Characteristics vs. Data Rate  With the aim of unifying all the systems under a global telecommunications standard, ITU-R defined IMT-2000, which establishes five modes of operation. The most important developments have been considered on Direct Spread IMT-DS and m ulti- carrier CDMA IMT-TC. Table 2 . 3G Standards as defined within IMT-2000  UTRA FDD – WCDMA (UMTS air interface) is the implementation of IMT-DS and is the 3G upgrade of GSM/GPRS 2G networks. Its advantages include robustness to frequency, selective fading and the need for less power to achieve the determined performance level. High Speed Downlink Packet Access (HSDPA) is an improvement to this technology. It introduces a faster set- up procedure, supports Voice over IP services, and introduces a 15 to 50 per cent improvement in efficiency . In contrast, CDMA systems have been implemented to meet IMT-MC standards as IS-95 ANSI/TIA/EIA-41 networks have improved. IS-2000 CDMA 1X-RTT is the first 3G approach whereby high data rates can be achieved. This is due to the assigned supplemented channels and based on the availability network resources. Based on CDMA technology has developed to provide: 1X-EVDO (Evolution Data Only), and 1X-EVDV which combines 1X-EVDO with traditional voice services i n a single carrier. However, 3G systems present technical weaknesses such as a difficulty in increasing bandwidth and thus achieving higher transmission data rates to support multimedia services, the limitation of available spectrum and the impossibility of roaming between different technologies and frequency bands. The alternative to 3G mobile services are the wireless networks. The approach for each form of technology is different; the first is focused on mobility and voice services with enhanced data transmissions support, whilst the second focuses on high rates of data transmission with few mobility capabilities. The most popular wireless network technologies are Wi-Fi 802.11a/b/g, WiMax 802.16, and Mobile-Fi 802.20. 4G
The next generation of mobile communications has been given a number of names such as ‘Beyond 3G’ ( B3G), ‘4G’ or ‘MAGIC’ (Multimedia Anytime Anywhere Global Mobility Support Integrated Wireless Solution and Customized Personal Service) . No standard has yet been defined for 4G services, but all the approaches consider integration as the key factor and the use of IP as the main protocol. An important aspect of 4G networks is the possibility of accessing any service anywhere, anytime, under any conditions. Furthermore, 4G will offer users an oriented service which delivers information even if location or terminals are changed ; users will not notice service interruptions when changes occur between network technologies. The optimal technical characteristics in a 4G network are listed in T able 3 . Table 3 . 4G System Optimal Technical Characteristics. [6, 13, 1] In order to achieve the demanding characteristics for 4G, it is important to consider developments in mobile terminals, network systems and the services offered Firstly, mobile terminals must be fully internetworking compatible . This means that the MS must be capable of detecting different network technologies, selecting the best one based on its service necessities, and, finally, reconfiguring itself to be able to operate under the chosen network . Furthermore, mobile terminals need to consider new user input/output interfaces, include location awareness and improve processing capabilities. Secondly, the network must support terminal mobility and internetworking capacity, offering a good, reliable and secure service. Full mobile IP deployment must be considered and the network must be capable of performing, not only horizontal handovers, but also vertical handovers between different network technologies. In addition, the network needs a high degree of reliability, an accurate security level, and a strong capacity for fault tolerance. Traffic management, overlay scenarios between different technologies, and hierarchical structures need to be considered in order to guarantee quality of service. The system must consider a strong and reliable security level with the flexibility to operate within different technologies. Finally, services offered must be focused on the concept of personal mobility rather than terminal mobility, introducing a new business architecture concentrated on new billing schemes that contemplate many services with many providers.
Techniques for Achieving 4G
All the characteristics required for 4G systems demand a consideration of newer and more powerful technologies. These techniques cover coding schemes, modulation and multiple access technologies, security measures, smart antennas, software definite radio, and IP mobility capacity. Advanced Coding Schemes The aim of coding schemes is to improve network performance and increase capacity to, as close as possible, the Shannon capacity limit. New schemes such as concatenated codes and Low Density Parity Checks (LDPC) can introduce meaningful improvements to network capacity and quality. Concatenated codes occur by combining two or more coding techniques such as Reed-Solomon and convolution codes. The result is an improvement in error correction and detection, offering greater reliability to the network. Data can be transmitted at a faster rate, so increasing the network throughput . On the other hand, LDPC is a technique that requires less computational complexity while offering similar performance to concatenated codes. There is the additional advantage that the patent licence has expired, reducing implementation costs. In conclusion, both coding scheme techniques offer improvements in network capacity and performance that need to be considered in order to approach characteristics demanded by 4G. Multiple Access Techniques Multiple access techniques control how users share limited recourses efficiently; this is critical to the development of 4G technology and the achievement of the demanding characteristics specified. As a result, new techniques are being developed that will take into account the most important strengths of the preceding technologies and avoid their weaknesses. For example, CDMA introduces good performance in fading environments, efficient frequency re-use, and the inclusion of soft hand off capabilities but it is susceptible to interference caused by other users which degrades system performance and limits channel capacity. Based on these characteristics, CDMA has been the principal technique adopted by 3G services. CDMA variants such as MC-CDMA, DS-CDMA, and TD-CDMA have been developed as an approach for data rate improvements. The main advantage of MC-CDMA is that it is suitable for severe frequency selective fading channels. It also has a good BER response when there is low interference between users and out- band radiation. The most significant weakness of this technique is that it requires a high linear amplifier, which results in inefficient power consumption. Secondly, DS-CDMA offers the best data transmission rates, but receiver complexity is increased due to the use of Rake diversity to compensate for fading effects. Thirdly, a characteristic of TD-CDMA is that it employs the same BW for up- and downlink transmissions. This makes it a good alternative for asymmetric transmissions but it introduces delays and a long overhead is required for synchronization. Another media access technique is SDMA, based on space diversity, which improves data throughput. However, it is not always possible to distinguish users based on their location which means that the characteristic of diversity is lost. New techniques such as MC/DS-CDMA, TDD-CDMA, and OFDM have been developed as alternatives for 4G systems. The first of these, MC/DS-CDMA, was developed as a result of the concern with the high data rates and strong fading reject capabilities of MC-CDMA and DS-CDMA. L inear amplifiers are still needed due to the problem of power inefficiency that occurs . TDD-CDMA introduces important spectral efficiency, and like its predecessor, TD-CDMA, it uses the same spectrum for uplink/downlink transmissions, is compatible with 4G burst traffic, and provides extra capacity gain based on asymmetric transmissions. Finally, OFDM, which is similar to FDMA, is an important technique and has been improved in that users share different orthogonal channels. T he base station, based on bandwidth requirements, can assign subcarriers to different users. Other important improvements introduced by OFDM are high bandwidth efficiency, inter symbol interference mitigation, and reduction of multi- fading effects. The result of these is that no channel equalization is needed. In the case of deep fading environments in OFDM networks, specific user data can be completely corrupted. In such a case , MIMO is an alternative and can mitigate the deep fading problem.
Security Measures In wireless networks, security is the pre-eminent key that needs to be considered in 4G networks. As a result of the integration capability, 4G networks must include strong and complex security policies but be flexible enough to be compatible with all the supporting technologies. Contrarily, security techniques must be transparent from the user’s point of view. Smart antennas and MIMO technologies. Wireless communication is limited by the effects of multipath fading and delay spread. To mitigate these effects, smart antennas and MIMO technologies have been developed to take advantage of the diversity that can be introduced by antenna arrays. Smart antennas use advance beam- forming techniques to create constructive interference where there is a user or group of users and increase the range of coverage and throughput. Similarly , MIMO techniques can achieve high spectral efficiency in rich scattering environments by means of the effective suppression of multipath fading. If M x M refers to the antennas at the transmitter and receiver and each one is fed by a different multipath, the capacity can be increased M times. T he use of M antenna elements increases system gain in the M factor where, in case of multipath fading environments, diversity gain might also be considered . Based o n this characteristic, an array of M antennas can cancel N interferences where M>N . When antenna array cannot be implemented, cooperative techniques might be used to simulate MIMO environments, whereby mobile nodes are selected as relays and retransmit information to achieve spatial diversity. In conclusion, the use of both beam- forming antennas and MIMO configuration could introduce important improvements in the system required for 4G deployment. Software Defined Radio Software Defined Radio or SDR is an important technique that allows mobile terminals to be configured to operate under a specified network characteristic such as channel conditions, traffic load, interference, etc.  The aim of SDR is to guarantee that a single radio transceiver will be able to operate in any environment, to ensure that this technique will provide the interoperability and integration features required by 4G. Mobile IPV6 Mobile IPv6 is the protocol version for IPv6 networks that maintains a connection between the terminal and the network and facilitates mobility. There are two scenarios to be considered. The first is when the mobile terminal is connected to its home network and the second is when that terminal is connected through a foreign agent. In the latter, a special router forwards packets though a virtual bidirectional tunnel established between the mobile node and the home agent.  This characteristic is essential to guarantee full mobility characteristics for a user connected to an IP network, which is the case of 4G Networks. Other Technologies Other important technologies that should be considered in an approach to the development of 4G systems are Ultra- Wide Band (UWB) and Long-Term Evolution (LTE). UWB provides high data rates minimizing BER. This technology is based in the transmission of ultra- short duration wide band pulses and has the advantage of minimizing inter symbol interference and presents multipath immunity . LTE UMTS V8.0 offers higher data rates than HSDPA and is backwards compatible with UMTS, GMS/GPRS networks. [3, 16]. This technology introduces the Super 3G concept as a smooth step between 3G and 4G systems . LTD increases data rates and improves spectrum efficiency. Conclusion
4G has more than one specification; it could be considered to be a group of service requirements of several technologies focused on achieving the demanding characteristics of 4G. The essential factor in the next generation development is integration. As a result, communication criteria will change, the needs of the users will be the important, and their communication demands will determine the characteristics of technology. In order to accomplish this, a complete set of interconnected and compatible technologies needs to be developed, using different techniques. It is clear that there will not be one predominant technology; the next generation is based on good management and a combination of advanced techniques rather than the development of new schemes. It must be recognised that people want to communicate each other. Communication technologies can be considered to be a tool that allows the user to share information. Therefore mobility, personalization, and flexibility are the key to satisfying people’s needs to communicate . This will be through the use of a new generation technology. References
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