There are numerous features of 3G which needed to be modified for the future applications of the mobile communication networks and their service sectors. These collections of advanced versions, along with some others new advanced features, have been proposed for the forth coming 4G system. Of course, the complete picture is not clear yet, though it is believed that by the end of 2010 a better view of the complete 4G features will be known. Here we have captured the main features of the existing experimental 4G system and some of the essential future versions.
As per the announcements of the 4G working groups, the infrastructure and the transreceiver terminals of 4G system will have almost similar structures like that of 2G and 3G except for some advanced features. The previous legacy systems will be in place to keep the existing users. The major change in the infrastructure for 4G will be "all packet-based system" and the technology on which it will be based is the IPv6. There are some other proposals for an open platform in which the new innovations and evolutions of the future can fit. One of the first technology really fulfilling the 4G requirements as set by the ITU-R will be LTE Advanced as currently standardized by 3GPP. LTE Advanced will be an evolution of the 3GPP Long Term Evolution. The higher data rates needed are for instance, achieved by the aggregation of multiple LTE carriers that are currently limited to 20MHz bandwidth and there are many such changes have been recommended. In the following sections we have listed some of the important features of the 4G systems.
1 OFDM Based Physical Layer
The main aim of 4G technology is to Provide high speed wireless broadband services. Airport lounges, cafés, railway stations, conference arenas, and other such locations are required to have high speed internet services; in those places, 4G can play an important role. 4G is equipped with the proper arrangements at the physical layer to meet all the demands of those various scenarios. There are many difficulties, however, in providing high speed wireless internet services in these environments, such as multipath fading and the inter-symbol interferences generated by the system itself. As a result, OFDM technology is used to handle this problem.
2 Inter-Symbol Interference Due to Time Delay
In a multipath environment, the signals and their delayed versions arrive at different times. When the time delay between the different delayed signals is a large enough fraction of the transmitted signal's symbol period (actual time allotted for one symbol transmission), a transmitted symbol may arrive at the receiver during the next symbol period. This is well known as inter-symbol interference (or ISI). At higher data rates, the symbol period or duration is shorter; hence, it takes only a small time delay to introduce ISI. In case of broadband wireless, ISI is a big problem and reduces the quality of service significantly. In conventional situations, statistical equalization is the method for dealing with ISI, but at high data rates it is quite complex and requires considerable amount of processing power. OFDM appears as a better solution for controlling ISI in broadband systems like 4G
OFDM deals with this problem in a very intelligent way by introducing a guard interval before each OFDM symbol. This guard interval is the duration in which no information is transmitted. Digitally, it is nothing but a certain number of zeros transmitted between each couple of symbols. Whatever signal comes during that interval is discarded by the receiver, but when the guard interval is properly chosen then the OFDM signal can be kept undistorted.
3 Effective Use of Bandwidth through OFDM
OFDM has the ability to optimize the consumption of resources. Extraneous bandwidths in the form of guard bands can, with proper implementation, be reduced to zero. Due to the orthogonal nature of the carriers used for different channels, it is possible to overlap the bands on each other and still recover them in the receiver without losing any quality. Because of this, OFDM is very effective in saving bandwidth. In low bandwidth systems where the demand for spectrum is very high, OFDM comes naturally as the first choice. The bandwidth saving has been shown in Figure 1
Figure 1: OFDM and bandwidth use
Besides the above advantages, OFDM based systems provide other facilities for digitalization and protocol supports. Processes like error correction and interleaving are easily supported by OFDM.
4 Software Defined Radio for the 4G System
Software defined radio (SDR) is an emerging radio technology that can be used in various digital networks and can be controlled and programmed through software.
According to the SDR Forum, SDR technology is "radio that provides software control of a variety of modulation techniques, wide-band or narrow-band operation, communication security functions (such as hopping), and waveform requirements of current and evolving standards over a broad frequency range." SDR has the ability to support wireless applications in various networks like Bluetooth, WLAN, GPS, radar, WCDMA and GPRS.
Currently, all of the major operations such as modulation, demodulation, coding, decoding, interference management, channel allocation and capacity management are done through the control software. One of the biggest advantages of the SDR is that it can ensure a secure communication network through implementation of encryption systems like the AES (Advanced Encryption Standard). This means that SDR is very reliable and useful for military and other high-level, secret communications. Due to these features, SRD is the most suitable method of data handling at the higher levels in 4G. With the link protocol standards now moving into 3G and 4G, networks differ dramatically in many ways. This is a big problem for both consumers and service vendors; while it can be handled through upgrading the handset, upgrading is usually not a good choice due to the high cost. Additionally, the wireless network operators face many interfacing problems during the migration of a network from one generation to another. Finally, the use of incompatible systems in different countries can hinder global communication. Through the use of SDR, all these scenarios can be handled smoothly.
The SDR system uses a generic hardware platform which has its own programmable units, microprocessors, digital signal processors, field programmable gate array and analog RF modules. The software modules of the SDR that implement link layer protocols and modulation/demodulation operations are called radio applications, and these applications provide link layer services to higher layer communication protocols such as WAP and TCP/IP. SDR has the ability to significantly reduce the life-cycle costs and can also support advanced capabilities in different portable networks. The SDR technology is also reconfigurable; it allows several software modules to co-exist, and also permits dynamic configuration on the handset as well as in the back-end equipment. As a result of this flexibility, the problem of discrepancies due to legacy handsets is solved, and the extra cost for a new handset is not required. SDR can also handle the implementation of multi-mode, multi-band and multi-standard terminals. All of these demonstrate that SDR is clearly the most desirable technology for 4G.
5 MIMO Antenna Systems for 4G
4G like its predecessor 3G would use the advanced versions of the MIMO Antennas. The antennas used for the 3G system were smart enough to take care of many advanced operations at the signal level. This system must continue for 4G as well, and may even be made more sophisticated for 4G, as the number of signal-level decisions would be far greater in the case of 4G compared to 3G
6 IPv6 Based Packet Transmission
The all-packet infrastructure is quite popular in the wireless communication, and now it is also true for the 4G systems as well. The biggest difference between 3G and 4G is the all-IP network (AIPN) structure of 4G, which means that all communication will be controlled by TCP/IP protocols. As a result, the whole communication will be packet switched and the circuit switching part will be taken out of this advanced version. Not only can this make the system compatible with all digital devices, but internet access will be quite flexible and high data rates can be achieved. According to the 3GPP LTE team, this target will be achieved by the end of 2008. Similarly, the 3GPP2 LTE teams are also busy trying to keep pace with their competitors.
7 Presence of TDD and FDD
TDD (Time Division Duplex) and FDD (Frequency Division Duplex) are different modes of CDMA. In FDD transmission mode, both the transmitter and the receiver transmit simultaneously. This simultaneous transmission is possible because they are both on different frequencies. In TDD mode of operation either transmitter or receiver can transmit at one time. This is because they use the same frequency for the transmission.
At present all the major 3G Networks are using FDD mode of operation, but in the 4G system both the FDD and TDD will co-exist.
In the FDD mode of operation, the uplink and downlink use separate frequency bands. These carriers have a bandwidth of 5 MHz and are divided into 10-ms radio frames; each frame further id divided into 15 time slots. The frequency allocation consists of one frequency band at 1920-1980 MHz and one at 2110-2170 MHz. These frequency bands are used in FDD mode both by the UE (user equipment) and the Network. The lower frequency band is used for the Uplink (UL) transmission and the upper frequency band is used for the Downlink (DL) transmission. The frequency separation is specified with 190 MHz for the fixed frequency duplex mode and with 134.8MHz to 245.8MHz for the variable frequency duplex mode.
The TDD mode differs from the FDD mode in that both the uplink and the downlink use the same frequency carrier. There are 15 time slots in a radio frame that can be dynamically allocated between uplink and downlink directions. Thus the channel capacity of these links can be different which is very advantageous especially when people are downloading stuff on their mobiles. The chip rate of the normal TDD mode is also 3.84 Mbps, but there exists also a "narrowband" version of TDD known as TD-SCDMA. The carrier bandwidth of TD-SCDMA is 1.6 MHz and the chip rate 1.28 Mbps. TD-SCDMA has been proposed by China and potentially has a large market-share in China if implemented.
8 Self-Organizing Characteristics of 4G
The resource and duty management operations of 4G would be quite different from the present scenario. Extensive automation in the system and self-organizing characteristics can create an intelligent management. This is a quite strange feature unique to 4G that could lead the system to a complete new level.
9 Two-Tier Coverage
In case of 4G, the geographical coverage would consist of at least two tiers. The normal coverage would be through normal macro cells, but in order to handle the traffic and resources properly during the peak-hours, microcells would be kept in place. Depending on the traffic distribution, the transmission and control duties are switched to the appropriate cells. In some hot spots the coverage layering would be composed of multiple layers to improve the quality of service and resource management.
10 4G Uplink and down Link Frequencies (Proposed)
Though the spectrum of 4G is still under planning, we have a rough idea about the uplink and downlink frequencies from the early developers. OFDM is used to divide the whole spectrum or bandwidth into thousands of small narrow bands. each having different frequencies. By doing this, the system becomes resistant to multipath fading and thus capable of providing better quality of service.
The 4G system also uses OFDMA for the downlink and single carrier FDMA (or SC-FDMA) for the uplink. It optimizes the data rate by using four MIMO antennas per station, which we have seen can provide tremendously high data rates. The channel coding schemes are chosen to be suitable for the OFDM signals. Turbo codes are preferred in this application.
10.1 Downlink
The OFDM system for the downlink uses maximum 2048 subcarriers. The subcarrier spacing in OFDM downlink is 15 kHz. The mobile device must have the ability to receive all the 2048 subcarriers but the base station needs only 72 subcarriers for transmission. The transmission is divided into sub frames of 1.0 ms duration and each time slot is of 0.5 ms duration. The net length of a radio frame is 10ms. For downlink the popular modulation formats are QPSK, 16 QAM, 64 QAM and 256 QAM. The spectrum for the downlink has not been finalized; but it is expected to be wider than the mobile WiMAX and in the similar range of WiMAX.
10.2 Uplink
For uplink, the proposed multiplexing method is SC-FDMA, and proposed modulation methods are QPSK, 16 QAM and sometimes 64 QAM. SC-FDMA is used to suppress the high PAPR, as in the case of OFDMA. For high data rates the constellation size may go up to 256 QAM. Of course it is still under review and the current road map is considering 64 QAM as the proper choice. Uplink spectrum of 4G would be in the same range as the WiMAX but it would have more bandwidth for faster data rate.
No comments:
Post a Comment