TechnologiesMigrationfrom3Gto4G英文论文

TechnologiesMigrationfrom3Gto4G-英文论文 Introduction The evolution of communication systems begins with the first-generation 1G analog mobile communication systems, passes through the second generation 2G digital communication system and third generation 3G currently the worlds mainstream mobile technology and ends with the fourth generation systems 4G of high ability. This report offers a critical view on the features of main technologies coding, modulation, signal processing and MA that underpin the migration from 3G to 4G. 1. The third generation and fourth generation communications systems 3G is an International Telecommunication Union ITU standard for third generation mobile telephone systems under the International Mobile Telecommunications programme, IMT-20003G wireless system was developed in the late 1990s and it is not only provided the high transmission speed from 125kbps to 2Mbps 1, but also included many services, such as global roaming, superior voice quality, data always add-on and packet routing. The 4G of wireless technology is still underway and stands to be the upcoming wireless devices standard. Some of these technologies are derived from 3G and hence are evolutionary, while other 4G techniques are totally new concepts and can be thought of as revolutionary. The key difference between 4G and 3G technologies is the improved data transfer rates and security 2 2. Coding techniques In terms of coding of 3G systems, turbo coding is a relatively new channel coding technique, which has been shown to have a performance close to the Shannon limit than what was previously achieved with block and convolutional codes 3. A typical turbo code system, a turbo encoder consists of two identical constituent Recursive Systematic Convolutional RSC encoders with an interleaver preceding the second constituent encoder as shown in Fig.1a and turbo decoding principle is shown in Fig.1b 4a. Turbo encoderb. Turbo decoderFig.1 A typical turbo code system 4 For almost all simulated services and environments, the turbo code implementation of the considered service shows a gain of approximately 1 to 3dB compared to the concatenated codes and the performance within 0.5dB of the Shannon limit has the bit error probability of 10-6 3, which come the closest to approaching the Shannon limit. The main drawbacks are the relatively high decoding complexity and latency. Thus, turbo coding may unsuitable for some applications like digital telephony and optical transmission applications, but it still useful where information transfer is not time-sensitive, such as satellite applications. The sensitivity of Turbo Codes to prediction errors may cause the system to produce much less favourable results than expected. Currently in 4G systems, the advanced coding schemes include concatenated codes and low-density-parity-check LDPC codes. The 4G providers of advanced cellular technology in FEC Forward Error Correction are adopting Concatenated Coding which has the capability of multiple QoS Quality of Service levels. It prefers to adopt Concatenated Coding to Convolution Coding method because Concatenated Coding combines of two or more coding techniques, such as a Reed-Solomon and a Convolution Code so that the system has more transition capacity and less BER. This combination improves error correction and combines error correction with error detection. However, it also adds the complexity of the system 1 Low-density parity-check LDPC codes forecast for 4G systems because of their excellent error correction performance and highly parallel decoding scheme. It can achieve high quality of data transmission with the comparatively low coding and decoding complexity. 3. Modulation techniquesFrom a modulation perspective, current 3G wide-band code-division multiple-access CDMA systems mainly use M-ary quadrature amplitude modulation M-QAM which modifies both the phase and amplitude to generate communications symbols, for high-speed downlink packet access HSPDA due to its high spectral efficiency 5. In terms of power efficiency, QAM is superior to M-ary PSK. Table 1 lists the bandwidth and power efficiencies of a QAM signal for several of M, assuming optimum raised cosine rolloff filtering in AWGN 6.M4166425610244096B123456Eb/N0 for BER10-610.51518.5242833.5Table 1. Bandwidth and Power Efficiency of QAM 6 Thus, it can be seen that the increased density will create a higher propensity for errors. The use of dense signal constellation is usually restricted to short range transmissions over relatively “clean” channels 1 Usually a fixed modulation technique cannot achieve the best spectral efficiency because the system has to be built with a modulation scheme considering the worst case scenario. Future 4G modulation scheme is changed dynamically based on the current channel estimates. Adaptive modulation and coding techniques AMC are key to 4G success in achieving higher data rates. The principle of AMC is to change the modulation and coding format transport format according to instantaneous variations in the channel conditions, subject to system restrictions. AMC extends the systems ability to adapt to good channel conditions and makes it closer to reach the Shannon limit. For a system with AMC, users close to the cell site are typically assigned higher order modulation with higher code rates e.g. 64 QAM with R3/4 Turbo Codes. On the other hand, users close to the cell boundary, are assigned lower order modulation with lower code rates e.g. QPSK with R1/2 Turbo Codes 7 Adaptive modulation based M-ary PSK, M-ary QAM, M-ary CPM continuous phase modulation, M-ary MHPM and GMSK systems applied to a Turbo coded MCCDMA system has been suggested for use in 4G. Fig.2 and Fig.3 show the BER performance of all these digital modulation techniques having been compared under a Rayleigh fast fading channel environment. Fig. 2 BER performance of 16 QAM based MC-CDMA system for a given number of CDMA users 8.Fig. 3 BER performance of 8PSK based MC-CDMA system for a given number of CDMA users 8.Fig.4 BER performance of a 4QAM QPSK and b 2QAM BPSK based MC-CDMA system for a given number of CDMA useres 8 These simulation results Fig.2 through 4 of BER performance under different channel conditions and different number of CDMA users are prerequisite to the deloyment of adaptibe modulation based system. It clearly shows higher order modulation formats and a higher spectral efficiency need better channel conditions and poorer channel conditions need lower-order modulations 8.4. Signal processing With the demand of higher data rates, 3G long-term-evolution 3G-LTE specifications will require complex signal-processing techniques such as multiple-input, multiple-output MIMO along with new radio modulation technologies like orthogonal frequency-division multiple access OFDMA and multicarrier code-division multiple access MC-CDMA, specifically, using OFDMA for the downlink and SC-FDMA for the uplink due to its low PAPR properties 9. In 3G cellular systems, MIMO is being widely considered for CDMA2000 3GPP2 and WCDMA 3GPP 10. MIMO is a smart antenna system that incorporates multiple antennas at the input and/or output. It allows data to be transmitted at the same time using the same frequency is multiplexed spatially at the transmit end and combined at the received end 1. The multidimensional MIMO channel can be exploited to increase the diversity of the system or to provide parallel spatial channels, which is known as spatial multiplexing 10 One of the greatest challenges facing MIMO in the context of 3G is that present MIMO systems do not cope gracefully with high levels of interference. Besides, the increasing the spectral ef?ciency with MIMO appears to require reducing the interference level, which traditionally requires increased frequency reuse or other spectral ef?ciency reducing measures 10. Future design of MIMO systems will focus on minimizing transmit power and rate in order to reduce the interference caused to neighbouring cells and increasing the amount of data transmitted and hence requires a larger received Signal-to-interference-plus-noise ratio SINR 10. 5. Multiple Accesses5.1 3G MA 3G wireless multiple access techniques were widely based on CDMA and Wideband-CDMA W-CDMA.5.1.1 CDMA CDMA allows each user to transmit over a wideband spectrum. It is more robust against multipath fading and provides various desirable system features such as universal frequency reuse, soft handoffs between adjacent cells, and softer handoffs between sectors of the same cell 11. However, it should be noted that the performance of CDMA systems depends crucially on accurate power control, as the channel attenuation of nearby and cell edge users can differ by many tens of dBs. Besides, in CDMA, all users share the same RF bandwidth at the same time and are distinguished from each other only by spreading code. Increasing the number of users in a CDMA system increases the total level of interference 125.1.2 W-CDMA To enhance CDMA receiver performance further, W-CDMA has been adopted as one of the 3G wireless standards by the Third Generation Partnership Project 3GPP. The receiver of CDMA system can resolve and combine multipath components to improve SNR. W-CSMA allows all users to transmit at the same time and to share the same RF carrier and uses wider bandwidth than CDMA systems 13. However, it should be noted that as the data speed increases the coverage area of the cell will become smaller. This can be improved by High Speed Downlink Packet Access HSDPA.5.2 4G MA 4G demands a better multiple access technique for reducing the MAI Multiple Access Interference and ISI Inter Symbol Interference and thus improve the bit error performance 10. Future 4G MA schemes includes: OFDM coupled with TDMA OFDM-TDMA, a variable spreading factor VSF-orthogonal frequency/code division multiplexing technique VFS-OFCDM, and Multicarrier CDMA MC-CDMA 15.2.2 OFDM-TDMA OFDMA is a multiple access scheme based on the well-known orthogonal frequency-division multiplexing OFDM modulation technique. Its main principle is to split the data stream to be transmitted onto a high number of narrowband orthogonal subcarriers by means of an inverse fast Fourier transform IFFT operation together with the guard interval called cyclic prefix CP, which allows for an increased symbol period and avoids ISI and ICI 14. OFDM has the capability to cancel multi-path distortion in a spectrally efficient manner. When combined with TDMA, it allows users to transmit OFDM-like signals in assigned time slots 1. However, OFDM also presents some drawbacks: sensitivity to Doppler-shift, synchronization problems, and inefficient power consumption due to high peak-to-average ratio PAPR 14.5.2.3 VFS-OFCDM MA One of the main characteristics of VSF-OFCDM is that time-domain and frequency- domain spreading factors can be adaptively controlled according to the propagation conditions, channel load and radio parameters 15. However, the main disadvantage of multicarrier systems is that they suffer from performance degradation due to carrier frequency offset CFO, which can cause a reduction in the desired signal amplitude as well as loss of orthogonally between subcarriers and results in ICI 165.2.4 MC-CDMA Multi-Carrier Modulation MCM improves the capacity of system by making transmission more robust to frequency selective fading and enhances spectral efficiency, but it has drawbacks on synchronisation, CFO and PAPR. To gain the advantages of both schemes CDMA & MCM, a combination known as MC-CDMA was proposed. The basic idea of CDMA is to maintain a sense of orthogonally among the users in order to eliminate the MAI. This is done by employing orthogonal spreading codes to spread the data sequence 17. MC-CDMA systems have higher data rates and greater access robustness to multipath fading 1. 6. Summary6.1 Limitations of 3G Although 3G has made a great progress within its higher capacity, higher quality of communication and more applications of multimedia, it still has many drawbacks. Some of the reasons for a new generation of mobile communications are listed as follows: ?Difficulty of CDMA to provide higher data rates?Need for continuously increasing data rate and bandwidth to meet the multimedia requirements ?Limitation of spectrum and its allocation ?Inability to roam between different services?To provide a seamless transport end-to-end mechanism No uniform standard for mobile wireless accessing into the network air interface ?To introduce a better system with reduces cost 106.2 3G vs. 4G TechnologiesKey Parameters3rd Generation 3G4th Generation 4GSpeed3G has the ability to utilize circuit/packet data at higher bit rates 2.4G can support data rates up to 20 to 100 Mbps in mobile mode 2.Bandwidth5 to 20 MHz 2.100MHz to above 2.Switching Design BasisPacket switching or circuit switchingPacket-switchingMA TechnologiesCDMACDMA-2000,WCDMAOFDM-TDMA, VSF-OFCDM, MC-CDMAFrequency Band1800 to 2400MHz 22 to 8 GHz 2Coding TechniquesMainly use Turbo codingConcatenated codes and low-density-parity-check LDPC codesModulation TechniquesMainly use M-QAMM-ary PSK, M-ary QAM, M-ary CPM, M-ary MHPM and GMSKReferences1: Roberts, M. 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