LTE-Advanced: Future of Mobile Broadband

Transcription

1 LTE-Advanced: Future of Mobile Broadband Third Generation Partnership Project (3GPP) a group of telecommunication associations working towards the development and maintenance of a Global System for Mobile communication (GSM) including evolved radio access technologies, has started working on Long-Term Evolution advanced (LTE-Advanced) in order to achieve the requirements of next generation technology. The key goals for this evolution are increased data rate, improved spectrum efficiency, improved coverage and reduced latency. The end results of these goals are significantly improving service provisioning and reduction of operator costs for different traffic scenarios. The requirements for LTE-Advanced are agreed and the radio interface techniques are currently under discussion. One of the most important requirements for LTEAdvanced is to support LTE and enhancement in the frequency spectrum. Layered OFDMA radio access is used to attain higher level requirements such as system performance and full backward compatibility. Moreover, key radio access technologies such as fast inter-cell radio resource management, multi-antenna transmissions with more antennas for coverage, and enhanced techniques are employed to achieve a high level of cell-edge spectrum efficiency.

2 About the Authors K. N Shantha Kumar K. N Shantha Kumar, who has a masters degree in VLSI design and embedded system, has over 9 years of experience in design and development of hardware, software and system integration. Madhu Kata Madhu Kata with Masters Degree in VLSI, has over three years of experience in design and development of Linux Device Drivers, development of protocol stacks in Layer1 (L1) and Layer2 (L2) for WCDMA and LTE. Paruchuri Chaitanya Paruchuri Chaitanya with Masters Degree in Electronics, has over two years of experience in design & development of wireless Medical devices and development of LTE Layer1 (L1) layer. Dinesh Mukkollu Dinesh Mukkollu with Masters Degree in Digital Communication, has over two years of experience in development of protocol stacks in Wimax and LTE. 1

3 Table of Contents 1. Third Generation Wireless Systems 3 2. Radio Interface Concepts of LTE 3 3. Evolution of LTE-Advanced 7 4. Advantages and key features of LTE- Advanced Comparision between LTE and LTE-Advanced Conclusion Reference 13 2

4 Third Generation Wireless Systems Third generation (3G) wireless systems partnership project Long Term Evolution (LTE), based on radio access technology is taking momentum and continuing to grow at an accelerated pace. However, it is necessary to further develop the future demands for mobile broadband services through higher data rates, shorter delays, and even greater capacity. In parallel to these activities related to the evolution of current 3G wireless technologies, there is also an increased research effort on future radio access, referred to as fourth-generation (4G) radio access. Such future radio access is anticipated to take the performance and service provisioning of wireless systems a step further, providing data rates up to 100 Mbps with wide-area coverage and up to 1 Gbps with local-area coverage, fulfilling the requirements for Beyond IMT-2000 systems [1][2]. To meet the challenges of major enhancements to LTE-Advanced which will be introduced in release 10, 3GPP has initiated the study item on LTE-A, aiming at achieving additional substantial leaps in terms of service provisioning and cost reduction[3][4]. Mobility LTE-Adv Low Speed Med Speed High Speed AMPSETACS, ITACS 1 G CDMA/GSM/TDMA 2 G CDMA2000 EV-DO/ DV W-CDMA/HSDPA 3 G LTE 3.x G 4 G ~14.4 Kbps ~400 Kbps ~40 Mbps 150 Mbps 500 Mbps Data Rates Figure 1 : Evolution of Radio Access Technologies In this paper, we first address some of the radio interface concepts of Release 8 LTE and then provide the major differences between LTE and LTE-A. Later we will discuss some of the advantages and key features of LTE-advanced. Radio Interface Concepts Of LTE The ability to provide a high bit rate is a key measure for LTE. LTE is designed to meet the requirements of peak data rate up to 150 Mbps in down-link, 75 Mbps at up-link. The characteristics of LTE will be cellular coverage, mobility, scalable bandwidth of 1.3, 3, 5, 10, 15, 20 MHz, FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing). 3

5 Diffracted wave LTE-Advanced: Future of Mobile Broadband The down-link by OFDMA (Orthogonal Frequency Division Multiplexing Access), up-link by SCFDMA (Single Carrier Frequency Division Multiplexing Access), MIMO (Multiple Input Multiple Output), and modulations by 16 QAM, 64 QAM technologies are used by LTE for meeting the data rate requirements mentioned above. A.Down-link OFDMA OFDMA is a multi-user version of a digital modulation scheme called Orthogonal Frequency-Division Multiplexing (OFDM). In OFDM the signal is first split into independent sub-carriers and these closely-spaced orthogonal subcarriers are used to carry the data. The data is divided into several parallel data streams or channels, one for each subcarrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional singlecarrier modulation schemes of the same bandwidth. A general analogy for OFDM can be of many small lamps in a hall rather than a single big lamp. The advantage is that light will be distributed across the hall equally as compared to a single lamp and increase redundancy a defect in one lamp will not affect the light in the hall. The primary advantage of OFDM over single-carrier scheme is its ability to cope with severe channel conditions without complex equalization filters. For example, attenuation of high frequencies in a long copper wire, narrowband interference, and frequency-selective fading due to multipath. Reflected wave Figure 2 : Multi Path Fading With the help of OFDM, channel equalization is simplified as OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. With the duration of each symbol being long, it is feasible to insert a guard interval between the OFDM, making it possible to handle time-spreading and eliminate inter-symbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency. As the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system. 4

6 In an OFDM symbol the cyclic prefix, transmitted during the guard interval, consists of the end of the OFDM symbol as shown in the following figure. The guard interval is used so that the receiver will integrate over an integer number of sinusoid cycles for each of the multipath when it performs OFDM demodulation with the FFT. CP Data 1 CP Data 2 Cyclic Prefix Cyclic Prefix Figure 3: OFDM Symbol with Cyclic Prefix In OFDM, the available bandwidth is divided into a large number of smaller bandwidths using Fast Fourier Transforms (FFTs) that are mathematically orthogonal. Reconstruction of the band is performed by the Inverse Fast Fourier Transform (IFFT). FFTs and IFFTs are well-defined algorithms that can be implemented very efficiently when sized as powers of 2. Typical FFT sizes for OFDM systems are 512, 1024, and For example, a 10-MHz bandwidth allocation may be divided into 1,024 smaller bands, whereas a 5-MHz allocation would be divided into 512 smaller bands. These smaller bands are referred to as subcarriers and are typically on the order of 10 KHz. The multiple access techniques selected for LTE are OFDMA in down-link and SC-FDMA in up-link. In OFDMA, the data is transmitted over a large number of orthogonal narrow band channels. By inserting the cyclic prefix, the received signal, even after undergoing multipath propagation, can be detected by a low complexity single tap equalizer in the UE. OFDMA provides easy bandwidth scalability by configuration of the number of the subcarriers. This allows the base station to dynamically adjust the bandwidth usage according to the system requirements. In addition, because each user consumes only a portion of the total bandwidth, the signal power of each user can also be modulated according to the current system requirements. Quality of service (QoS) is another feature that can be adapted for different users depending on their specific application, such as voice, streaming video, or Internet access. The drawback of OFDMA is the relatively large peak to average power ratio (PAPR), which tends to reduce the efficiency of the radio frequency (RF) power amplifier [10]. Frequency Reference Sub carriers User 1 User 2 User 3 User 4 Figure 4 : OFDMA sub carriers 5

7 Carriers Carriers Time Time User 1 User 2 User 3 User 4 Figure 5 : Bandwidth allocation OFDM Vs OFDMA B. Uplink Single-Carrier FDMA with Dynamic Bandwidth To improve the RF transmission power efficiency in the UE, Single Carrier Frequency Division Multiple Access (SC- FDMA) is used. SC-FDMA has similar performance and essentially the same overall structure as those of an OFDMA system. One prominent advantage of SC-FDMA over OFDMA is that the SC-FDMA signal has lower peak-to-average power ratio (PAPR). In the up-link communications low PAPR greatly benefits the User Equipment (UE) in terms of transmit power efficiency. Guard intervals with cyclic repetition are introduced between blocks of symbols as in OFDM explained earlier. In OFDM, FFT is applied on the receiver side on each block of symbols, and IFFT on the transmitter side. In SC-FDMA, both FFT and IFFT are applied on the transmitter side, and also on the receiver side. However SC-FDMA requires transmissions in consecutive bands, and thus introduces restrictions on the frequency domain packet scheduling for individual users compared to OFDMA. C. Multi-Antenna Solutions Multiple Input Multiple Output (MIMO) is the major feature used to improve the performance of the LTE system, it allows in improving the spectral efficiency and data throughput. MIMO consists of multiple antennas on the receiver and transmitter to utilize the multipath effects. This reduces the interference and leads to high throughputs. Multipath occurs when the different signals arrive at the receiver at various times intervals. MIMO divides a data stream into multiple unique streams, transmits data streams in the same radio channel at the same time. The receiving end uses an algorithm or employs special signal processing to generate one signal that was originally transmitted from the multiple signals [7]. 6

8 Transmitter Receiver Data Streams Data Streams Figure 6: MIMO Block In LTE, the MIMO concepts vary from down-link to up-link to keep the terminal (UE) cost low. The base station either consists of two or four transmitting antennas and two receiving antennas on the terminal (UE) side for the down-link, and UE employs MU-MIMO (Multi User MIMO) for the up-link. With this scheme UE only have one transmit antenna which reduces the cost of the equipment. Interference due to transmission of data in the same channel by multiple mobile terminals is reduced by using mutually orthogonal pilot patterns. 4G UE Base Station Base Station A: DL Direction B: UL Direction 4G UE Evolution of LTE-ADVANCED Figure 7 : MIMO Tx and Rx Schemes LTE (4 X 2 MIMO) LTE-A should be real broadband wireless network that provides peak data rates equal to or greater than those for wired networks, i.e., FTTH (Fiber To The Home), while providing better QoS. The major high-level requirements of LTE- A are reduced network cost (cost per bit), better service provisioning and compatibility with 3GPP systems [8]. LTE-A being an evolution from LTE is backward compatible. Some of the major technology proposals of LTE-A are [8]: A. Asymmetric transmission bandwidth Access such as Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are the two most prevalent duplexing schemes used in fixed broadband wireless networks. FDD uses two distinct radio channels and supports 7

9 two-way radio communication and TDD uses a single frequency to transmit signals in both the downstream and upstream directions. Symmetric transmission results when the data in down-link and in the up-link are transmitted at the same data rate. This is one of the cases in voice transmission which transmits the same amount of data in both directions. However, for internet connections or broadcast data (for example, streaming video), it is likely that more data will be sent from the server to the UE (the down-ink). Based on the current and future traffic demands in cellular networks the required bandwidth in up-link will be narrower than that in down-link. So asymmetric transmission bandwidth will be a better solution for efficient utilization of the bandwidth. LTE Bandwidth Symmentric BW Asymmetric BW LTE DL BW (20 MHz) LTE Advanced Max BW 100 MHz LTE Advanced DL BW (20 MHz) LTE UL BW (20 MHz) LTE Advanced UL BW (10 MHz) Figure 8: Support of Asymmetric Bandwidths for LTE Advanced B. Layered OFDMA In layered structure, the entire system bandwidth comprises multiple basic frequency blocks. The bandwidth of basic frequency block is, MHz. Layered OFDMA radio access scheme in LTE-A will have layered transmission bandwidth, support of layered environments and control signal formats. The support of layered environments helps in achieving high data rate (user throughput), QoS, or widest coverage according to respective radio environments such as macro, micro, indoor, and hotspot cells. The control signal formats are a straightforward extensions of L1/L2 control signal formats of LTE to LTE-A. Independent control channel structure is used for each component carrier. Control channel supports only shared channel belonging to the same component carrier. C. Advanced Multi-cell Transmission/Reception Techniques In a multi-user multi-cell environment employing multi-transmission/reception antenna devices for each cell have multiple first units and a second units in wireless communication. The first units consists of a predetermined antenna and the second unit consists of the following sub units: 8

10 Estimation unit: Estimates channel information on signals from the individual first units and estimates information of noise and interference signals from adjacent cells. Calculation unit: Calculates the sum of transfer rates for each user group having at least one first unit using the information estimated by the estimation unit. Determination unit: Determines one user group by comparing the sum of the transfer rates of each user group calculated by the calculation unit. Feedback unit: Information on the user group determined by the determination unit is fed back to the first units of the corresponding cell. In LTE-A, the advanced multi-cell transmission/reception processes (also called as coordinated multipoint transmission/reception) helps in increasing frequency efficiency and cell edge user throughput. Faster handovers among different inter-cell sites are achieved by employing Inter-Cell Interference (ICI) management (that is, inter-cell interference coordination (ICIC) aiming at inter-cell orthogonalization). D. Enhanced Multi-antenna Transmission Techniques Mobile traffic in wireless communications has been increasing multi folds over the years, which amplifies the requirement of higher-order MIMO channel transmissions and higher peak frequency efficiency than LTE. In LTE-A, the MIMO scheme has to be further improved in the area of spectrum efficiency, average cell through put and cell edge performances. With multipoint transmission/reception, where antennas of multiple cell sites are utilized in such a way that the transmitting/receiving antennas of the serving cell and the neighboring cells can improve quality of the received signal at the UE/eNodeB and reduces the co-channel interferences from neighboring cells. Peak spectrum efficiency is directly proportional to the number of antennas used. In LTE-A the antenna configurations of 8x8 in DL and 4x4 in UL are planned. 4G UE 4G Base Station Fig a: DL Direction Base Station Fig b: UL Direction UE Figure 9 : MIMO Tx & Rx Schemes LTE-A (8 X 4 MIMO) E. Enhanced Techniques to Extend Coverage Area Remote Radio Requirements (RREs) using optical fiber should be used in LTE-A as effective technique to extend cell coverage. Layer 1 relays with non-regenerative transmission, that is, repeaters can also be used for improving coverage in existing cell areas. Layer 2 and Layer 3 relays can achieve wide coverage extension through an increase in Signal to Noise Ratio (SNR). 9

11 Optical Fiber LTE-Advanced: Future of Mobile Broadband Direct Connection to BS 4G Base Station Indirect Connection to BS UE RRE 4G UE Figure 10 : RRE using optical fibers F. Support of Larger Bandwidth in LTE-Advanced Peak data rates up to 1Gbps are expected from bandwidths of 100MHz. OFDM adds additional sub-carrier to increase bandwidth. The available bandwidth may not be continuous as a result of fragmented spectrum. To ensure backward compatibility to the current LTE, the control channels such as synchronization, broadcast, or PDCCH/PUCCH might be needed for every 20 MHz. LTE (20 MHz) 100 MHz Figure 11: Support of larger Bandwidths The above described technology proposals of LTE-A will help us to: Lower the total cost of network ownership Easily deploy the network Increase user throughput for fully multi-media feature services Achieve spectrum flexibility support scalable bandwidth and spectrum aggregation Achieve backward compatibility and inter-working with LTE with 3GPP legacy systems Enable extended multi-antenna deployments and denser infrastructure in a cost-efficient way 10

12 Advantages and Key Features Of LTE- Advanced A. Advantages Some advantages that are applicable to the 4th Generation mobile communications are also applicable to LTE-A. With average download speeds of 400 Kbps to 700 Kbps, the network offers enough bandwidth to enable cell phone users to surf and download data from the Internet. LTE-A should significantly lower the bit-cost for the end-users and the total cost of ownership for the operators. At the same time, LTE-A should meet new emerging challenges such as energy-efficient Radio Access Network (RAN) design, increase the flexibilities of network deployments, and off load networks from localized user communications. Regardless of the actual technology, the forthcoming technology will also be able to allow the complete interoperability among heterogeneous networks and associated technologies, thus providing clear advantages in terms of: Coverage: The user gets best QoS and widespread network coverage as there is network availability at any given time. Bandwidth: Sharing the resources among the various networks will reduce the problems of spectrum limitations of the third generation. B. Key Features 1. Friendliness and Personalization: User friendliness exemplifies and minimizes the interaction between applications and the user. Thanks to a well designed transparency that allows the person and the machine to interact naturally (for example, the integration of new speech interface is a great step to achieve this goal). 2. Heterogeneous Services: Services that are heterogeneous in nature (for example, different types of services such as audio, video etc.) such as quality and accessibility may not be the same due to the heterogeneity of the network. For instance, a user in proximity of the shopping mall but out of the coverage of a WLAN can still receive pop-up advertisements using the multi-hop ad hoc network setup in his surrounding. Therefore the dynamics of the network environment can change the number of users, terminals, topology, etc. 11

13 Comparision between LTE and LTE-advanced Comparison of performance requirements of LTE with some of the current agreements of LTE Advanced [8] are: Table 1: Difference between LTE and LTE-A Technology LTE LTE--A Peak data rate Down Link ( DL) Peak data rate Up Link (UL) Transmission bandwidth DL Transmission bandwidth UL Mobility Coverage Scalable Band Widths Capacity 150 Mbps 75 Mbps 20MHz 20MHz Optimized for low speeds(<15 km/hr) High Performance At speeds up to 120 km/hr Maintain Links at speeds up to 350 km/hr Full performance up to 5 km 1.3,3, 5, 10, and 20 MHz 200 active users per cell in 5 MHz. 1 Gbps 500 Mbps 100 MHz 40 MHz (requirements as defined by ITU) Same as that in LTE a) Same as LTE requirement b) Should be optimized or deployment in local areas/micro cell environments. Up to MHz 3 times higher than that in LTE 12

14 Conclusion LTE-A helps in integrating the existing networks, new networks, services and terminals to suit the escalating user demands. The technical features of LTE-A may be summarized with the word integration. LTE-Advanced will be standardized in the 3GPP specification Release 10 (Release 10 LTE-A) and will be designed to meet the 4G requirements as defined by ITU. LTE-A as a system needs to take many features into considerations due to optimizations at each level which involves lots of complexity and challenging implementation. Numerous changes on the physical layer can be expected to support larger bandwidths with more flexible allocations and to make use of further enhanced antenna technologies. Coordinated base stations, scheduling, MIMO, interference management and suppression will also require changes on the network architecture. References [1] S. Parkvall et al. Evolving 3G Mobile Systems Broadband and Broadcast Services in WCDMA, IEEE Communications Magazine, February [2] 3GPP, RP , Proposed Study Item on Evolved UTRA and UTRAN, [3] D. Astely et al., A Future-Radio-Access Framework, Journal on Selected Areas in Communications, Special Issue on 4G Wireless Systems, to appear [4] E. Mino Diaz, et al., The WINNER project: Research for new Radio Interfaces for better Mobile Services, IEICE Transactions, Japan, Vol. E87-A, No. 10, October 2004 [5] X. Yu, G. Chen, M. Chen, and X. Gao, Toward Beyond 3G: The FuTURE Project in China, IEEE Communications Magazine, pp 70-75, January 2005 [6] 3GPP, TR , Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description, [7] H. Ekström et al., Technical Solutions for the 3G Long-term Evolution, IEEE Communications Magazine, March [8] 3GPP, TR , Requirements for further advancements for E-UTRA (LTE-Advanced), [9] Progress on LTE Advanced - the new 4G standard Eiko Seidel, Chief Technical Officer Nomor Research GmbH, Munich, Germany. [10] IEEE Communications Magazine. April

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