Technical innovation and challenges of LTE

Technical innovation and challenges of LTE

Abstract: LTE, as a revolutionary broadband mobile communication standard, deeply explores the spatial channels from the frequency domain, space domain and other dimensions, while adopting an adaptive system design and a simple all-IP flat network architecture to provide a powerful time-frequency Resources. Faced with extremely flexible systems, there are still challenges in how to efficiently use these time-frequency resources and how to achieve true co-frequency networking. It takes arduous efforts to fully realize the expected potential of LTE technology. This article analyzes the technical innovations of the 3GPP Long Term Evolution (LTE) standard and the challenges facing R & D.
1 Introduction With the standardization of 3GPP LTE (Long Term Evolution) technology nearing completion, LTE R & D and industrialization have entered a critical stage. Major mobile communication manufacturers both in China and internationally have developed TD-LTE or FDD LTE R & D prototypes, and have conducted a series of proof-of-concept tests based on these prototypes. Some of the more aggressive European and American operators have also signed contracts for pre-commercial networks with equipment manufacturers with relatively rapid development schedules, preparing to deploy city-level LTE test networks.
As an innovative technology with tremendous potential, LTE will undoubtedly gradually thin the profits in the traditional voice market today, providing a valuable opportunity for the wireless communication industry to expand into the mobile Internet market. But at the same time, LTE, as a new standard that fully adopts revolutionary technology, also poses a series of challenges to the communications industry. Therefore, in the early stage of LTE industrialization, it is beneficial to have a clear understanding of the essence and technical challenges of LTE technology innovation.
2 LTE standardization progress
LTE is called "EvoluTIon", which is actually "RevoluTIon". Most of the core technologies used in the 3G system are not used, and a large number of innovative technologies and new system designs are adopted. Since 3GPP launched the LTE project in November 2004, 3GPP has gone all out to promote LTE research with frequent meetings. The formulation of the requirements was completed in only half a year, the research phase (Study Item, SI) was completed in September 2006, and the standard formulation of the work phase (Work Item, WI) was basically completed by the end of 2008, forming the LTE standard A finished version-R8 version. As of March 2009, the core standard of LTE has basically stabilized. Although there are continuous detailed updates, it has little effect on device development. However, the radio frequency and terminal test specifications have not yet been completely completed, which also has a certain impact on the development and testing of LTE systems and terminals.
From the beginning of 2009, 3GPP started the standardization of LTE R9. R9 will be a "shorter" version and is expected to be completed by the end of 2009. In addition to revising R8, R9 will also undergo certain enhancements and application extensions based on the LTE core standard. The more important tasks include:
(1) Dual-stream beamforming: That is, based on the single-stream beamforming (Beamforming) adopted by R8 LTE and spatial multiplexing technology, it is extended to dual-stream beamforming to increase the peak rate.
(2) LTE-based positioning technology: Base station positioning (PosiTIoning) technology based on LTE standard.
(3) Home base station technology based on LTE: The LTE core standard has made preliminary considerations on the home base station (Home eNodeB), but there is still much room for optimization. R9 will further optimize the LTE standard for home base stations.
The long-term evolution of LTE-the research stage of LTE-Advanced technology is also in full swing. Based on this research, 3GPP will submit IMT-Advanced candidate technology proposals to ITU in October 2010. Based on the LTE core standard, LTE-Advanced will adopt carrier enhancement (Carrier AggregaTIon) technology, coordinated multipoint (CoMP) technology, relay (Relay) technology, and uplink MIMO technology in several aspects.
3 Technical innovation of LTE
3.1 Innovation one in the field of LTE technology innovation: using frequency division multiple access system instead of code division system
The LTE system abandons the CDMA (Code Division Multiple Access) technology that the 3G system has long adopted, and adopts multi-branch technology with OFDMA (Orthogonal Frequency Division Multiple Access) as the core. The key of OFDMA technology is to achieve orthogonal transmission in the cell, so that the system can allocate a certain period of "clean" bandwidth for specific users within a specific time, thus providing a basis for achieving higher peak rates. Relatively speaking, the CDMA system faces the problem of "inter-user interference" even inside the cell, so it may be more difficult than the OFDMA system when achieving high peak rates.
The uplink of LTE system adopts SC-FDMA (Single Carrier Frequency Division Multiple Access) technology, which is an improved OFDMA technology, which can take into account the low peak-to-average ratio (PAPR) of single carrier transmission while maintaining the orthogonal transmission characteristics of OFDMA ) Characteristics, so as to obtain better terminal power amplifier efficiency and lower power amplifier cost. Innovation 2: MIMO (multi-antenna technology) technology is adopted
The LTE system is by far the most comprehensive wireless communication system using MIMO technology. Compared with IEEE 802.16e which only uses spatial diversity technology, LTE uses various MIMO transmission modes, including:
(1) Downlink MIMO mode ● Transmit diversity: By repeatedly sending different versions of a data stream on multiple antennas to obtain diversity gain to improve cell coverage, it is suitable for large-spacing antenna arrays.
● Spatial multiplexing: The multiplexing gain is obtained by sending multiple data streams on multiple antennas in parallel to improve the peak rate and cell throughput.
● Beamforming: Through wave interference in multiple antenna array elements, beams with performance energy concentration in a specified direction can obtain beamforming gain to improve cell coverage. It is suitable for small-pitch antenna arrays.
● Spatial multiple access: Similar to the spatial multiplexing mechanism, multiple parallel data streams are used by multiple users to increase system user capacity.
(2) Uplink MIMO mode spatial multiple access: Uplink is only supported by space division multiple access mode due to the limitation of the number of terminal transmit antennas and transmit power amplifiers.
Innovation 3: Flat network
The LTE system cancels the important network element RNC (Central Control Node) in the UMTS system, leaving only one layer of RAN nodes-eNodeB. The eNodeB and the core network implement more flexible multiple connections through the S1-flex interface based on IP routing The adjacent eNodeBs implement Mesh connection through the X2 interface.
3.2 The essence of LTE technology innovation
The essence of LTE technology innovation is to further dig deep into the wireless channel resources and further simplify the network structure. In terms of wireless channel resource mining, it mainly extends to two dimensions: (1) frequency domain expansion
The LTE system uses OFDMA / FDMA, which is a more natural large-bandwidth solution than CDMA, and can be expanded directly to a larger bandwidth by increasing the number of subcarriers. Using this extension method, in principle, no matter what bandwidth, can be achieved through a unified framework. Compared with the dual-cell HSPA + (Duel-cell HSPA +) system bandwidth of 10 MHz, the bandwidth supported by LTE has been increased to 20 MHz.
(2) Airspace expansion
The LTE system adopts the adaptive MIMO transmission of the same frame, which can adaptively switch between various modes of space diversity, space division multiplexing, beam forming, space multiplexing, and single antenna transmission according to channel conditions and needs, so that Maximize the use of actual channel capacity. Compared with Duel-cell HSPA + 's 2-antenna MIMO, LTE MIMO transmission can support up to 4 antennas (see Figure 1).

In terms of network structure simplification, in order to reduce the transmission delay of the system and meet the needs of users always online, LTE has simplified the vertical network layer to the greatest extent. Intuitively, this design is equivalent to shortening the distance between the network and the user, making the network closer, faster, simpler and more transparent to the user.
The simplification of the vertical network structure will decentralize many network functions (such as handover) to the eNodeB level. LTE is solved by enhancing the horizontal network connection, that is, the newly added X2 interface is used to realize the handover between adjacent cells, and the mobility management is optimized. In addition, the entire network adopts an all-IP structure, and IP connections between network elements are realized through routers, so that IP data services can be realized more optimally.
4 Background of LTE Technology Innovation Background One: The Needs of Mobile Internet Business Development
(1) From voice optimization to data optimization The focus of next-generation broadband wireless system optimization shifts from optimizing for voice services to optimizing for data services. Therefore, in addition to focusing on narrowband services, the system also pays more attention to improving the efficiency of broadband services.
(2) From coverage optimization to capacity optimization, the main requirement of voice services for the system is to ensure continuous coverage of basic services, while data services focus more on improving the throughput of services in certain "hot spots."
(3) From user capacity optimization to data rate capacity optimization In the mobile Internet era, data services mainly use metering or monthly subscription, so the revenue of operators depends not only on the number of users, but also on the ability to provide business traffic, so In addition to increasing user capacity, the system pays more attention to improving the data rate and throughput of the system. (4) From uniform capacity distribution to uneven capacity distribution It is predicted that in the future 80% to 90% of the data service capacity requirements of the system will be concentrated in indoor and hot zones. This uneven distribution of service capacity is a requirement for uniform coverage of the system It provides greater flexibility. The system does not need to pursue complete uniform coverage like a voice cellular system, allowing a certain performance difference between the "hot zone" and the "hot zone".
Many of the above backgrounds determine the direction of LTE technology innovation, that is, the technology with large bandwidth, high peak rate, and high intra-cell throughput such as OFDMA / MIMO is selected as the core.
Background two: the integration of broadband wireless access and broadband mobile communications In recent years, the traditional communications industry and the traditional IT industry have coincidentally recognized the importance of the ubiquitous mobile Internet market. As broadband wireless access and broadband mobile communications The penetration of different directions into the same market makes the boundaries of the two technologies more and more blurred, showing a trend of convergence (see Figure 2).

(1) Broadband access mobility: from large bandwidth to variable bandwidth (effectively supporting small bandwidth); from fixed access to support low- and medium-speed mobile; from isolated hotspot coverage to multi-cell network support for handover; from The evolution of data services to support voice services at the same time; from portable terminals supported by laptops to mobile terminals supported by mobile phones.
(2) Broadband mobile communications: from 5 MHz bandwidth to 20 MHz bandwidth; from high-speed mobile to low-speed mobile optimization; from circuit switching / packet switching to full packet domain; from cellular network to hot spot coverage ; Terminal form has evolved from mobile terminals to portable and mobile terminals.
Background three: OFDMA and MIMO technology reserves are mature to the end of the 20th century. Academia has accumulated a wealth of technical reserves in the realization of OFDM, MIMO theory, algorithms, software and hardware foundation. Various international research and standardization work, some have set technical indicators for LTE, some have provided technical reserves for LTE, some have verified device achievability for LTE, some have provided lessons and lessons for LTE, and some have Competitive pressure has been exerted to promote the development of LTE projects from all aspects.
5 Technical challenges facing LTE
The LTE standard is close to completion, but LTE research and development has just begun, and it is still unknown whether the device implementation can achieve the expected performance of the LTE standard. The LTE standard defines stronger capabilities than the 3G standard, but it also poses greater challenges to device research and development, including:
(1) Challenges brought by OFDM / SC-FDMA technology.
(2) Challenges brought by MIMO technology. (3) Challenges brought by LTE networking technology.
OFDM and MIMO systems have brought unprecedented and abundant four-dimensional air interface resources to the LTE system-frequency domain, time domain, code domain and space domain, which can be flexibly scheduled and adaptive at 4 latitudes, making the LTE system more Strong technical potential, but whether these resources can be used well and this flexible system is well managed is a problem that needs to be solved.
The tremendous flexibility of the LTE standard has objectively resulted in a lower degree of assurance of the device development quality than 3G. The optimization of LTE devices depends more on the manufacturers' R & D capabilities. The flexibility of the LTE system depends more on the implementation of the MAC layer. Therefore, in the LTE standard, the pure physical layer technology has a lower degree of protection for device capabilities, and the performance of the system depends more on the optimization of the MAC layer scheduling and resource allocation algorithm.
5.1 Challenges brought by OFDM / SC-FDMA technology
(1) The theory that OFDMA system has higher spectrum efficiency than CDMA system has not been finalized in academia and industry. If OFDM may obtain higher spectral efficiency, it must be derived from its orthogonal transmission characteristics, but the OFDM system needs to insert CP (cyclic prefix) to avoid inter-user interference, introducing a certain additional overhead, so it is not free of cost Solve the problem of multi-user interference. The problem of multi-user interference in CDMA systems is relatively troublesome to solve (such as the use of joint detection technology). Even though OFDMA can achieve higher spectrum efficiency in a cell, its lack of inherent inter-cell multiple access capability may make it more difficult to achieve high spectrum efficiency in a multi-cell network.
(2) The OFDMA system is more scalable than the CDMA system. Because the OFDMA system increases the bandwidth by increasing the number of subcarriers, and uses frequency domain balanced reception on each subcarrier, the receiver complexity of the OFDMA system is linear with the bandwidth Growth, the increase in complexity under a larger system bandwidth can also be tolerated. The CDMA system can only expand the bandwidth by increasing the chip rate, causing the receiver complexity to increase exponentially with the bandwidth. Therefore, OFDMA system has better ability to realize large bandwidth than CDMA system.
In terms of flexibility in bandwidth allocation, OFDMA is not as flexible as in theory. Although in principle, the OFDMA system supports subcarrier-level bandwidth allocation, in fact, in order to reduce control signaling overhead, the system can only support subband-level allocation.
(3) The OFDMA system is more conducive to the realization of MIMO. The OFDMA system avoids the trouble of multipath interference and can use simple equalization to correct the channel distortion. Therefore, it is possible to avoid the inter-symbol interference and the inter-antenna interference of the MIMO system from mixing together. Realize simpler MIMO signal reception. Relatively speaking, when MIMO technology is used in a CDMA system, inter-symbol interference, multi-user interference, and inter-antenna interference may be mixed together, which will increase the difficulty of interference cancellation. However, the above conclusion has a great relationship with the type of receiver. When a simple receiver is used, the complexity of the OFDM + MIMO receiver is indeed significantly smaller than that of the CDMA + MIMO receiver.
(4) The OFDMA system has higher scheduling gain. The efficiency of the frequency division system greatly depends on the optimization of the scheduling algorithm. The resource allocation of the LTE system in 6 dimensions of time, frequency, space, code, user, and cell is complicated to the scheduler. Higher requirements have been put forward. In addition, cross-layer optimization problems caused by multiple QoS levels and fairness will further increase the complexity.
An optimized scheduler should be able to select the appropriate time slot, appropriate resource block, appropriate modulation and coding format, and appropriate MIMO format for multiple users to meet their QoS requirements, while taking into account fairness, while also avoiding the cell Interference, spatial pairing (when using multi-user MIMO). If a fully optimized algorithm is used, the complexity is too high, and if a suboptimal algorithm is used, it will have a negative impact on the performance of scheduling. 5.2 Challenges brought by MIMO technology The network planning of traditional cellular systems is used to choosing the base station site to cover the commanding heights with more LOS (line-of-sight) channels. Generally, the correlation between wireless channel antennas in this scenario is high, which is not conducive to MIMO. The application of technology (the contradiction can be alleviated by using orthogonally polarized antenna arrays). Multi-stream spatial multiplexing and space division multiple access usually need to be applied in areas with higher SINR (signal to interference and noise ratio).
In various wireless environments, you need to choose between various MIMO configurations, such as between spatial multiplexing and beamforming; choose between large-pitch antenna arrays and small-pitch antenna arrays; choose various specific The configuration of the antenna array, such as the number of array elements, whether to use dual-polarized array, whether to use optical fiber to extend, etc.
In terms of baseband complexity, there is a trade-off between the performance and complexity of the MIMO interference cancellation receiver, and a compromise between the degree of optimization of the transmitted signal and the amount of measurement feedback. In the implementation of RRU (remote radio frequency unit), you need to consider the complexity of the RRU implementation of the MIMO system and the implementation complexity of the Ir interface (the interface between the BBU (baseband processing unit) and RRU).
5.3 Challenges brought by LTE networking technology (1) OFDMA itself is just a multiple access technology within a cell, and the LTE system potentially also supports certain code division multiple access operations, that is, using low code rate channel coding + repetition coding + cell scrambling code to fulfill.
(2) For LTE systems, more efficient inter-cell multiple access relies on intelligent scheduling between cells.
(3) The LTE system overlaps and covers a large number of macros, microcells, indoors, and home base stations, making the interference structure extremely complicated. It is difficult to rely solely on interference scheduling to solve the problem.
(4) The use of LTE system poses new challenges to network planning and network optimization technology from concept to method. The new technologies and new features adopted by LTE cause the adjustable parameters to increase exponentially, and the choice of site for MIMO technology is also very different from that of non-MIMO systems. LTE / 2G / 3G joint networking and joint network planning and network optimization will further complicate this issue.
LTE standardization is nearing completion, but LTE system research and development is still in the early stages, facing many new challenges, and still requires arduous efforts to fully realize the expected potential of LTE technology and demonstrate the technical advantages of LTE.

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