1、 Rep. ITU-R M.2040 1 REPORT ITU-R M.2040 Adaptive antennas concepts and key technical aspects (Question ITU-R 224/8) (2004) 1 Introduction This Report identifies the key adaptive antenna concepts and describes their technical aspects. The traditional approach to the analysis and design of wireless s
2、ystems has generally been to address antenna systems separately from other key systems aspects, such as: propagation issues; interference mitigation techniques; system organization (access techniques, power control, etc.); modulation. Adaptive antenna technologies are best implemented with an overal
3、l system approach, where all the system components, including the antenna system, are integrated in an optimal way, leading to substantial coverage improvements (e.g. larger coverage area, reduced “holes” in coverage) for each cell, vastly superior mitigation of interference problems, and substantia
4、l system capacity improvements. This Report reviews the various concepts of adaptive antennas, including the concept of “spatial channels”, provides a theoretical analysis of the potential of the technology and identifies the key characteristics. Attached as Annex 1 is a glossary of relevant termino
5、logy for adaptive antenna systems. 1.1 Related Recommendations The following ITU-R Recommendations may be useful in that they address mobile systems to which the concepts considered here may be considered appropriate to a greater or lesser degree: Recommendation ITU-R M.622: Technical and operationa
6、l characteristics of analogue cellular systems for public land mobile telephone use Recommendation ITU-R M.1032: Technical and operational characteristics of land mobile systems using multi-channel access techniques without central controller Recommendation ITU-R M.1033: Technical and operational ch
7、aracteristics of cordless tele-phones and cordless telecommunication systems Recommendation ITU-R M.1073 Digital cellular land mobile telecommunication systems Recommendation ITU-R M.1074: Integration of public mobile radiocommunication systems Recommendation ITU-R M.1221: Technical and operational
8、requirements for cellular multimode mobile radio stations 2 Rep. ITU-R M.2040 Recommendation ITU-R M.1457: Detailed specifications of the radio interfaces of International Mobile Telecommunications-2000 (IMT-2000) Recommendation ITU-R M.1678: Adaptive antennas for mobile systems Recommendation ITU-R
9、 SM.856: New spectrally efficient techniques and systems. 2 Antennas and adaptive antenna concepts 2.1 Antennas and coverage Adaptive antennas may be defined as an array of antennas and associated signal processing that together are able to change its radiation pattern dynamically to adjust to noise
10、, interference and multipath. Adaptive antennas are used to enhance received signal-to-interference noise ratios (SINR) and may also be considered as forming beams for transmission. Likewise, switched beam systems use a number of fixed beams at an antenna site. The receiver selects the beam that pro
11、vides the greatest signal enhancement and interference reduction. Switched beam systems may not offer the degree of performance improvement offered by adaptive systems, but they are much less complex and are easier to retro-fit to existing wireless technologies. Finally smart antennas are similarly
12、defined as systems that can include both adaptive antenna and switched beam technologies. A glossary of relevant adaptive antenna terms is provided in Annex 1; this section provides further discussion on the terminology and its general usage. The reader is cautioned that there is some variation in t
13、erminologies here; for example, non-adaptive or non-switched systems are sometimes termed smart simply due to the incorporation of masthead RF electronics, and often the terms adaptive and beam-forming are used rather loosely or narrowly. (For example, Recommendation ITU-R SM.856, seemingly the only
14、 other ITU-R Recommendation that mentions any aspect of adaptive antennas, uses the term adaptive rather than fully adaptive correctly, but briefly describes an example of a very narrow and specific interpretation of this as used in an earlier system at VHF.) Further, care is needed when the term ad
15、aptive is applied in discussing land mobile systems, but used alone without a further descriptor e.g. as applied to dynamic control of modulation or of bandwidth resources or of coding, power or other attributes of an air interface protocol. Adequate for simple RF environments where no specific know
16、ledge of the users location is available, the omnidirectional approach scatters signals, reaching target users with only a tiny fraction of the overall energy radiated into the environment (or, conversely, for emissions from the users towards the base station (BS). Given this limitation, omnidirecti
17、onal strategies attempt to overcome propagation challenges by simply boosting the power level of the signals. In settings where numerous users (hence, interferers) are relatively close to each other, this makes a bad situation worse in that the vast majority of the RF signal energy becomes a source
18、of potential interference for other users in the same or adjacent cells, rather than increasing the amount of information conveyed by the link. Rep. ITU-R M.2040 3 In uplink applications (user to BS), omnidirectional antennas offer no gain advantage for the signals of served users, limiting the rang
19、e of the systems. Also, this single element approach has no multi-path mitigation capabilities. Therefore omnidirectional strategies directly and adversely impact spectral efficiency, limiting frequency reuse. A single antenna can also be constructed to have certain fixed preferential transmission a
20、nd reception directions: today many conventional antenna systems split or “sectorize” cells. Sectorized antenna systems take a traditional cell area and subdivide it into “sectors” that are covered using multiple directional antennas sited at the BS location. Operationally, each sector is treated as
21、 a different cell. Directional antennas have higher gain than omnidirectional antennas, all other things being equal, and hence the range of these sectors is generally greater than that obtained with an omnidirectional antenna. Sectorized cells can improve channel reuse by confining the interference
22、 presented by the BS and its users to the rest of the network, and are widely used for this purpose. As many as six sectors per cell have been used in commercial service. 2.2 Adaptive antenna systems and diversity Understanding diversity is an important element in this context. As noted by Winters 1
23、, the three primary cellular wireless system impairments may be grouped under three categories viz. multipath fading, delay spread and co-channel interference. As explained later, multiple antennas, M in number, can generally provide increased gain of M and additionally effect diversity gain against
24、 multipath fading. The gain M may be considered as the reduction in required receive power for a given average output S/N (independent of the environment) whereas the diversity gain component (only possible with multipath evident) is the reduction in required average output S/N for a given error rat
25、e in the presence of fading. So space diversity antenna systems incorporate two or more antenna elements whose physical separation is used to combat the negative effects of multipath. Diversity offers an improvement in the effective strength of the received signal by using one of two methods: Switch
26、ed diversity: Assuming that at least one antenna will be in a favourable location at a given moment, this system continually switches between antennas (connecting each of the receiving channels to the most favourably located antenna) to select the antenna with the maximum signal energy. While reduci
27、ng signal fading, switched diversity does not increase gain since a single antenna is used at any time, and does not provide interference mitigation. Diversity combining: This approach coherently combines the signals from each antenna to produce gain: Maximal ratio combining systems combine the outp
28、uts of all the antennas to maximize the ratio of combined received signal energy to noise. 4 Rep. ITU-R M.2040 In contrast to switched diversity systems, diversity combining uses all antenna elements at all times for each user, creating an effective antenna pattern that dynamically adjusts to the pr
29、opagation environment. Diversity combining is not guaranteed to maximize the gain for any particular user, however. As the algorithms that determine the combining strategy attempt to maximize total signal energy, rather than that of a particular user, the effective antenna pattern may in fact provid
30、e peak gain to radiators other than the desired user (e.g. co-channel users in other cells). This is especially true in the high interference environments that are typical of a heavily loaded cellular system. 2.3 Antenna systems and interference More sophisticated antenna systems can mitigate the ot
31、her major limiting factor in cellular wireless systems: co-channel interference. For transmission purposes, the objective is to concentrate RF power toward each user of a radio channel only when required, therefore limiting the interference to other users in adjacent cells. For reception, the idea i
32、s to provide peak gain in the direction of the desired user while simultaneously reducing interference from other co-channel and adjacent channel users. This assumes an antenna system with instant beam steering capabilities: This can be achieved with phased array technology, in particular with digit
33、al beam-forming techniques. In addition, using a larger number of simple antenna elements gives a new dimension to the treatment of diversity as well. 2.4 Smart antenna systems The advent of powerful and low-cost digital signal processors, general-purpose processors and application-specific integrat
34、ed circuits (ASICs), as well as the development by several companies and research entities of software-based signal-processing techniques have made advanced adaptive antenna systems a practical reality for cellular communications systems: arrays of multiple antennas, combined with digital beam-formi
35、ng techniques and advanced low cost baseband signal processing open a new and promising area for enhancing wireless communication systems. Useful reference material on adaptive antennas will be found in 12. At the heart of an adaptive antenna system is an array of antenna elements (two or more, typi
36、cally four to 12), whose inputs are combined to adaptively control signal transmission and/or reception. Antenna elements can be arranged in linear, circular, planar, or random configurations and are most often installed at the BS site, although they may also be implemented in the mobile terminal. W
37、hen an adaptive antenna directs its main lobe with enhanced gain to serve a user in a particular direction, the antenna system side lobes and nulls (or directions of minimal gain) are directed in varying directions from the centre of the main lobe. Different switched-beam and adaptive smart antenna
38、systems control the lobes and the nulls with varying degrees of accuracy and flexibility. This has direct consequences in term of system performance. Rep. ITU-R M.2040 5 2.4.1 Switched-beam antenna Switched-beam antenna systems form multiple fixed beams with heightened sensitivity in particular dire
39、ctions. These antenna systems detect signal strength, choosing from one of several predetermined, fixed beams, based on weighted combinations of antenna outputs with the greatest output power in the remote users channel, and switching from one beam to another as the mobile moves through the sector.
40、These choices are driven by RF or baseband digital signal processing techniques. Switched beam systems can be thought of as a “micro-sectorization” strategy. 2.4.2 Adaptive array antenna Adaptive antenna technology represents the most advanced and productive approach to date for significant improvem
41、ents to spectrum efficiency. Using a variety of signal-processing algorithms, an adaptive system effectively identifies and tracks all the relevant signals and interferers present in order to dynamically minimize interference and maximize reception of the signals of interest. In the same manner as a
42、 switched-beam system, an adaptive system will attempt to increase gain based on the users signal as received at the various elements in the array. However, only the adaptive system provides optimal gain while simultaneously mitigating interference. Diversity combining also continuously adapts the a
43、ntenna pattern in response to the environment. The difference between it and the adaptive antenna method is fundamentally in the richness of the models on which the two systems processing strategies are based. In a diversity system, the model is simply that there is a single user in the cell on the
44、radio channel of interest. In the adaptive system, the model is extended to include the presence of interferers and, often, temporal history regarding the users propagation characteristics. With this second model, it is possible to discriminate users from interferers, even at low SINR, and provide r
45、eliable gain and interference mitigation. The adaptive antenna systems approach to communication between a user and the BS in effect takes advantage of the spatial dimension, adapting to the RF environment including the constellation of users and other emitters as it changes, according to predefined
46、 strategies. This approach continuously updates the BS systems radiation and reception patterns, based on changes in both the desired and interfering signals relative configuration. In particular, the ability to efficiently track users through antenna main lobes and interferers through nulls ensures
47、 that the link budget is constantly maximized. By implementing the smart antenna strategies digitally, it is possible for the BS to support a separate, tailored, strategy for each active channel in the system via a single array and set of radio electronics. The difference between the two approaches
48、adaptive and switched beam is illustrated in Fig. 1, which shows how the adaptive algorithms behave with respect to interferers and the desired signal. 6 Rep. ITU-R M.2040 Rap 2040-01FIGURE 1Difference between switched beam and adaptive beamSwitched stategy Adaptive stategy2.4.3 Spatial processing:
49、the fully adaptive approach Utilizing sophisticated algorithms and powerful processing hardware and microprocessors, “spatial processing” takes the frequency reuse advantage resulting from interference suppression to a new level. In essence, spatial processing (spatial division multiple access. (SDMA) dynamically creates a different notional beam structure for each user and assigns frequency/channels on an ongoing basis in real time. Spatial processing maximizes the use of multiple antennas to usefully combine signals in space, through methods that transcend the
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