蜂窝系统切换技术中英文对照外文翻译文献Word文档下载推荐.docx

1、 原文：Handoff in Cellular SystemsCellular SystemDeployment ScenariosThe radio propagation environment and related handoff challenges are different in different cellular structures. A handoff algorithm with fixed parameters cannot perform well in different system environments. Specific characteristics

2、of the communication systems should be taken into account while designing handoff algorithms. Several basic cellular structures （e.g., macrocells, microcells, and overlay systems） and special architectures （e.g., underlays, multichannel bandwidth systems,and evolutionary architectures） are described

3、 next. Integrated cordless and cellular systems, integrated cellular systems, and integrated terrestrial and satellite systems are also described.MacrocellsMacrocell radii are in several kilometers. Due to the low cellcrossing rate, centralized handoff is possible despite the large number of MSs the

4、 MSC has to manage. The signal quality in the uplink and downlink is approximately the same. The transition region between the BSs is large; handoff schemes should allow some delay to avoid flip-flopping. However, the delay should be short enough to preserve the signal quality because the interferen

5、ce increases as the MS penetrates the new cell. This cell penetration is called cell dragging. Macrocells have relatively gentle path loss characteristics . The averaging interval （i.e., the time period used to average the signal strength variations） should be long enough to get rid of fadingfluctua

6、tions. First- and second-generation cellular systems provide wide-area coverage even in cities using macrocells .Typically, a BS transceiver in a macrocell transmits high output power with the antenna mounted several meters high on a tower to illuminate a large area. MicrocellsSome capacity improvem

7、ent techniques （e.g., larger bandwidths, improved methods for speech coding, channel coding,and modulation） will not be sufficient to satisfy the required service demand. The use of microcells is considered the single most effective means of increasing the capacity of cellular systems.Microcells inc

8、rease capacity, but radio resource management becomes more difficult. Microcells can be classified as one-, two-, or threedimensional,depending on whether they are along a road or a highway, covering an area such as a number of adjacent roads,or located in multilevel buildings, respectively . Microc

9、ells can be classified as hot spots （service areas with a higher traffic density or areas that are covered poorly）, downtown clustered microcells （contiguous areas serving pedestrians and mobiles）, and in-building 3-D cells （serving office buildings and pedestrians）.Typically, a BS transceiver in a

10、microcell transmits low output power with the antenna mounted at lamppost level （approximately 5 m above ground）.The MS also transmits low power, which leads to longer battery life. Since BS antennas have lower heights compared to the surrounding buildings, RF signals propagate mostly along the stre

11、ets.The antenna may cover 100200 m in each street direction, serving a few city blocks. This propagation environment has low time dispersion, which allows high data rates.Microcells are more sensitive to the traffic and interference than macrocells due to short-term variations （e.g., traffic and int

12、erferencevariations）,medium/long-term alterations （e.g., new buildings）, and incremental growth of the radio network （e.g., new BSs） . The number of handoffs per cell is increased by an order of magnitude, and the time available to make a handoff is decreased. Using an umbrella cell is one way to re

13、duce the handoff rate. Due to the increase in the microcell boundary crossings and expected high traffic loads, a higher degree of decentralization of the handoff process becomes necessary.Microcells encounter a propagation phenomenon called the corner effect. The corner effect is characterized by a

14、 sudden large drop （e.g., 2030 dB） in signal strength （e.g., at 1020 m distance） when a mobile turns around a corner. The corner effect is due to the loss of the line of sight （LOS） component from the serving BS to the MS. The corner effect demands a faster handoff and can change the signal quality

15、very fast. The corner effect is hard to predict. A long measurement averaging interval is not desirable due to the corner effect. Moving obstacles can temporarily hinder the path between a BS and an MS,which resembles the corner effect. Reference studies the properties of symmetrical cell plans in a

16、 Manhattan-type environment. Cell plans affect signal-to-interference ratio （SIR） performance in the uplink and downlink significantly. Symmetrical cell plans have four nearest co-channel BSs located at the same distance. Such cell plans can be classified into half-square （HS）, full-square （FS）, and

17、 rectangular （R） cell plans. These cell plans are described next.Half-Square Cell Plan This cell planplaces BSs with omnidirectional antennas at each intersection, and each BS covers half a block in all four directions. This cell plan avoids the street corner effect and provides the highest capacity

18、. This cell plan has only LOS handoffs. Figure 2 shows an example of a half-square cell plan in a microcellular system.Full-Square Cell Plan There is a BSwith an omnidirectional antenna located at every other intersection, and each BS coversa block in all four directions. It is possible for an MS to

19、 experience the street corner effect for this cell plan. The FS cell plan can have LOS or NLOS handoffs. Figure 3 shows an example of a fullsquare cell plan in a microcellular system.Rectangular Cell Plan Each BS covers a fraction of either a horizontal or vertical street with the BS located in the

20、middle of the cell. This cell plan can easily be adapted to market penetration. Fewer BSs with high transmit power can be used initially. As user density increases, new BSs can be added with reduced transmit power from appropriate BSs.The street corner effect is possible for this cell plan. The R ce

21、ll plan can have LOS or NLOS handoffs. Figure 4 shows an example of a rectangular cell plan in a microcellular system. Macrocell/Microcell Overlays Congestion of certain microcells, the lack of service of microcells in some areas, and high speed of some users are some reasons for higher handoff rate

22、s and signaling load for microcells. To alleviate some of these problems, a mixed-cell architecture （called an overlay/underlay system） consisting of largesize macrocells （called umbrella cells or overlay cells） and small-size microcells（called underlay cells） can be used. Figure 5 illustrates an ov

23、erlay system.The macrocell/microcell overlay architecture provides a balance between maximizing the number of users per unit area and minimizing the network control load associated with handoff. Macrocells provide wide-area coverage beyond microcell service areas and ensure better intercell handoff.

24、Microcells provide capacity due to greater frequency reuse and cover areas with high traffic density （called hot spots）. Examples of hot spots include an airport,a railway station, or a parking lot. In less congested areas （e.g., areas beyond a city center or outside the main streets of a city） traf

25、fic demand is not very high, and macrocells can provide adequate coverage in such areas. Macrocells also serve highspeed MSs and the areas not covered by microcells （e.g., dueto lack of channels or the MS being out of the microcell range）. Also, after the microcellular system is used to its fullest

26、extent, the overflow traffic can be routed to macrocells.One of the important issues for the overlay/underlay system is the determination of optimum distribution of channels in the macrocells and microcells. Reference evaluates four approaches to sharing the available spectrum between the two tiers.

27、 Approach 1 uses TDMA for microcell and CDMA for macrocell. Approach 2 uses CDMA for microcell and TDMA for macrocell. Approach 3 uses TDMA in both tiers, while approach 4 uses orthogonal frequency channels in both tiers.The overlay/underlay system has several advantages over a pure microcell system

28、: The BSs are required only in high traffic load areas. Since it is not necessary to cover the whole service area with microcells,infrastructure costs are saved. The number of handoffs in an overlay system is much less than in a microcell system because fast-moving vehicles can be connected to the o

29、verlay macrocell. Both calling from an MS and location registration can easily be done through the microcell system.There are several classes of umbrella cells. In one class, orthogonal channels are distributed between microcells and macrocells.In another class, microcells use channels that are temp

30、orarily unused by macrocells. In yet another class,microcells reuse the channels already assigned to macrocells and use slightly higher transmit power levels to counteract the interference from the macrocells.Within the overlay/underlay system environment, four types of handovers need to be managed

31、19: microcell to microcell, microcell to macrocell, macrocell to macrocell, and macrocell to microcell.Reference describes combined cell splitting and overlaying. Reuse of channels in the two cells is done by establishing an overlaid small cell served by the same cell site as the large cell. Small c

32、ells reuse the split cells channels because of the large distance between the split cell and the small inner cell, while the large cell cannot reuse these channels. Overlaid cells are approximately 50 percent more spectrally efficient than segmenting （the process of distributing the channels among t

33、he small- and largesize cells to avoid interference）.A practical approach for implementation of a microcell system overlaid with an existing macrocell system is proposed in . This reference introduces channel segregation （a self-organized dynamic channel assignment）and automatic transmit power control