DiversityAntennas.docx

1、DiversityAntennas GSM Products DivisionDiversity Antenna Systems ForGSM900/GSM1800/PCS1900 NetworksIssue AJun XiangProduct ManagementGSM Products DivisionEmail xiangjPhone +44(0)1793565680Fax +44(0)1793514215About this issueThis document has been requested by a number of staff in Motorola system/fie

2、ld engineering groups.It is a technical primer intended to give a rational background to the issues of diversity antenna application and selection. The content allows for a very wide variety of experience and application knowledge.It is intended for this document to cover all the fundamental informa

3、tion that Motorola GSM system/field engineers need to implement a diversity antenna system. The complete package will include: basic principles, applications, system specifications, Motorola preferred antenna vendors, procedures for the diversity antenna trial and analysis tool.There is however an u

4、rgent need for earlier release of the document, hence some parts have not been completed in this issue. Ultimately this document will include practical recommendations derived from all Motorola experiences. These recommendations will also incorporate the most commercially advantageous equipment sele

5、ctions based on global purchasing advantage.This version covers section 1 to 6 and Appendix D.The antenna vendors listed in Appendix A are a preliminary selection based on a purely technical analysis. This is because the antenna vendors need to be commercially assessed by the Wireless Network Produc

6、ts Division (WNP). GPD and WNP are working together to finalise this process within the next few weeks. The finalised antenna recommendations will be a shorter list and will be published in the next issue of this document.Appendix C describes the procedures for the diversity antenna trial and method

7、 for data acquisition and post-processing. The GSM research group of GPD and various of other groups are currently working on this project. Once the measurement tool has been developed, large scale measurements will be organised. The typical diversity gain with the Motorola BTS equipment can then be

8、 derived from the measured results and published in Appendix B.Contents1 Scope2 Introduction2.1 The needs2.2 Concept of diversity antenna systems2.3 Realisation of the diversity antenna systems3 Spatial diversity antenna systems3.1 Basic principles3.2 Configurations3.3 Specifications4 Polarisation d

9、iversity antenna systems4.1 Basic principles4.2 Types of dualpolarisation antennas4.3 Configurations4.4 Specifications5 Summary6 ReferencesAppendix A Provisional recommended dualpolarisation antennas and vendorsAppendix B Typical diversity gainAppendix C Generic test procedures and analysis toolAppe

10、ndix D Generic specifications for dualpolarisation antennas1 ScopeThis document describes the principle, application and practical configuration of diversity antenna systems for GSM900/GSM1800/PCS1900 networks.The aim of the document is to provide fundamental information regarding the diversity ante

11、nna system as a guideline to Motorola GSM system/field engineers in the process of selecting, implementing and testing the diversity antenna system.This document is intended for internal use only purpose.2 Introduction2.1 The needsA diversity antenna system is an essential part of the front end RF d

12、istribution system at a cellular base site. The use of a diversity antenna system enhances the ability of a cellular network to combat the multipath fading and increases the signal strength or signal-to-noise (S/N) ratio. The ultimate aim is to enable the network to offer high transmission quality.

13、The distinct phenomenon in the mobile propagation environment is multipath fading. Overcoming the multipath fading to maintain an acceptable radio transmission quality is therefore one of the key elements that drive the architecture design of any mobile communication systems.In a typical cellular ra

14、dio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths. This is because the direct path between the transmitter and the receiver is obstructed by buildings and other objects. The signal that arrives at the receiver is therefore largely by

15、 way of scattering, either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions.When various incoming radiowaves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength as sh

16、own in Figure 1. The signal fluctuations are known as multipath fading. The fluctuation can very often be so large that the radio link suffers from either severe deterioration of transmission quality or loss of the communication.Diversity techniques have been recognised as an effective means which e

17、nhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include In the time domain - interleaving In the frequency domain - frequency hopping In the spatial domain - spatial diversity In the polarisation domain - polarisati

18、on diversity.Figure 1Illustration of the multipath fadingThe fundamental principle of the previously mentioned techniques is to create various copies of signals that carry the same or consecutive pieces of information but arrive at the receiver through different transmission media (Figure 2) in term

19、s of time, frequency, location and polarisation. Those diversified signals experience different fading and hence the probability that they all suffer from a deep fade are considerably reduced. Optimal use of the diversified incoming signals will then result in an improvement of the radio transmissio

20、n quality.2.2 Concept of diversity antenna systems Of those techniques described earlier, spatial and polarisation diversity techniques are realised through antenna systems.In general, a diversity antenna system provides a number of receiving branches or ports (typically 2 in the present network, th

21、is will be assumed in the rest of this document) from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal (Figure 3) with improved quality in terms of signal strength or signal-to-noise ratio

22、(S/N). The use of the combined signal for demodulation will then lead to a lower transmission error rate.Figure 2Independent fading derived from different transmission mediaThe performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between br

23、anches.Correlation between two receiving branchesThe branch correlation coefficient () represents the degree of similarity between the signals from two different receiving branches. The correlation coefficient ranges from 0 to 1. =1 means the signals from two different branches behave exactly the sa

24、me. In this case, the signals are coherent.=0 means the signals from two different branches behave completely different. In this case, the signals are uncorrelated.To achieve the best performance, a diversity antenna system is required to provide uncorrelated signals. In the scenario that the signal

25、s are uncorrelated, the two signals fluctuate differently as shown in figure 3. As a result, the probability that both signals simultaneously experience deep fades is considerably reduced. By appropriate combining the two uncorrelated signals, the receiver is able to derive a combined signal with in

26、creased signal strength or S/N and hence an improved Bit-Error-Rate (BER) performance.The improvement of the transmission quality decreases as the decorrelation between the two branches increases. For =0, the diversity antenna becomes ineffective in combating the multipath fading.(a)(b)Figure 3Diver

27、sity combiningAn example of selection combiningThe process by which a receiver combines the two inputs to produce a signal with good quality for demodulation is called diversity combining. The simplest combining technique is the selection combining which selects the best signal from the two receivin

28、g branches. The technique widely used in GSM is however the maximal ratio combining, which aims at maximising S/N.In reality, however, it is not always practical to have a diversity antenna system which guarantees =0. Extensive research in this field has revealed that a diversity antenna system can

29、perform satisfactorily provided that 0.7. 0.7 is therefore one of the fundamental criteria for the design of a diversity antenna system.Signal level differenceThe second key parameter for a good diversity antenna system is the mean signal level difference. The difference is a statistical parameter w

30、hich indicates the balance of the signal strengths from the two receiving branches.It has been found that a balance of the signal strengths between the two branches is essential for a diversity antenna system to operate to its maximum efficiency.In a real system, the statistical balance can be verif

31、ied by comparing the mean values of the two signals measured over a lengthy period, as in Figure (4). If the ratio between the median values is 0dB, the two receiving branches are statistically balanced. This is an ideal scenario for a diversity system to operate. The performance of the diversity sy

32、stem will deteriorate while the ratio increases or decreases from 0dB. Figure 4Illustration of the signal level differenceTable 1 1 lists the diversity gain (G) achieved by the maximal ratio combining versus the signal level difference (). The table shows that the signal level difference must be less than 4dB in order to achieve 3 dB diversity gain.Since the diversity gain is normally assumed to be 3 dB in a link budget analysis,