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Effects of adaptive antennas on TDD/FDD coexistence
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- 4 Some radio interface design considerations
- 4.2 Downlink
- 4.3 Subscriber unit
- 4.4 Optimal new radio interfaces
- 4.5 Duplexing methods
- 4.6 Carrier bandwidth
3 Effects of adaptive antennas on TDD/FDD coexistenceThe direct effect of coexistence is due to the fact that the RF energy radiated by transmitters is focused in specific areas of the cell and is not constant over time. This characteristic plays a major role in determining the likelihood of interference in coexistence scenarios, especially in the context of TDD/FDD mixed system deployments. While an absolute worst case may look prohibitive, the statistical factor introduced by the use of adaptive antennas determines the percentage of time that the worst case happens. If this percentage is satisfactorily small, any established coexistence rules may for example be relaxed, thus helping the economics of deployment. The Monte Carlo approach to statistical analysis can be used to study the improvements that can be obtained from adaptive antennas here. In realistic IMT-2000 system simulations, very significant improvements were determined in terms of safe coexistence distances (much reduced), or reduced RF isolation requirement on base stations. In some cases any additional isolation is now sufficiently modest that it should be readily achieved simply by adopting coexistence-friendly site engineering practices, and in other situations coexistence cases otherwise impossible become practical. 4 Some radio interface design considerations4.1 UplinkTiming alignment, or timing discrepancy, between signal bits can change because of phase variations and the arrived frequency can be offset from the transmit frequency because of differences in the local oscillators and multipath signals cause fluctuations in the received signal levels. The net result is high bit error rates or an unrecoverable signal, if not corrected. Attempts to correct these anomalies can be made at the antenna by using switched antenna diversity, or optimal ratio combining. However, analysing the signal after it leaves the demodulator and using signal feedback mechanisms to continually correct the signal is where the greatest enhancements can be achieved. The degrees of freedom to correct these anomalies increase if different samples of the same signal can be analysed from multiple antenna/receiver chains as the same signal impinging on different antenna elements have slightly different characteristics, i.e. phase difference and amplitude. These together with other temporal parameters can be analysed separately and together to optimize the ultimate signal that is forwarded to the switching network. Another figure of merit that can be improved significantly when analysing signals in this manner is carrier-to-interference rejection (C/I), or in other words, the interference from signals that are not of interest can be isolated and rejected to clean-up the desired signal, leading to better cell coverage and call quality. To achieve these performance improvements, the cost of adding power amplifiers in a multiple antenna base station, has the potential to be lower than the cost of the power amplifier in a conventional single antenna base station. Amplifier costs increase disproportionately with the output power and the cost of multiple lower power amplifiers, which may be integrated at the antenna, cost less today than one higher power amplifier having equivalent effective radiated power. 4.2 DownlinkUsing reciprocity, it is possible to achieve the same performance gains in the downlink direction. This is straightforward with time division duplex but becomes more difficult when used in a frequency division duplex environment, although not insurmountable. Broadcast signalling channels also present special challenges, but can be accommodated with innovative solutions. 4.3 Subscriber unitFor certain applications it may be economically feasible to incorporate adaptive antenna processing at the subscriber unit thereby increasing uplink gain and reducing system-wide uplink interference. Significant benefits are also possible through simpler uplink strategies when adaptive antennas are present at the base station, only. Implementation of uplink power control will improve the overall performance of the network since less interference equates to more capacity. Tightly coupling subscriber power control into an overall performance enhancing strategy can significantly enhance network performance. 4.4 Optimal new radio interfacesOver the last decade, there have been many efforts to increase the efficiency of wireless networks. This effort has largely focused on modulation types, channel coding and access methods. Below are listed some areas in which careful design with consideration to adaptive antennas can more closely approach the global optimum. 4.5 Duplexing methodsAs mentioned above, the benefits of TDD over FDD are evident due to the significantly reduced de correlation between uplink and downlink channels. Also one of the initial drawbacks to TDD technology were the problems associated with the rapid switching between transmit and receive, limiting the e.i.r.p. of a TDD base station. However, with the combined signal from various independent transmitting elements the overall power level that can now effectively be rapidly switched on and off is significantly increased. As for terrestrial wide-area, full high-speed mobility systems the use of paired bands and FDD transmission might be advantageous, while shorter-range, slower mobility systems and TDD transmission in unpaired bands can be advantageous to handle asymmetric traffic. 4.6 Carrier bandwidthThe decision to incorporate adaptive antennas will affect the choice of carrier bandwidth. Adaptive antenna systems can most effectively control the RF environment when the number of significant (as seen at a given base station) co-channel in-cell and out-of-cell users is modest. It may be better to partition the users into slices of spectrum rather than have large number of users sharing the “whole” spectrum. One may also wish to consider the signal correlation variation across the bandwidth, which tends to decrease as the channel bandwidth increases making the adaptive antenna processing more complex. Directory: dms pub -> itu-r -> oth oth -> Section 1a vocabulary, characteristics and general technologies for land mobile service oth -> U. S. Mobile eas for atsc mobile dtv itu-r -> Report itu-r sa. 2312-0 (09/2014) itu-r -> Report itu-r itu-r -> 3rd itu inter-regional workshop on wrc-15 preparation geneva, 1 – 3 September 2015 oth -> Radiocommunication Study Groups oth -> Itu-r workshop on Emergency Broadcasting Speakers Biographies Lynn Claudy oth -> Coordination and notification procedures introduction Download 496.69 Kb. Share with your friends:
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