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Comparison of the alternative technologies to provide asymmetric traffic capability
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- 2.5 Comparison of normalized spectrum usage for FDD and TDD
2.4 Comparison of the alternative technologies to provide asymmetric traffic capabilityAsymmetric modulation has the following advantages: – No additional frequency allocations or time slot allocations are required. – A better throughput for the same channel, however, only for lower range or cases with higher signal/noise-ratio (link adaptation necessary). – This method could be used to enhance both TDD and FDD systems in areas with favourable radio conditions. Potential disadvantages to be considered are: – Influence of direction on system margin. – Only limited possible ratio of asymmetry. – Trade-off between coverage and maximum available data rate. – Link adaptation is required. – Mainly applicable for packet services. – More complicated planning and implementation. – If capacity on one link can be improved, the same method does also increase capacity on the other link; therefore, no inherent advantage with regard to improving asymmetric service provision. Application of additional techniques provides the following advantages: – Improvements can be applied to both, FDD and TDD. – Improvement of capacity by adaptive antennas in the downlink only by transmit antenna for simple terminals. – Adaptive transmit antennas are more effective for TDD than for FDD. Potential disadvantages to be considered are: – More capacity will be needed in the downlink, which would increase terminal receiver complexity compared to the base station for advanced detection schemes. – These methods would put higher complexity in the terminal than in the base station. The selection of a suitable duplex arrangement depends on the application (short range, wide area, flexibility, etc.). The duplex scheme is only one of several aspects to be considered (other aspects are, e.g. interference sensitivity, switching point flexibility, etc.). 2.5 Comparison of normalized spectrum usage for FDD and TDDThe spectrum usage can be characterized by the total average throughput in downlink and uplink normalized to the allocated total bandwidth for the downlink and uplink (normalized throughput) for a given spectrum allocation and traffic asymmetry. The spectrum allocation is fixed, where the traffic asymmetry may vary. For FDD a symmetric spectrum allocation is assumed here. In FDD the throughput in both links can be adjusted independently between 0 < auplink < 100% and 0 < adownlink < 100%. The spectrum usage for traffic asymmetry is maximized if one of the links is completely used with auplink or adownlink equal 100% and the other parameter may vary with respect to the ratio of asymmetry with less than 100%. In the case if both auplink and adownlink are less than 100%, the spectrum is used less efficiently. In the case of TDD auplink and adownlink cannot be adjusted independently, because under optimum conditions the sum of both equals 100%. If the time slots can be adjusted according to the traffic asymmetry and all time slots are used, then the spectrum usage is optimal. Under these optimum assumptions for FDD and TDD Fig. 12 shows a comparison of the spectrum usage for both duplex schemes versus auplink and adownlink. The adjustment of the location of the switching point between uplink and downlink transmission in TDD may be constrained (e.g., to maintain synchronization with other cells, traffic asymmetry from other users in the same cell). In that case, the throughput of TDD would approach that of FDD. FIGURE 12
The selection of a suitable duplex arrangement depends on the application (short range, wide area, flexibility, spectrum availability of potential new paired or unpaired spectrum etc.). The duplex scheme is only one of several aspects to be considered (other aspects are, e.g. interference sensitivity, switching point flexibility, etc.). This Annex provides information about the trade-off between efficient spectrum usage and maximum throughput per direction including other advantages and disadvantages, which have different impacts depending on requirements. The spectrum availability will not be known before WRC 07. Therefore, no general guideline can be derived yet. Annex 3 Advanced system innovation using TDD Members of the TDD Coalition, for example, have developed important technical innovations through the implementation of TDD. System advantages can be obtained from the use of reciprocal channels – a unique feature of TDD systems. Channel reciprocity for single carrier frequency shared by uplink and downlink allows an easier access to channel-state information for advanced signal processing techniques. For instance, channel reciprocity ensures that the fading on the uplink and downlink are highly correlated. Since the channel characteristics are same in both directions, any signal processing resources for doing space/time/equalization/frequency processing can be shared between the transmitter and receiver. Hence, TDD is a uniquely suited technology for advanced signal processing in the areas of open-loop power control, novel multipath and antenna combining, and time-space processing techniques, with a lower cost adder. For example, adaptive antenna arrays can be added by implementing advanced signal processing at the base station and sharing the channel weighting information with the subscriber units. This allows the spectral efficiency of the system to be increased by an order of magnitude without increasing the CPE cost. Another example is mesh networks. These innovative systems, using an architecture hitherto more common for military HF systems, are better facilitated by TDD implementation. The network and frequency planning in a substantial FDD mesh (MP-MP) system is significantly more difficult than for conventional multi-way P-P or P-MP (area coordinated) systems. Another area of innovation is in media access control (MAC). The TDD operation allows for highly dynamic and various configurations of physical layer time frame. TDD systems have a much higher flexibility to handle the dynamic up/down traffic, since the boundary between uplink and downlink duty cycle could be adaptively adjusted to accommodate the service requirements. Dynamic TDD systems are far more bandwidth efficient than the traditional FDD systems for the future data-centric multimedia traffic. By implementing intelligent (“channel-aware”) media access control (MAC) protocols and use of the superior architecture provided by TDD, throughput multiplication, statistical multiplexing gain, and reduction of packet delays can be achieved. 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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