RAN Design Planning

The FDD LTE and TD-LTE versions of the 3GPP standard are very similar from both operational, functional and coverage perspectives. Existing UEs and LTE devices can support both the FDD and TDD interfaces through a single chipset without any additional cost, allowing TD-LTE to benefit from the wide availability of FDD LTE devices that will be able to support TD-LTE as well. What makes LTE-TDD so important is the TDD spectrum availability, more than existing FDD, where in most cases it is cheaper and under-utilized. Moreover there is also the possibility to use the unlicensed band of 4 GHz-5 GHz.

It is often considered among planners that that TD-LTE has less coverage range than LTE FDD system in the same condition. However, this is not always true since the coverage of radio system could be considered as control channel coverage and traffic channel coverage. The control channel coverage is the minimum requirement for UE to access the network; therefore the coverage range is relatively large. In this sense, TD-LTE and LTE FDD have similar coverage. But since LTE system is a Mobile Broadband (MBB) data oriented network, it is strongly required cell edge users to access the network with a minimum required MBB data service fully guaranteed. Since LTE-TDD (TD-LTE) system contains a special subframe which could not transmit uplink data, and also considering the TDD frame structure, it is safe to conclude that the efficient uplink bandwidth of TD-LTE system is therefore lower than FDD LTE system. Therefore, for the same minimum cell edge user uplink data rate, TD-LTE needs to decrease link budget to compensate for the losses in efficient bandwidth. In this sense, TD-LTE has lower uplink coverage range. Concluding it is important to keep in mind the fact the LTE-TDD UL coverage range strongly depends on d differs with the subframe configuration and system requirements of cell edge users.

Internet application today has shown asymmetric character between downlink and uplink; therefore in order to bring flexibility and efficiently utilize time-frequency resources, TD-LTE system has defined different downlink to uplink allocations. LTE FDD system is somehow problematic on the traffic requirements since it allocates the same bandwidth to both uplink and downlink transmission keeping a similar spectral efficiency of uplink and downlink for a given scenario. However not always the user peak data rate and system capacity are compatible in a certain ratio since actual data traffic does not always follow the pattern given by the system but fluctuates according to various unpredictable factors like time or location. As a result system experiences a waste of resources over time/frequency in either direction. That makes LTE FDD system totally solid and inflexible in the diverse traffic pattern requirement satisfaction for typical Mobile Broadband MBB services and applications. To overcome such a waste of resources and make the system more flexible to different asymmetric scenarios, 3GPP has specified under LTE-TDD (TD-LTE) technology 7 different uplink and downlink subframe configuration. TD-LTE system could support by proper static configuration both strong uplink biased scenarios and downlink biased scenarios, as well as symmetric applications.