FDD. Most probably spectrum filling up issues.

Thanks ismat for the informative link.

Thanks Sayed.

LTE TDD is Spectraly Efficient . Moreover the Telecom Ecosystem in China and India pushes for LTE TDD. No matter your are in TDD or FDD future enhancement such as LTE-A will really push you for carrier aggregation.

TDD or FDD depends of frequency plan and what is the organisation of the spectrum. For LTE it will be question about the legacy spectrum and possibility to change from FDD to TDD. If LTE goes down to 1.8GHz or 900MHz, existing FDD concept will have to be considered and probably kept unchanged. For 700MHz and 800MHz bands TDD might look more efficinet and there are no legacy issues.

TDD is more interesting also for other two aspects:

1. channel reciprocity

2. more suitable for device-to-device communications, which is a main feature of future 4G systems, i..e., LTE-Advances

The TDD mode is preferred for the following reasons:

1- It enables dynamic allocation of DL and UL resources to efficiently support asymmetric DL/UL traffic (adaptation of DL:UL ratio to DL/UL traffic).

2- It ensures channel reciprocity for better support of link adaptation; MIMO and other closed-loop advanced antenna techniques such as transmit beam-forming.

3- Unlike FDD, which requires a pair of channels, TDD only requires a single channel for both downlink and uplink providing greater flexibility for adaptation to varied global spectrum allocations.

4- Transceiver designs for TDD implementations are less complex and therefore less expensive (restrictions in the number of DL/UL switching points).

What is the reason to have 4x4 MIMO kind of configuration more adapted in TDD rather than in FDD? What is the reasoning to not have 4x4 MIMO deployment in FDD?