FDD and TDD LTE frequency bands

FDD spectrum requires pair bands, one of the uplink and one for the downlink, and TDD requires a single band as uplink and downlink are on the same frequency but time separated. As a result, there are different LTE band allocations for TDD and FDD. In some cases these bands may overlap,and it is therefore feasible, although unlikely that both TDD and FDD transmissions could be present on a particular LTE frequency band.

The greater likelihood is that a single UE or mobile will need to detect whether a TDD or FDD transmission should be made on a given band. UEs that roam may encounter both types on the same band. They will therefore need to detect what type of transmission is being made on that particular LTE band in its current location.

The different LTE frequency allocations or LTE frequency bands are allocated numbers. 
Currently the LTE bands between 1 & 22 are for paired spectrum, i.e. FDD, and LTEbands
between 33 & 41 are for unpaired spectrum, i.e. TDD. 

                                 
FDD LTE frequency band allocations 

There is a large number of allocations or radio spectrum that has been reserved for FDD, 
frequency division duplex, LTE use. 
The FDDLTE frequency bands are paired to allow simultaneous transmission on two 
frequencies. The bands also have a sufficient separation to enable the transmitted signals
 not to unduly impair the receiver performance. If the signals are too close then the receiver 
may be "blocked" and the sensitivity impaired. The separation must be sufficient to enable 
the roll-off of the antenna filtering to give sufficient attenuation of the transmitted signal within
 the receive band.

TDD LTE frequency band allocations 
With the interest in TDD LTE, there are several unpaired frequency allocations that are 
being prepared for LTR TDD use. The TDD LTE allocations are unpaired because the 
uplink and downlink share the same frequency, being time multiplexed.
                              
There are regular additions to the LTE frequency bands / LTE spectrum allocations as a
 result of negotiations at the ITU regulatory meetings. These LTE allocations are resulting
 in part from the digital dividend, and also from the pressure caused by the ever growing 
need for mobile communications. Many of the new LTE spectrum allocations are relatively
 small, often 10 - 20MHz in bandwidth, and this is a cause for concern. With LTE-Advanced needing bandwidths of 100 MHz, channel aggregation over a wide set of
 frequencies many be needed, and this has been recognised as a significant technological 
problem. . . . . . . . .