LTE - Resource Block Flexible Bandwidth

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A transmitted OFDMA signal can be carried by a number of parallel subcarriers. Each LTE subcarrier is 15 kHz. Twelve subcarriers (180 kHz) are grouped into a resource block. The downlink has an unused central subcarrier. Depending on the total deployed bandwidth, LTE supports a varying number of resource blocks.
The following illustration shows resource block definition:




A resource block is limited in both the frequency and time domains. One resource block is 12 subcarriers during one slot (0.5 ms).
In the downlink, the time-frequency plane of OFDMA structure is used to its full potential. The scheduler can allocate resource blocks anywhere, even non-contiguously.
 A variant of OFDMA is used in the uplink. This variant requires the scheduled bandwidth to be contiguous, forming in effect a single carrier. The method, called SC-FDMA, can be considered a separate multiple access method.

A user is scheduled every Transmission Time Interval (TTI) of 1 ms, indicating a minimum of two consecutive resource blocks in time at every scheduling instance. The minimum scheduling in the frequency dimension is 12 subcarriers that is the width of one resource block in the frequency dimension. The scheduler is free to schedule users both in the frequency and time domain. Show in Figure as example of two users scheduled in the time and frequency domain for the downlink and the uplink:





The defined LTE bandwidths in 3GPP are the following:
In Table  Bandwidths and Resource Blocks Specified in 3GPP

Bandwidth
Number of Resource Blocks nRB
1.4 MHz
6
3.0 MHz
15
5.0 MHz
25
10.0 MHz
50
15.0 MHz
75
20.0 MHz
100



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