So far, we have concentrated our efforts in describing LTE-FDD. However, as TD-LTE gets deployed in the USA, more blogs on this technology will be published.

In this occasion, we will describe the frame structure of both, LTE-FDD and LTE-TDD.

Let us describe the LTE-FDD structure first.


The duration of one LTE radio frame is 10 ms. One frame is divided into 10 subframes of 1 ms each, and each subframe is divided into two slots of 0.5 ms each. Each slot contains either six or seven OFDM symbols, depending on the Cyclic Prefix (CP) length. The useful symbol time is 1/15 kHz= 66.6 mircosec. Since normal CP is about 4.69 microsec long, seven OFDM symbols can be placed in the 0.5-ms slot as each symbol occupies (66.6 + 4.69) = 71.29 microseconds. When extended CP (=16.67 microsec) is used the total OFDM symbol time is (66.6 + 16.67) = 83.27 microseconds. Six OFDM symbols can then be placed in the 0.5-ms slot. Frames are useful to send system information. Subframes facilitate resource allocation and slots are useful for synchronization. Frequency hopping is possible at the subframe and slot levels.

In LTE, radio resources are allocated in units of Physical Resource Blocks (PRBs). Each PRB contains 12 subcarriers and one slot. If the normal Cyclic Prefix is used, a PRB will contain 12 subcarriers over seven symbols. If the extended CP is used, the PRB contains only six symbols. The UE is specified allocation for the first slot of a subframe. There is implicit allocation for the second slot of the subframe. For example, if the eNB specifies one RB as the resource allocation for the UE, the UE actually uses two RBs, one RB in each of the two slots of a subframe. When frequency hopping is turned on, the actual PRBs that carry the UE data can be different in the two slots. In a 10 MHz spectrum bandwidth, there are 600 usable subcarriers and 50 PRBs.



Frame structure Type 2 is applicable to TDD is as shown in the figure. Each radio frame of 10 ms in length consists of two half-frames of 5 ms in length. Each half-frame consists of eight slots of the length Ts=5 ms and three special fields DwPTS, GP, and UpPTS of 1 ms in length.

Different configurations, numbered zero to six, are defined in the standard for the subframe number allocated for the uplink and downlink transmission. Subframe 1 in all configurations and subframe 6 in configurations 0, 1, 2 and 6 consist of DwPTS, GP and UpPTS. All other subframes are defined as two slots.

Switch-point periodicities of 5 ms and 10 ms are supported. The standard defines the table for the uplink and downlink allocations for switch-point periodicity. In the case of a 5-ms switch-point periodicity, UpPTS and subframes 2 and 7 are reserved for uplink transmission.

In the case of a 10-ms switch-point periodicity, UpPTS and subframe 2 are reserved for uplink transmission and subframes 7 to 9 are reserved for downlink transmission.

Subframe 0 and 5 are always for the DL. The subframe following the special SF is always for the UL. The DwPTS field carries synchronization and user data as well as the downlink control channel for transmitting scheduling and control information. The UpPTS field is used for transmitting the PRACH and the Sounding Reference Signal (SRS).