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Frames, Slots and Symbols of LTE

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Frames, Slots and Symbols of LTE

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In LTE, the time axis is divided into entities. These time entities have the following hierarchy, as illustrated in Figure 10.

- The fundamental time unit of LTE transmission is a radio frame, which has a duration of 10 ms.
- Each radio frame is divided into 10 subframes (each being 1ms long). Subframes are the fundamental time unit for most LTE processing, like scheduling.
- Each subframe consists of two slots (illustrated in Figure 11), which are each 0.5 ms long.
- Each slot consists of 7 (or 6) symbols.
Duration of the different units is often given in terms of the sampling time $T_s = 1/30,720,000 s$
which is the reference time for defining any slot in LTE and also called as
*book keeping time*.

Now consider the details of a symbol. Since the modulation format is OFDM (regular OFDM for the downlink, and DFT- precoded OFDM in the uplink), multiple subcarriers are present.

- The regular spacing between the subcarriers is $f = 15 KHz $. Therefore an OFDM symbol duration (without cyclic prefix) is $1/15K = 67 \mu s = 2048 T_s$.
- One subcarrier, for the duration of 1 OFDM symbol, is called a
*resource element*. - Number of OFDM symbols into one slot = $15630/ 2048 = 7.6 $. That is 6 or 7 OFDM
symbols can fit in to one time slot, depending on the duration of the cyclic prefix. LB
represents the
*load block*or*OFDM symbol*. - In the “regular” case, the duration of the cyclic prefix is 160 $T_s$ in the first OFDM symbol and 144 $T_s$ for the subsequent symbols. Therefore, $\text {one time slot} = 160T_s +(6 \times 144T_s)+(7 \times 2048)= 15360 T_s = 0.5ms$
- A long or extended cyclic prefix is $512 T_s$, so that only a total of 6 OFDM symbols fit into one slot. Such a long cyclic prefix is used in environments with large delay spread and/or for Multicast/Broadcast over Single Frequency Network (MB-SFN).

In this chapter “normal-length” cyclic prefix is considered unless otherwise stated.
Time/frequency resources are assigned to different users as integer multiples of a *Resource
Block* (RB).

- Specifically, an RB is 12 subcarriers ($12 \times15kHz=180 kHz$) over the duration of one slot.
- For the uplink, only contiguous RBs can be assigned to one MS.
- Furthermore, the number of RBs has to be decomposable into factors of 2, 3, and 5; this is done to ensure an efficient implementation. With that prescription, any of the necessary DFTs can be composed of radix-2, radix-3, and radix-5 butterfly structures.

Allocation of resource blocks to a MS for downlink is illustrated in Figure 12.

- In the TDD case, subframes can be assigned flexibly to uplink and downlink, with the exception of subframes 0 and 5, which are always used for the downlink, and subframe 2, which is always used for the uplink.
- For every transition from downlink to uplink, a guard interval is kept to avoid collisions between the packets on the air. Therefore there are subframes that contain three distinct parts: a Downlink Pilot Time Slot (DwPTS), Uplink Pilot Time Slot (UpPTS), and a guard interval between them.
- A guard interval is not necessary for a transition from uplink to downlink.

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