4G/LTE - BL/CE
BL/CE(LTE-M1) operates in 1.4 Mhz frequency block called Narrowband. The location of the Narrowband may vary in each subframe. It means that UE should need to tune to a difference frequency at every subframe. As you may guess, every freuqency retuning would take a certain period of time. It implies that we would need a certain period of guard period when transitions from one subframe to next subframe. Guard Period is a period during which no transmission and no reception occurs.
This guard period varies depending on whether it is downlink or uplink, and varies with different situations within downlink or uplink.
Uplink Guard Period
Type of uplink guard periods varies depending on the characteristics of data carried by a subframe and next subframe as described below (Following description is based on 36.211-5.2.5).
Case 1 : PUSCH in First Subframe and PUSCH in Second Subframe
Case 2 : PUCCH in First Subframe and PUCCH in Second Subframe
Case 3 : Shortened PUCCH in First Subframe and PUSCH in Second Subframe
Case 4 : Normal PUCCH in First Subframe and PUSCH in Second Subframe
Case 5 : Normal PUSCH in First Subframe and PUCCH in Second Subframe
Downlink Guard Period
Downlink guard period is required as in the following condition (refer to 36.211-6.2.8).
The guard period is defined as follows
 3GPP TS 36.211 V13.2.0 (2016-06)