Downlink Slot Structure | Radio Interface Basics

This section introduces the LTE DL structure with respect to time. In the time domain, the DL is divided into slots and frames. A radio frame is the largest unit and lasts 10 ms. As both duplex modes, FDD and TDD, use similar timings, this makes a UE starting to synchronize to a LTE cell unaware of the duplex method used. Thus, TDD and FDD both introduce radio frames of 10 ms timing.

Both duplex methods define different types of radio frames:

• Frame type 1 is the frame type used with FDD.
• Frame type 2 is the frame type used with TDD.

One radio frame is split into 10 subframes of 1 ms duration. A subframe is also the most important unit of scheduling and for physical control channel durations the PDCCH is found in the first OFDM symbols of each subframe. Furthermore, subframes are divided into two slots of 0.5 ms. The smallest unit is an OFDM symbol. Depending on the CP length (normal or extended) used, the length will be seven or six OFDM symbols transmitted within one slot, resulting in 14 or 12 OFDM symbols within one subframe, respectively. Figure 1 shows one radio frame of type 1 (FDD) with 20 slots and 10 subframes. The length of one radio frame is defined as Tf = 307 200 × T_s whereT_s is the sampling period. Therefore, the base sampling frequency of 30.72 MHz (T_s = 1/30.72 MHz) is used.

 
Figure 1: Frame structure type 1 used with FDD TS36.211. Reproduced with permission from © 3GPP^™

Figure 2 shows a complete radio frame of type 1 as used with FDD. It illustrates all 10 subframes with areas for the PDSCH. The first OFDM symbols are allocated by the PDCCH. In the area of the PDCCH, additional physical channels for control information are embedded. These channels are the Physical HARQ Indicator Channel (PHICH) and the Physical Control Format Indicator Channel (PCFICH).

 
Figure 2: Downlink FDD radio frame (normal cyclic prefix) with PDCCH, PDSCH, PBCH, reference signals, and synchronization signals. Reproduced with permission from Nomor

The PCFICH indicates the number of OFDM symbols allocated for the complete PDCCH in its current subframe. Thus, the PDCCH is transmitted on a variable number of OFDM symbols depending on how much control or scheduling information has to be transmitted, and the resulting number of OFDM symbols which are used for the PDSCH is variable as well. The PDCCH allocates between 1 and 3 OFDM symbols and the PDSCH between 13 and 11 for normal CP and between 11 and 9 OFDM symbols for extended CP, respectively.

In the middle (around the DC subcarrier) of the bandwidth are six RBs used for some common signals and channels in some subframes. These locations are used for initial cell search and cell synchronization. This central location enables a bandwidth-independent cell and frame synchronization as well as initial cell access. Two step hierarchical synchronization signals are defined and located in the first and sixth subframes. The Primary Synchronization Signal (PSS) is transmitted on the seventh OFDM symbol and the Secondary Synchronization Signal (SSS) is transmitted on the sixth OFDM symbol of subframes described above. PSS and SSS can be seen in Figure 2 shows the bandwidth of the synchronization signals. Only 62 subcarriers of the 72 provided by six allocated RBs are used for synchronization signals. 

Resources for the PBCH are allocated apart from the SSS. The PBCH is transmitted in each first subframe of all radio frames. Other than the synchronization signals, the PBCH uses all 72 subcarriers of the six central RBs 

Reference signals are needed by the channel estimation process in order to correct the wireless channel distortion in the signal at the receiver. Four sets of reference signals are specified for each transmit antenna as LTE defines multi-antenna transmissions (DL MIMO). The receiver needs to know the propagation conditions of each transmit antenna for using the complete MIMO gain. Therefore, a defined signal (reference signal) is transmitted from each individual antenna completely independently and all other transmit antennas do not transmit any signals on those specific frequencies and time resources (REs). Figure 3 illustrates the frequency and time RE used for transmitting reference signals depending on the number of antennas configured.

 
Figure 3: Mapping of downlink reference signals (normal cyclic prefix) (TS36.211). Reproduced with permission from © 3GPP^™

Frame type 2 is used with TDD. The general slot structure of a TDD frame is similar to a FDD frame, since a mobile is not aware of the duplex method before synchronizing to the cell. A radio frame of type 2 has a duration of 10 ms as its FDD equivalent. The TDD radio frame is also divided into 10 transmission time intervals of 1 ms duration called subframes. Furthermore, TDD subframes are split as well into two slots of 0.5 ms period.

TDD systems switch on one frequency between DL and UL transmission. Therefore, TDD needs defined switch points and guard intervals between UL and DL transmission.

LTE defines two basic switch point interval durations of 5 and 10 ms. A switch point is a designated subframe divided into three zones which are already known from basic UMTS TDD: Downlink Pilot Time Slot (DwPTS), GP, and Uplink Pilot Time Slot (UpPTS). Other subframes are used for either UL or DL transmission. Operators can decide from UL–DL configurations which subframes are used as DL and UL depending on the UL and DL traffic mixture of a network. Seven different UL and DL structures are defined. The possible UL–DL configurations with the switch points and the subframe used for UL and DL transmission are given in Table 1.

Table 1: Uplink–downlink configurations for TDD. Reproduced with permission from © 3GPP^™ 

Uplink-downlink configuration	Downlink-uplink switch-point periodicity (ms)
		Subframe number
		0	1	2	3	4	5	6	7	8	9
0	5	D	S	U	U	U	D	S	U	U	U
1	5	D	S	U	U	D	D	S	U	U	D
2	5	D	S	U	D	D	D	S	U	D	D
3	10	D	S	U	U	U	D	D	D	D	D
4	10	D	S	U	U	D	D	D	D	D	D
5	10	D	S	U	D	D	D	D	D	D	D
6	5	D	S	U	U	U	D	S	U	U	D
S = Swithching point.

Figure 4 shows a radio frame with frame structure type 2 used with TDD. This is an example with a 5 ms switch-point periodicity.

 
Figure 4: Frame structure type 2 used with TDD (for 5 ms switch-point periodicity) (TS36.211).