MTC Features

More constrains are added on the MTC UE to reduce its cost and complexity. The features of the physical downlink control channel for MTC are summarized as:

• The design of the physical downlink control channel for MTC is based on EPDCCH. • The introduction of new DCI messages to R13 for low complexity UEs.

• The use of a narrowband (within 6 PRBs) control channel. • Its usage for other UEs in enhanced coverage.

• The demodulation of the control channel shall be based on CRS and/or DMRS.

2.8.1.1 Modes Of Operation

In Rel-13, the CAT-M UE is referred to as Bandwidth-reduced Low-complexity and Coverage Extension user equipments (BL/CE UE). BL/CE UEs can operate in two different modes of operation: CE Mode A and CE Mode B, each depends on the coverage extension level seen by the UE. The operation mode is determined by the eNodeB and sent to the UE with radio resource control message. The UE selects one of 4 different CE levels, namely level 1,2,3 and 4, based on the reference signal received power. Each level is distinguished by a different random access channel configuration and the time and frequency resources used by the UE for the random access transmissions. For CE Mode A, associated with CE level 1 and 2, a UE would have an equivalent coverage as that of UE CAT-0. This mode requires small number of repetition of different channels, or in the best case no repetitions at all.

Also, the power control mechanism used for Mode A is the same as the legacy users. For the repeated transmissions, the used transmit power remains unchanged during the repetition period. For CE Mode B, the UE is in CE level 3 or 4, and is covered with extended coverage of 15 dB with reference to CAT-0 (equivalent to 155.7 dB Maximum Coupling Loss (MCL) between the transmitter and the receiver). CE Mode B features a medium or large number of repetitions, and the transmission power of the uplink channels is set to the maximum value. Hence, no power control is needed, which makes the control information sent to this category has a unique format as well.

2.8.1.2 Narrowband Concept

The bandwidth choice of a certain base station is configured once and remains unchanged during operation, hance, it would be a good choice to use a bandwidth unit that can be a common divisor of the available bandwidth options in the legacy LTE. Then the choices become limited to 6 RBs or one RB. However, in order for the MTC devices to capture the signature of the LTE signal, it will have to receive the synchronization signals PSS and SSS. These signals occupy the central 6 RBs of the bandwidth of the base station, which makes it desirable for the MTC devices to take 6 RBs as the basic unit of bandwidth. Not only that, but also as mentioned before, the legacy PDCCH is mapped to the full occupied bandwidth, which makes decoding PDCCH is not possible in MTC applications, since the UE is limited to bandwidth of only 1.4 MHz. However, the enhanced version EPDCCH, uses one PRB pair as the basic resource unit, which makes it a good candidate to be used to control MTC UE. Furthermore, the use of only one PRB for the MTC control channels based on EPDCCH was reported to be insufficient, as it could provide the required converge enhancement (CE) [83]. It was concluded in [84] that, increasing the bandwidth of the basic EPDCCH to 6 PRBs in the 1.4 MHz bandwidth on the MTC UE along with repetition will be sufficient to have a good coverage of -14 dB. Furthermore, coverage enhancement can be achieved by employing EPDCCH that supports beamforming which increases coverage by directing the power of the base station towards the UE. For these reasons, it is agreed on to use the 6RBs (or one narrowband (NB)) as the basic bandwidth unit for MTC. A narrowband is defined as six non-overlapping consecutive physical resource blocks in the frequency domain. The total

NBn NBn+1 NBn+2 NBn-1 NBn-2 Slot # 0 Slot # 1 Slot # 10 Slot # 11 Slot # 18 Slot # 19 Sub-frame # 0 Sub-frame # 5 Sub-frame # 9 1 PRB 0.5ms 1 ms PSS Symbol SSS Symbol Freq. _Time

Figure 2.4: Frame structure of LTE-MTC systems for FDD with Normal CP type showing the synchronization signals. The concept of narrowband is highlighted.

number of downlink narrowbands in the downlink transmission bandwidth configured in the cell is given by NDL

N B = ⌈ NDL

RB

6 ⌋ and are numbered nN B = 0, ..., N DL

N B− 1 in order of increasing

physical resource-block number. The narrowband concept and numbering is highlighted in Fig. 2.4.

2.8.1.3 Frequency Hopping

With the small bandwidth offered to MTC UE and the use of single receiver chain, the fre- quency and spatial diversity are taken away. To retrieve some of the lost frequency diversity, the frequency hopping concept is added to the MTC LTE system. In other words, MTC transmissions will hop from one NB to another to make use of transmitting over different channels. Hence, providing frequency diversity. However, frequency hopping shall introduce new challenges due to the fact that the UE will have to re-tune its RF chain every time it hops. The re-tuning consumes some time and might affect the overall behaviour. Therefore a guard period is needed for re-tuning. Rel-13 specifies the re-tuning guard period as the time duration of two orthogonal frequency division multiplexing (OFDM) symbols. No transmis- sion or reception is done by the UE during the guard period. The time of the guard period varies depending on whether it is downlink or uplink.

2.8.1.4 Repetitions

As mentioned before, the limitations forced on the MTC LTE directly affects the downlink control channel, which in turn, has a direct impact on the performance of the downlink. For the downlink to have sufficient link budget to support the prospected coverage enhancement, an enhanced version of the distributed EPDCCH is reused as the base control signal for MTC. Thus, enhancement is obtained by transiting repeated copies of the same signal over time. As a direct result of the use of such repetition code, the link performance can be enhanced through time diversity and boost the control signal energy. On the other hand, the repetition has the effect of increased decoding time (more latency), which requires more wake-up time of the MTC device. The maximum number of repetitions for different physical channels and signals is given in Table 2.2.

Table 2.2: Maximum number of repetitions for different channels.

CAT-M Channel Mode A Repetitions Mode B Repetitions

PSS/SSS 1 1 PBCH 1 5 MPDCCH 16 256 PDSCH 32 2048 PUSCH 32 2048 PUCCH 8 32 PRACH 32 128