# Narrowband Internet of Things (NB-IoT):
## 3 Narrowband-IoT Protocol Stack
### Media Access Control Layer
#### 3.2.2 Link Adaptation  
Like in LTE, NB-IoT link adaptation involves adaptive modulation and coding schemes as well as adaptive power allocation. However, the modulation schemes are limited to QPSK to enable low complexity and hence reduce the overall power consumption. To extend the coverage and increase the link reliability, a repetition number of up to 128 times is introduced. In the literature, it is seen that NB-IoT link adaptation has several issues; potential solutions are also proposed, as summarized below.  
In [31], the author formulated the scheduling issue such that the resource assignment must be in a specific format taking into an account reserved signaling resources and capabilities of the NB-IoT UE. They proposed a solution that incorporates two parameters which are (i) offset index selection (k0) and (ii) UE specific and common search space configuration. The offset index selection was chosen because with the limited k0 and varying size of payloads, it is critical to adapt the scheduling process for high resource use to accommodate more devices at the same time. Additionally, UE specific and common search space configuration were chosen because it decides the timing of NPDCCH and NPDSCH for different UEs, hence it can consequently improve the overall scheduling efficiency.  
In [73], the authors presented a basic NB-IoT scheduler for NB-IoT system and analyzed the enhancements on average delay, optimal resource usage, and processing time. The proposed algorithm demonstrated that shorter NPDCCH period selection may reduce the UE average delay and optimize the overall system resource usage. Also, the model shows that the scheduling delay (k0) should be determined before the allocation of subcarriers. However, the model does not elaborate the type of configuration used since the choice of configuration such as single tone or multi-tones have a direct impact on periodicity and transmission delay and hence can directly impact the system performance.  
In [74], the authors analyzed NB-IoT repetition number and bandwidth allocation and proposed analytic expressions based on SNR, bandwidth, and energy per bit that can be derived from Shannon theorem in order to characterize the impact of the repetition number as well as bandwidth allocation to different UEs. Additionally, their work proposed an algorithm for link adaptation. The algorithm exploits resource unit number, repetition as well as bandwidth. Their results show that reducing bandwidth and performing repetitions could enhance the coverage. However, the work did not consider the actual impact of channel parameters as well as NB-IoT UE impairments such as CFO which may lead to transmission errors.  
In [75], the authors proposed a new NB-IoT link adaptation scheme with the consideration of the repetition factor. They claim that their proposed two-dimensional scheme is composed of Inner Loop Link Adaptation that copes with BLER by periodically adjusting the repetition number and outer loop link adaptation which coordinates the MCS and repetition number. This is because 20 dB coverage enhancement beyond LTE can be achieved by the repetition of transmitted data. So, in this work, they proposed an algorithm that dynamically chooses MCS and repetition number based on estimated real-time channel state information (CSI). However, their algorithm does not elaborate on the different NB-IoT power classes and to which range their respective coverage could be enhanced.