# An Overview of Key Technologies in Physical Layer Security
## 4 Challenges and Promising PLS Solutions  
### Challenges and Limitations in PLS  
The individual PLS techniques discussed above have their own different pros and cons. The major challenge is the ability to implement an optimal secure transmit precoding algorithm that can maximize the achievable secrecy rates without any counter cost. We outline some of the notable challenges in detail, and show how they are addressed, in the subsequent section.  
One problem prevalent in PLS techniques, specifically beamforming, is the leakage of transmitted signals into the eavesdropper's subspace. In beamforming, the transmitted signal is steered in the desired direction to the legitimate receiver. The transmit power is highly concentrated in the main lobe beam, but some of the power is lost in the minor side lobes. This leakage makes it possible for eavesdroppers who are in the vicinity to decode the transmitted signal since the finite number of transmit antennas could only provide a limited amount of spatial directivity [52]. It has also been shown that transmit beamforming focuses only on enhancing the quality of the main channel [89]. Beamforming does not take into consideration the possibility that the eavesdropper can have a favorable channel when compared to the main channel. Therefore, it can be concluded that even though the design of an optimal beamforming vector for the intended receiver is fairly easy, it is cumbersome and computationally expensive to come up with a perfect balance between getting rid of signal leakage and obtaining the optimal signal power.  
It has been shown in Section 3.3 that AN precoding offers provable security in the physical layer. However, this comes at an additional cost of extra energy requirement. The approach relies on the generation of AN signals. A fraction of the power used for transmission of information signal power is used to generate the AN signal. This consumes the transmission power which could have been used to improve the channel capacity and receiver's SNR. Therefore, there is a trade-off between capacity and secrecy rate capacity by means of the transmit power available. On the other hand, there is also a higher power consumption in both convex optimization and zero-forcing, owing to the computational requirements.  
Figure 16: Bit error rate (BER) and secrecy rate performance metrics simulations for different precoding schemes.  
PLS precoding schemes make an assumption of the knowledge or availability of the unintended user's CSI. This is the fundamental limitation of such PLS techniques. In practice it is very difficult for the transmitter to obtain the CSI of the eavesdropper. This is due to the fact that the eavesdropper does not naturally cooperate with the transmitter to send CSI feedback. Therefore, this assumption is entirely valid for theoretical systems. Most PLS secure precoding techniques assume that an eavesdropper has limited resources. To be more specific, the eavesdropper is commonly assumed to have a smaller number of antennas when compared to those of the intended receiver. Even though it has been proposed that secrecy against an eavesdropper with more antennas than the transmitter is possible in [90], such a solution requires additional power assumptions. Hence, secrecy against resourceful eavesdroppers remains a major challenge in PLS precoding.