## 3 Communication in the 60 Ghz Band
### Link Adaptation  
In wireless communication systems, the quality of a wireless signal received by nodes depends on a number of factors: the distance between the nodes, path loss exponent, log-normal shadowing, short term (Rayleigh) fading and noise. In order to improve system capacity and peak data rate, the signal transmitted to and by a particular node is modified to account for the signal quality variation through a process commonly referred to as link adaptation. This is also known as adaptive modulation and coding (AMC) techniques [24].  
In communication systems, the use of AMC techniques allows the system to achieve higher spectral efficiency by dynamically changing the modulation and coding schemes based on the channel statistics so as to improve overall spectral efficiency. In other words, it is utilized to set the modulation and coding in order to reflect the features of the wireless link and to maximize throughput. Moreover, AMC has been widely used to match transmission parameters to time-varying channel conditions in order to enhance the spectral efficiency while adhering to a target error performance over wireless channels [25; 26].  
A number of research works have been conducted on link adaptation, and new link adaptation schemes have been proposed. [PERSON] et al. [27] introduced a Receiver-Based Auto-Rate (RBAR) protocol based on the Request-To-Send (RTS) and Clear-To-Send (CTS) mechanism by adjusting the IEEE 802.11 standard. The basic idea of RBAR is that the receiver estimates the wireless channel quality using a sample of the instantaneously received signal strength at the end of the RTS reception, then selects the appropriate  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**MCS Index** & **Modulation** & **Code Rate** & **PHY Rate (Mbps)** & **Notes** \\ \hline
1 & pi/2-BPSK & 1/2 & 385 & Single Carrier Phy 2 repeated frames \\ \hline
2 & pi/2-BPSK & 1/2 & 770 & Single Carrier Phy \\ \hline
3 & pi/2-BPSK & 5/8 & 962.5 & Single Carrier Phy \\ \hline
4 & pi/2-BPSK & 3/4 & 1155 & Single Carrier Phy \\ \hline
5 & pi/2-BPSK & 13/16 & 1251.2 & Single Carrier Phy \\ \hline
6 & pi/2-QPSK & 1/2 & 1540 & Single Carrier Phy \\ \hline
7 & pi/2-QPSK & 5/8 & 1925 & Single Carrier Phy \\ \hline
8 & pi/2-QPSK & 3/4 & 2310 & Single Carrier Phy \\ \hline
9 & pi/2-QPSK & 13/16 & 2502.5 & Single Carrier Phy \\ \hline
10 & pi/2-16-QAM & 1/2 & 3080 & Single Carrier Phy \\ \hline
11 & pi/2-16-QAM & 5/8 & 3850 & Single Carrier Phy \\ \hline
12 & pi/2-16-QAM & 3/4 & 4620 & Single Carrier Phy \\ \hline \hline \end{tabular}
\end{table}
Table 1: SC_PHY Modulation and Coding Schemes.  
transmission rate based on this estimate, which feeds back to the transmitter, and finally the transmitter responds to the receipt of the CTS by transmitting the data packet at the rate chosen by the receiver [28]. [PERSON] et al. [29] presented the Auto Rate Fall-back (ARF) protocol for IEEE 802.11, which is used in Lucent's WaveLAN devices. With ARF, the sender selects the best rate based on information retrieved from previous data packet transmissions and incrementally increases or decreases the rate after a number of consecutive successes or losses.