# Physical Layer Latency Management Mechanisms: A Study for Millimeter-Wave Wi-Fi
## 5 Results
### Including Physical Layer Latency and Channel Noise  
The next step in the analysis includes RX DBB data processing, further increasing latency in addition to the finite transmission rate. The decoder may execute up to 10 iterations and conclude codeword processing at any time if the early exit criterion is met. Only decision threshold demapping is used.  
Figure 9(a) builds on the ideal case results by inducing additional delays within the PHY and studying the quality of the data, received over the AWGN channel. The observed BER values indicate that (a) while transmission at MCSs with higher indexes is less latent, it may provoke data loss and (b) below 3.5 dB \(\frac{E_{k}}{N_{0}}\), the received data is highly erroneous. The BER results and the incurred number of LDPC decoder iterations together demonstrate that in the presence of fewer errors, the decoder is more likely to conduct fewer iterations. This is the case of the early exit stop criterion, mentioned in Section 4.1, terminates execution when it does not detect any more errors in the received codeword. The results are lower data propagation delays and a higher decoder throughput, in accordance with Equations (4) and (5). The manifestation on MPDU latency is more pronounced at MCSs with higher indexes, where the decoder persists at conducting a high amount of iterations till relatively high \(\frac{E_{k}}{N_{0}}\) values. An example is the rightmost cluster consisting of three curves in Figure 9(b). The three are associated with 64 QAM modulation. The middle cluster is associated with 16 QAM, while the leftmost contains both QPSK and BPSK curves. Figure 9(c) shows how decoder time delays are reflected in MPDU latency. Excluding MCS at index 9, illustrated in olive green, the MCSs at the nine highest indexes can all become a bottleneck. The latency curve clusters, affected by the bottleneck, are in accordance with those in Figure 9(b). The largest difference is that BPSK modulated data is not affected and that, when using QPSK modulation, only the curves corresponding to MCS indexes 7 and 8 show a visible increase in latency. This is due to a combination of already relatively low MPDU latency and a high delay caused by the decoder at lower code rates. All latency values in Figure 9(c) stabilize towards 15 dB, where the average amount of iterations for all MCSs in Figure 9(b) falls to 1.  
In time-critical applications where latency is the main optimization metric, the bottleneck decoder makes transmission at MCS 12.5 a viable option only beyond 11 dB \(\frac{E_{k}}{N_{0}}\), after the intersection with MCS 12.4. A similar pattern repeats itself for all 9 MCSs that suffer from the decoder becoming a bottleneck during the reception of 262 kB long payloads. Transmission at other MCSs also suffers from the decoder inducing a data propagation delay. However, it never becomes a bottleneck, as the major part of latency originates from the finite transmission rate. This is reflected in the seemingly horizontal lines associated with MCS indexes 2-6 and 9. Note should be taken, that the decoder's data propagation delay during short sequence reception may become significant in comparison to the total packet latency. Furthermore, the number of decoder iterations is in practice limited to about 5 [44; 47] to help avoid the decoder becoming a bottleneck.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline
**A-PPDU** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** & **12.1** & **12.3** & **12.4** & **12.5** \\ \hline
0 & 2.73 & 2.18 & 1.82 & 1.68 & 1.36 & 1.09 & 0.91 & 0.84 & 0.68 & 0.55 & 0.46 & 0.42 & 0.37 & 0.31 & 0.28 \\
1 & 5.45 & 4.36 & 3.63 & 3.36 & 2.73 & 2.18 & 1.82 & 1.68 & 1.37 & 1.09 & 0.91 & 0.84 & 0.73 & 0.61 & 0.56 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Total MPDU latency without aggregation (0) and when appending a single A-PPDU (1), per MCS index. All values are in milliseconds.