Role of preamble in 802.11a packet

Conventional Packet Oriented Networks Like IEEE 802.11a Precede Each Data Transmission with a PHY Layer Preamble and Header



The
purposes of the PHY preamble are:
• Signal detection
• Antenna diversity selection
• Setting AGC
• Frequency offset estimation
• Timing synchronization
• Channel estimation

Note that the address of the intended recipient is not in the PHY preamble.It is actually in the packet data and is interpreted at the MAC layer.

Each packet is addressed to a single recipient.

However, the randomized backoff period of the CSMA multiplexing scheme is idle time and therefore represents an inefficiency. The PHY preamble is also network overhead and further reduces efficiency, particularly for shorter packets.

The typical real-world efficiency of an 802.11a system is approximately 50 percent. In other words, for a network with a nominal data rate of 54 Mbps, the typical throughput is about 25 – 30 Mbps. Some of the inefficiencies can be mitigated by abandoning the CSMA multiplexing scheme and adopting a scheduled approach to packet transmission. Indeed, subsequent versions of the 802.11 protocol include this feature. Inefficiencies due to dedicated ACK packets can also be reduced by acknowledging packets in groups rather than individually.

In spite of potential improvements, it remains difficult to drive packet-oriented network efficiency much beyond 65 to 70 percent.

Removing any row containing NaN in MATLAB

X(any(isnan(X),2),:) = [];

>> X = [ 1 2 3; 3 4 5; NaN 3 NaN]

X =

     1     2     3
     3     4     5
   NaN     3   NaN

>> X(any(isnan(X),2),:) = []

X =

     1     2     3
     3     4     5

One can make a function for it also

function X = exciseRows(X)
X(any(isnan(X),2),:) = [];