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802.11 n and MIMO

802.11 n is a major improvement over previous models of 802.11 a/b/g. The new features are :-
1. Support for MIMO
MIMO uses multiple antennas for Tx and Rx using spatial multiplexing. Spatial multiplexing is multiplexing the same frequency channel using multiple antennas, each sending a part of the information to a receiver which also has multiple antennas listening on the incoming signal. Each antenna receives part of the signal and spatial signatures are taken into account to reconstruct the total signal using on-board DSP equipment. This is a major advance, in the sense that we have higher throughput without using TDM or FDM, which were the traditional ways of increasing throughput.

MIMO, of course, requires multiplicity of antennas at both sender and receiver. It should be noted that the Wireless Access Point (WAP) deals with one client at a time in a given frequency band ( there is dual band : 2.4 Ghz and 5 ghz support, which hence can deal with two clients simultaneously, but still having the restriction of having a single client per frequency band)

We tested MIMO throughput using a 802.11n capable WAP and Client to obtain the following interesting results:-

Experimental Setup

WAP : RouterStation Pro ( 3 Tx antennas, 3 Rx antennas)
Client 1 : Macbook Pro Airport Extreme ( 2x2 MIMO)
Client 2: Intel 6200 WLAN (3x3 MIMO)

WAP is running licensed version of DD-WRT v24 and clients are running UNIX based Operating systems. We use iperf to measure TCP throughput between a iperf server initiated at the WAP and iperf client initiated at the client. It should be noted that, iperf measures TCP throughput, which will be lesser than the bandwidth or actual data transmitted which includes preambles, headers, etc.

Also, it should be noted that , we have obstructions in the path between the WAP and clients resulting in reduced SNR. But, over the set of experiments, these factors stay invariant, so we can do a reasonable comparison

The WAP supports configuration of Tx,Rx antennas used for MIMO.

Macbook Pro and WAP experiments

A1. Wireless N ( 5 Ghz channel) initiated between Macbook pro and WAP. Antenna configuration Tx - 1 , Rx - 1 at WAP and Tx -2 , Rx -2 at laptop.

Throughput obtained is 28.1 mbps

B1. Wireless N ( 5 Ghz channel) initiated between Macbook pro and WAP. Antenna configuration Tx - 1+2+3 , Rx - 1+2+3 at WAP and Tx -2 , Rx -2 at laptop.

Throughput obtained is 38.6 mbps

C1. Wireless N ( 5 Ghz channel) initiated between Macbook pro and WAP. Antenna configuration Tx - 1+2, Rx - 1+2 at WAP and Tx -2 , Rx -2 at laptop.

Throughput obtained is 64 mbps

D1. experiment B with external antennas at WAP

Throughput obtained is 45 mbps

Intel 6200 WLAN and WAP experiments

Note : high constant noise in path

A2. Wireless N ( 5 Ghz channel) initiated between Intel WLAN 6200 and WAP. Antenna configuration Tx - 1+2, Rx - 1+2 at WAP and Tx -3 , Rx -3 at WLAN 6200.

Throughput obtained is 11.5 mbps

B2. Wireless N ( 5 Ghz channel) initiated between Intel WLAN 6200 and WAP. Antenna configuration Tx - 1+2+3, Rx - 1+2+3 at WAP and Tx -3 , Rx -3 at WLAN 6200.

Throughput obtained is 23.6 mbps

Conclusions:-

1. Throughput is maximized when the Tx, Rx count is the same at receiver and sender nodes.

2. (obvious) External Antenna increases throughput for same experiment