Archive for the ‘MIMO’ Tag

How Stuff Works – 802.11n and Spatial Multiplexing

This is the third post in my “How Stuff Works” series.  The first two posting discussed MIMO and channel bonding.  This post looks at another technical improvement that leads to greater speed in 802.11n networks – spatial multiplexing.

It is helpful to take a quick look at a classic 802.11 transmitter.

802.11 Classic Transmitter

802.11 Classic Transmitter

In this scenario, only one data stream is sent from the transmitter to the receiver (represented by the orange line).

With spatial multiplexing, multiple data streams are transmitted at the same time.  They are transmitted on the same channel, but by different antenna.  They are recombined at the receiver using MIMO signal processing.  This is represented in the diagram above with two spatial streams – an orange colored one and a navy blue colored one.

Spatial Multiplexing - Two Streams

Spatial Multiplexing - Two Streams

Spatial multiplexing doubles, triples, or quadruples the data rate depending on the number of transmit antennas.   Remember, you may hear three numbers when referring to 802.11n or MIMO networks – the first is the number of transmit antenna, the second is the number of receive antenna, and the third is the number of spatial streams.   For example, a 3×3x2 system has two spatial streams.

Gigabit Wireless Expected in 2009?!?

While we are on the topic of 802.11n and speed, I thought this was an applicable story that appeared on Network World’s website earlier in the week.

It talks about Quantenna Communications plans to release a 4×4 MIMO chip set in Q3 of 2009 that is capable of 1 Gbps wireless bandwidth (600 Mbps of throughput).

With 100+ Mbps throughput currently provided by 802.11n, most desktop applications have more than enough bandwidth.   Gigabit Wireless would certainly eliminate speed as one of the obstacles to replacing Ethernet with wireless.

How Stuff Works – 802.11n MIMO

If you have been following my blog, you know that I have a poll regarding the greatest challenges to 802.11n deployment in the enterprise.  (Here’s your chance to rock the vote!)

One challenge for replacing desktop Ethernet with wireless is speed.  Perhaps the widest publicized enhancement to 802.11n is that of MIMO (“my-moe”) antennas, which stands for “Multiple Input, Multiple Output”.  How does MIMO work?

To answer that question, let’s look at how a classic 802.11 wireless transmitter operates:

802.11 Classic Transmitter

802.11 Classic Transmitter

In this case, the signal is sent out of one antenna.  The signal is received by both antennas at the other end, but only one signal is processed and sent up to the MAC layer.   Antenna diversity helps in the fact that the best signal is the one that is processed, but remember that it is still a single antenna that processes the receives and processes the RF energy.

Let’s compare that to a MIMO antenna structure:

MIMO Signal Processing

MIMO Signal Processing

In this case, we have three transmit antennas and three receive antennas (often referred to as 3×3 MIMO).  The black, green, and red lines above each represent their own signal.   With MIMO all three signals are received and processed up the stack.   This significantly improves the receiver’s “ability to hear” and it represented in the graph above by the orange line.

You may hear different implementations of MIMO such as 2×3 and 3×3.  The first number is the number of transmit antennas and the second number is the number of receive antennas.   If you hear 3x3x2, the last number refers to the number of spatial streams, which I will discuss in another post.

802.11n Challenge – Speed

First, vote on the greatest deployment challenge for 802.11n in the Enterprise (if you haven’t done so already).

Let’s take a closer look at one of these challenges – speed.

There are many technical improvements in 802.11n that make it significantly faster than 802.11 a/g.   In no particular order, here are some of the improvements:

  • MIMO (Multiple In, Multiple Out) Antennas
  • Spatial Multiplexing
  • Channel Bonding
  • Short Guard Interval
  • Frame Aggregation
  • Block Acknowledgments

It is my intent to have a blog post “tutorial” on each of these improvements, so check back shortly!