MIMO--Multiple Input Multiple Output

In the development of 3G and 4G networks increasing the data rates of wireless networks is one of the top priorities.

MIMO refers to the use of multiple antennas on both the transmitter or receiver to improve throughput and performance . It uses the phenomenon known as multipath (a RF signal bounces off walls and other structures and hits the antenna at different times) and organizes the mulitple occurences of the signal to achieve better performance.

In this MIMO primer the author describes a basic MIMO system which uses two, sometimes three antennas, to receive an RF signal. By measuring multiple signals over time, the signal processing in the system can reduce noise or interference from the multipath fading effect. It is used as part of the 802.11n specification to improve bandwidth.

Areas of future research into MIMO related to multi-user environments with multiple access schemes. Also, a great deal of research has yet to be implemented in hardware. For a further review, please check out this article.


Best regards,

Hall T.

Radio Technologies -- OFDM

OFDM stands for Orthogonal Frequency Division Multiplexing It is a digital modulation method that works well in challenging environments such as noisy or highly attenuated communication mediums. It translates multiple signals with each signal occupying its own frequency. A tutorial can be found here.

The technology has been around for over 40 years and patented in 1970 but found popularity in the early 2000s when bandwidth requirements for wireless demanded higher throughputs at a lower cost. In this article the author outlines a short history of OFDM and talks about why it was adopted. OFDM uses narrowband carriers with each signal transmitted in parallel but at a different frequency. By placing these signals called sub-carrieres closely but not overlapping. The term orthogonal refers to the mathematical relationship of the signals, and not the spatial. OFDM uses a guard interval to protect against multipath signal interference.

There are challenges with OFDM including a high average peak-to-average ratio. In this article the author indicates OFDM requires an outsized power amplifier and that power efficiency will be compromised and that heat dissipation will be an issue for the RF designer. OFDM also requires high linearity to avoid inter-modulation distortion and third order intermodulation products can pile up on the carrier. Non-linearity also shows up in adjacent channel interference. Also phase noise is an issue that must be controlled given the closely spaced carrier signals. These challenges impact the data rates one can achieve.

Best regards,

Hall T.

Finding Unused Spectrum in Your Area -- Show My White Space

The switch to digital television this past year has freed up spectrum for other purposes. To find out more about the white space in your area you can go to this link showing unused TV channels which is run by Spectrum Bridge.

Even Google founder Larry Page registers interest in the use of TV White Space. Google wants to organize the world’s information but the world needs electronic access for them to achieve that.

In searching on my home address in Austin, Texas I found 12 channels between channels 2 and 51 that were available. The application does a nice job in highlighting channels that are currently being used and may be used by the local TV stations. Personal portable devices cannot use channels 2 through 20.

Wireless microphones appear to be a major concern due to the interference they can cause. There are a number of FCC papers and positions on the use of wireless microphones.

There are several applications in the consumer space for the unused channel s including WiFi for the home, in home media distribution, home security, and cordless telephones. All of these applications point to potential uses of software defined radio.

Best regards,
Hall T.

Coopers Law and Shannon’s Theorum – Revisited

I had the opportunity to meet with Bill Bard of the University of Texas engineering department last week. Bill teaches a software defined radio course among other topics. During our discussion he posed the question – will we see more or fewer wireless standards in the coming years.

I made the prediction that we would continue to see new wireless standards emerge but that they would become less over time. The reason is that just as the web created the “long-tail” effect (the rise of many niches over the few hits) so software-defined radio would also create a “long-tail” effect but for wireless.

I’ve blogged before on Coopers Law which states that every 30 months the amount of information that can be transmitted over a given amount of radio spectrum doubles. This has held true since Marconi discovered the wireless signal in the late 1800s.

This is not to be confused with Shannon’s theorem which states in a mathematical formula the maximum information that can be transmitted over a communication channel.

While Shannon’s theorem refers to bits per hertz which has an upper limit, Coopers law takes into account frequency usage, spectrum allocation, and other variables related to wireless communication.

For those who don’t know the “long-tail” concept it was coined by Chris Anderson of Wired magazine who noticed that the web enabled niche players and gave them equal prominence as hit players. This applied to books, movies, etc. The internet distribution treated all equally unlike mass media (TV broadcasting for example) which favored hits over niche players.

Software defined radio levels the wireless field just as the internet leveled the media field. One no longer needs huge critical mass to create a standard and then adoption. Instead, software defined radio lets the user create their own wireless use case and with the power of standard PCs behind it, they can develop their own wireless protocols.

Changing the spectrum allocations is quite daunting given the government bureaucracy. But with some spectrum being used less than 0.3% of the time, pressure will mount to make use of that spectrum in some fashion. Software defined radio will provide a technical solution.

Best regards,
Hall T.