Commercial radio technology has reached an inflection point similar to the transition from analogue to digital which created a range of new digital technology. Now we are moving from single carrier technologies, where we transmit one digital symbol at a time, to the point where we are potentially transmitting hundreds of symbols simultaneously (OFDM) using multiple RF carriers.
The change has been driven by customer demand for more mobile services and the decreasing cost of the digital signal processing technology required to deploy high-bandwidth broadband wireless systems.
Single-input single-output (SISO)
A typical SISO radio has one transmitter and one receiver and information is sent over a single data channel. This configuration is used in almost all radio products today. While there might be an extra antenna for spatial diversity, there is a single up-converter and a single down-converter, a single demodulator/modulator in the radio, and a single data stream in the higher levels of the product's communications stack.
Multiple-input multiple-output (MIMO)
While MIMO represents a big change in radio architecture it is quite simple in principle. If you have one transmitter, you can transmit data from point A to point B. If you have four transmitters, the likelihood of the data getting there or the quality of the transmission would be increased, but the transmission would take up four times as much bandwidth.
MIMO takes four independent OFDM carriers, and puts them on top of each other, so really you have four separate transmissions, all sharing the same frequency. You can either carry more information in the same bandwidth as a single carrier or significantly improve the transmissions' signal to noise ratio. If you compare the spectral efficiency of a GSM signal of around 0.5bit/s/Hz to that of a WLAN 802.11n MIMO signal with an efficiency that is greater than 6bit/s/Hz you can see a significant improvement.
The challenge at the receivers is to be able to get back to four independent radio signals. While the principle is not new, the cost of the technology has decreased to the point that it is now commercially viable.
MIMO radio configuration
Figure 1 - MIMO radio configuration
Figure 1 (above) shows some typical MIMO configurations. A 2x2 system contains two transmitters and two receivers, a 4x4 has four transmitters and four receivers. Many commercial WLAN devices today employ a 3x2 configuration of three transmitters and two receivers.
While a great deal of signal processing is required the concept is simple. Taking the first example in Figure 2, you can see we need to resolve the received signal for the transmission of TX1 and TX2, but you can see RX1 is also receiving TX2 and the same is happening for RX2. To overcome this effect a known signal is transmitted in the form of a header. This allows the receiver to build a model of the channel (the transmission environment), which will be subtracted out from the data transmission giving us the original transmitted symbols of TX1 and TX2 plus some noise.
Hardware
Transmission of multiple signals requires accurate synchronisation of multiple channels in phase and sampling alignment.
Figure 2 - Three MIMO measurements on a two-channel system
Figure 2 shows three MIMO measurements on a two-channel system. The upper measurement is EVM across the OFDM sub-carriers on each channel shown by the yellow and red plots.
Underneath can be seen the channel frequency response over 60 sub-carriers. Finally, we see the constellation diagram plotting the resolved constellations of both TX1 and TX2in different colours.
The long term evolution of wireless
The long term evolution (LTE) of cellular devices, also known as ultra mobile broadband (UMB), is the migration from 3G technology to 4G technology. The current proposal is that the fourth generation of cellular type technology will be based on OFDM technology and MIMO radio configurations.
When choosing test equipment for testing today's radio standards such as GSM or W-CDMA, it is important to consider the evolution of wireless technology and have a clear upgrade path from SISO technology to MIMO technology, and ensure that your equipment purchases are forward compatible.
To be a winner in MIMO requires driving the cost per channel down to a point where it is commercially viable. The bandwidths are very large in some of the systems, so it is necessary get to a cost and performance point where it is possible not only to successfully transmit these signals but to be able to sell devices for less than $100.
This is the challenge for the industry. It is a significant change right now there is cost pressure just on the regular cell phone with one transmitter and one receiver if you go up to 4x4, a lot of work must be done in the industry to make this commercially viable. Some of it has already been proven with the wireless LAN work that's been done and currently with the second wave of WiMAX technology, but it is still a challenge and it is a big change to radio system design as we know it today.
Mark Elo is marketing director of RF products at Keithley Instruments