by Dr. Jan Prochnow, Rohde & Schwarz
MIMO (multiple-input multiple-output) is an effective technique for boosting data rates. Consequently, it forms an integral part of modern mobile radio standards, such as 3GPP Release 7 and EUTRA/LTE, as well as wireless local (802.11n) and regional (WiMAX) radio networks. There are two approaches to MIMO: diversity and multiplexing. This article explains both concepts and shows how to perform MIMO receiver tests with the R&S SMU200A or the R&S AMU200A.
A signal traveling from a transmit to a receive antenna is subject to reflection, which causes multipath propagation of the signal. The propagation paths add up, in part, constructively (i.e., they amplify each other), and, in part, destructively (i.e., they cancel each other out). All of the paths between a transmit and a receive antenna taken together are referred to as a channel. If the above type of superposition is present, the channel is referred to as a faded channel. If receive antennas are closely spaced, the incoming signals are correlated, which means they have traveled through similar fading channels.
A receive diversity or SIMO (single-input multiple-output) system has one transmit and usually two receive antennas. If the fading channels are sufficiently different, the combination of several receive signals will yield a better signal-to-noise ratio. That’s because the receiver can compensate destructive superposition at one antenna by taking advantage of the signals from the other antenna.
A transmit diversity or MISO (multiple-input single-output) system usually contains two transmit antennas and one receive antenna. Identical data contents with different coding (space time coding) are sent via the two transmit antennas. If parts of one signal are lost due to destructive superposition, the data contents can be retrieved from the other signal. For this, the fading channels must be sufficiently different from each other.
Multiplexing systems transmit different data contents simultaneously on the same carrier frequency by using several antennas. The signals are received in parallel by the receive antennas, which results in an increase of the overall data rate (Fig. 1).
A challenging task for a multiplexing system is to distinguish between the different signals at the receiver. As with diversity systems, this can be done on the basis of the different transmission channels. By way of channel estimation, the characteristics of the individual fading channels are determined. Based on this information, the receiver can differentiate between the transmit antennas and pick up the two data contents simultaneously.
Diversity as well as multiplexing systems only work if the fading channels between the transmit and the receive antennas are sufficiently different. Since the antennas are closely spaced, similar fading channels will result, which reduces system efficiency.
To simulate similar fading channels, the fading simulator must correlate the attenuation characteristics of the fading channels with one another. This means that performance tests on MIMO receivers require not only several vector signal generators, but also a multichannel fading simulator.
The R&S SMU200A vector signal generator and the R&S AMU200A baseband signal generator and fading simulator combine these complex functionalities in a single unit. The user can configure signals and define fading conditions conveniently via the generator’s intuitive user interface. This also does away with any cabling between signal generators and fading simulator.
2X2 MIMO SYSTEMS
Tests on 2x2 MIMO systems require four fading channels and two signal sources (Fig. 1, again). Using the R&S SMU-K74 or R&S AMU-K74 option, it’s now possible to perform such tests with a single unit.
Figure 2 shows the block diagram for a 2x2 MIMO test on the SMU200A user interface. Each block represents a functionality that’s configured in the associated menu. The baseband blocks on the left symbolise the two transmit signals, and the four fading blocks represent the fading channels, which are shown as blue arrows in Figure 1. The two RF blocks on the right can be used to configure the RF parameters of the two signals.
To simulate fading channels that are similar to one another, the attenuation characteristics of the fading channels can be mutually correlated. With four fading channels, correlation is configured by means of a 4x4 matrix. Figure 3 shows the window for entering the matrix elements on the AMU200A and SMU200A.
Each matrix element correlates one fading channel with another. The matrix elements are complex numbers, since correlation is defined by both magnitude and phase. The phase value is derived from the phase angle between the antennas. The SMU200A offers the standard matrix assignments defined in the test specifications, and allows users to create matrix assignments of their own. Thus, the instrument is already prepared to handle future expansions.
Featuring a dual-path concept and an integrated multichannel fading simulator, the AMU200A and the SMU200A are well-suited for testing diversity systems. The SMU-K74 and AMU-K74 options simulate up to four fading channels, which allows for tests on 2x2 MIMO receivers by means of a single instrument. This eliminates the need for cabling as well as level adjustment between signal generators and fading simulator.