Testing WLAN radio modules with 2×2 MIMO

Multiple input multiple output (MIMO) is a multi-antenna technology that can help to significantly accelerate WLAN transmissions and optimize the signal quality. However, WLAN radio modules require additional test procedures in production and development covering the specified MIMO techniques.

In MIMO transmissions, signals are sent via multiple transmit antennas to multiple receive antennas. Reflections along the path to the receiver produce the multipath propagation that MIMO requires. The IEEE standardization committee has specified various MIMO techniques for the 802.11n/ac/ax standards. They can be used to boost the data rate or the signal quality. For each transmission, the WLAN router and WLAN station negotiate which of the techniques to use. Key parameters include the prevailing transmission conditions, the requirements for the data rate and signal quality, and the device configuration.

2×2 MIMO for small devices

The WLAN modules in small devices such as smartphones or compact tablets are generally configured to handle a maximum of 2×2 MIMO. More than that is not possible due to the required minimum spacing between antennas of one half wavelength. In the 2.4 GHz frequency band, this is equal to one half of 12.5 cm. In addition, multi-antenna systems need more power for their RF amplifiers, which can increase the energy consumption as well as the temperature. The following examples illustrating MIMO techniques as well as test and measurement equipment are based on MIMO-capable devices with a maximum of two transmit and two receive antennas.

Higher data rates with spatial multiplexing

The MIMO technique of spatial multiplexing significantly increases the data rate compared to single-antenna systems, for example. The outgoing data stream is divided among several spatial streams that are simultaneously transmitted to the receiver on the same frequency by means of different transmit antennas.

Fig. 1: In spatial multiplexing, each receive antenna receives a sum signal consisting of all of the transmitted signals. The prevailing channel conditions are represented in a transmission matrix H with elements h_nm.

Source: Rohde & Schwarz

Decoding in 2×2 MIMO means the receiver has to solve a system of equations with two equations and two unknowns. This is only possible if the equations are linearly independent. In physical terms, this necessitates multipath propagation on separate, uncorrelated transmission paths.

In order to decode the transmission matrix H, the matrix elements must be known. The receiver autonomously determines these elements using open-loop channel estimation based on known bit sequences in the transmitted data packets. Under ideal conditions, 2×2 MIMO with spatial multiplexing can achieve double the data rate of single-antenna systems.

TX diversity for higher signal-to-noise ratio

In addition, various methods are specified that can improve the signal quality based on TX diversity. These methods involve using more transmit antennas than receive antennas in order to boost the signal-to-noise ratio. Transmission is thus less susceptible to interference, which also extends the range. The data rate can also be increased if the improved signal quality allows the use of higher-order modulation (16QAM/64QAM/256QAM/1024QAM). The signal-to-noise ratio can be increased in various ways.

Timing offset for stronger signals

In the method known as cyclic shift diversity (CSD) or cyclic delay diversity (CDD), a signal is transmitted several times with a defined timing offset. Variable signal delays arise due to multipath propagation over transmission paths of different lengths. A transmitted signal reaches the receiver with different timing offsets. If an AP now transmits the signal via multiple antennas with a defined timing offset, the various multipath signals are superimposed at the receive antenna, which ideally helps to boost the receive signal. This virtual echo in the transmitter increases the receiver’s frequency selectivity.

Space time block coding for more robust behavior

In space time block coding (STBC), the outgoing data stream is transmitted redundantly via two transmit antennas (Fig. 2), but is only received via a single antenna.

Fig. 2: In space time block coding, the modified outgoing data stream is transmitted via multiple antennas and received via a single antenna.

Source: Rohde & Schwarz

Compared to the original data stream, the redundant signal is permuted in time and complexly conjugated. This system is known as Alamouti’s code in honor of its developer, Siavash Alamouti, who created it for communications with two antennas.

As with spatial multiplexing, the two data streams can only be decoded here if there are separate, independent transmission channels from transmitter to receiver. Under ideal conditions, using two transmit antennas increases the signal-to-noise ratio by 3 dB, thereby doubling it.

Targeted beamforming 

The TX diversity technique known as beamforming works without multipath propagation. The transmit signal is sent with a timing offset or phase offset via multiple antennas or antenna arrays. Depending on the geometrical arrangement and spacing of the individual transmit antennas, the signal is amplified, attenuated or even canceled out in various directions. Data streams can be targeted at specific WLAN stations and suppressed for other stations.

Multi-user MIMO

Multi-user MIMO (MU-MIMO) is a typical application of beamforming. If only one access point (AP) and one WLAN station are using a MIMO technique, this is known as single-user MIMO (SU-MIMO). In MU-MIMO, however, the AP sends different MIMO streams to a number of WLAN stations which can have one or more receive antennas. Devices that support IEEE 802.11ax are configured accordingly, while small devices can only process two data streams simultaneously. In order for the receiving device to receive its intended data stream with high field strength, the APs use beamforming in MU-MIMO to target the signals at the desired receiver (Fig. 3).

Fig. 3: Multi-user MIMO system with spatial multiplexing based on beamforming.

Source: Rohde & Schwarz

The above techniques can be combined in a variety of ways. For example, the signal-to-noise ratio of a spatial diversity data stream can be improved with space time block coding. This results in a 2×2 MIMO stream being transmitted with 4×2 MIMO (four transmit antennas and two receive antennas).

Testing MIMO functionality

Every MIMO-capable WLAN module requires testing of the relevant functions all the way from development through series production. However, the test requirements that apply during the development process are significantly different from those in production.

Requirements for production tests

During production tests, transmitters and receivers must undergo calibration and inspection on a time-optimized basis. As the device under test (DUT), the WLAN module is remote-controlled via an electrical connection and operated in an artificial non-signaling mode. For such production tests, the test instrument only needs to be equipped with a signal generator and an analyzer. The test duration is primarily dependent on the tester’s measurement speed as well as the number of points and the test depth. If manufacturers try to minimize the test duration by defining as few measurement points as possible and only performing very superficial testing at these points, they run the risk that the product will no longer achieve the specified minimum quality. As a result, they must always keep the applicable risks in mind when attempting to streamline the test plan.

(cont.)