Part 1 of this series provided an overview of MIMO.
WLAN MIMO Example
The IEEE 802.11a/g/n/ac WLAN standards use orthogonal frequency-division multiplexing (OFDM) modulation. OFDM is a method of encoding digital data simultaneously on multiple subcarrier frequencies. Each subcarrier is used to transmit QAM or PSK encoded, unique digital data. The number of subcarriers varies by channel bandwidth and WLAN standard. For example, 802.11a contains 52 subcarriers in its 20 MHz channel bandwidth, and 802.11ac contains 484 subcarriers in its largest 160 MHz bandwidth.
In the time domain, WLAN signals are transmitted in frames, where each frame consists of training fields, signal fields, and data as shown in Figure 4. The short training field (STF) and long training field (LTF) are used to synchronize and equalize the channel. The signal field (SIG) contains logical information used to decode the data transmission. The payload data is variable-length and the last four bytes contain a Cyclic Redundancy Check (CRC).
Figure 4) Each WLAN frame format includes training fields, signal fields, and data.
Figure 4 shows four different frame types for the various WLAN protocols. The legacy fields (L-) are shown in green, the high-throughput fields (HT-) are shown in blue, and the very-high throughput fields (VTH-) are shown in orange. IEEE 802.11a/g protocols use the L fields only. IEEE802.11n supports a mixed mode of both L and HT fields, and a green-field mode that consists almost entirely of HT fields. IEEE 802.11ac uses the VHT mixed mode.
MIMO was introduced in WLAN protocols with the 802.11n standard as a way to increase data rates without requiring more RF bandwidth. The newest IEEE 802.11ac WLAN standard, which is still in draft format, will achieve up to 6.93 Gbps using up to eight MIMO channels. (Note that the legacy WLAN 802.11a/b/g protocols do not support MIMO.) When transmitting a legacy protocol, an 802.11n/ac system with multiple antennas often uses STBC in a MISO configuration to improve channel integrity.
The OFDM modulation of WLAN simplifies the MIMO channel estimation requirements. The modulation bandwidth for each subcarrier is narrow enough to reduce the equalization coefficients to a single complex coefficient (e.g. amplitude and phase do not vary over the subcarrier bandwidth). Within 802.11n/ac systems, MIMO channel estimation is accomplished using MIMO training sequences based upon the HT and VHT training fields (STF and LTF) shown in Figure 4.
The IEEE 802.11 WLAN specifications define a number of standardized compliance tests . Much research has been done on test optimization for RF devices and systems in a SISO configuration .Single Transmitter (SISO) Tests
Typically, a vector signal analyzer (VSA) is used to perform standard compliance tests upon signals generated by a WLAN transmitter . Standard transmitter tests include:
• Spectrum mask
• Spectral flatness
• Peak power
• Center frequency error
• Symbol clock frequency error
• Center frequency leakage
• Error vector magnitude (EVM)
In overview, WLAN protocol analysis software is used to analyze in-phase/quadrature (I/Q) data captured by a VSA and return the various measurement results listed above. Figure 5 shows an example of this type of protocol analysis software tool.
Figure 5) Software analyzes data captured by a VSA.
EVM (also called relative constellation error) is often used as a comprehensive measure of transmitter performance . EVM is a measure of how far the constellation points vary from their ideal locations and is degraded by any imperfection in the RF channel. The EVM thresholds for a WLAN transmitter for the various modulation coding schemes are shown in Figure 6.
Figure 6) Transmitter EVM SpecificationsMIMO Transmitter Tests
Testing MIMO transmitters is similar to testing a single transmitter with the added complexity of multiple channels. In addition to decoding MIMO-specific signal fields and training sequences, WLAN compliant testing for MIMO requires that composite EVM be calculated by averaging the individual EVM results for all spatial streams. In a composite EVM test, STBC is not used and consequently each transmitter simultaneously generates the same RF output signal. According to the specifications, each transmitter output port should be connected through a cable to a dedicated VSA input port. This test configuration returns individual and combined EVM performance, fulfilling the one additional MIMO test requirement of the IEEE 802.11 specifications . In practice, verifying a MIMO design may require more sophisticated tests and test equipment setup. Additional RFIC design verification tests will be discussed in the next section.Single Receiver (SISO) Tests
Typically, a vector signal generator (VSG) is used to generate RF signals into a WLAN receiver for standard compliance testing. In overview, receiver tests verify the dynamic range and linearity of the receiver. Standard receiver tests include:
• Minimum input level sensitivity
• Maximum input level
• Adjacent channel rejection (ACR)
• Non-adjacent channel rejection
• Clear channel assessment (CCA) sensitivity
Receiver minimum input level sensitivity defines the minimum input RF signal that meets a specified limit on packet error rate (PER). Successful demodulation requires a PER of less than 10%. The minimum sensitivity thresholds for a WLAN receiver for the various modulation coding schemes and modulation bandwidths are shown in Figure 7.
Figure 7) Receiver Minimum Sensitivity Specifications MIMO Receiver Tests
The IEEE 802.11 specifications require MIMO receivers to be tested as multiple single receivers in parallel. For example, the minimum input level sensitivity defines the threshold as the average power per receive port for a MIMO system. This test configuration requires each receiver port to be connected through a cable to a dedicated VSG port.
Most MIMO receivers are tested for additional characteristics including cross-coupling between receivers. Receiver isolation is measured by applying a signal to one receiver and measuring the coupled response on all other MIMO receivers. Typically, the spectrum of the long training sequence is used for isolation measurements by acquiring data that is time-gated around the long training symbols (LTS) within the packets. Additional RFIC design verification tests will be discussed in the next section.