Measurements for the new WLAN standard: IEEE 802.11ac
New technologies in the WLAN standard IEEE 802.11ac
Wireless communications have fundamentally changed our lives at home, at the office and in our leisure time. Using a smartphone or tablet PC to access the Internet over WLAN has become commonplace. And this trend will continue because WLAN is finding increasing support by devices such as TV set-top boxes and televisions, and mobile radio network operators are counting on WLAN offloading to reduce the burden on their own networks. There is a demand for ever-increasing data rates, for example to transmit videos in HD quality or to take advantage of cloud services.
These high data rates require larger signal bandwidths that are available only at the higher frequency ranges. As a result, the new WLAN standard IEEE 802.11ac also uses bandwidths of 80 MHz and 160 MHz in the 5 GHz band, making data rates of several Gbit/s possible. In the current WLAN standard IEEE 802.11n, the bandwidth is still limited to 40 MHz with a maximum data rate of 600 Mbit/s.
If a contiguous 160 MHz block is not available in the spectrum, it is also possible to combine two 80 MHz channels (channel bonding). The two 80 MHz channels can be combined non-contiguously or contiguously. Other functions introduced with WLAN IEEE 802.11ac are a higher-order modulation up to 256QAM and expanded support of multiple-in multiple-out (MIMO) antennas with up to eight antennas and multi-user MIMO.
High-order modulation ensures good signal quality
IEEE 802.11ac not only supports the modulation modes used up to now (BPSK, QPSK, 16QAM and 64QAM) but also 256QAM. The wireless device determines, on the basis of the measured signal quality, which modulation mode is used. If the signal quality improves or degrades during a connection, the system will select a higher-order or lower-order modulation. To allow 256QAM transmission at a 5/6 code rate, IEEE permits an error vector magnitude (EVM) of at most –32 dB (2.5 %). For 64QAM, up to –27 dB (4.4 %) is tolerated.
In the case of multi-user MIMO, a WLAN switch serves several users on the same frequency at the same time thanks to multipath propagation and beamforming through multi-antenna technology. Each user is assigned a spatially separate data stream. With single-user MIMO, every additional path between the transmitter and the receiver increases the signal-to-noise ratio, thus increasing the throughput for an individual receiver. Although multi-user MIMO improves the capacity of the entire system, the individual data streams lead to a higher noise level. An especially high signal-to-noise ratio, with an EVM below –32 dB, is required for demodulating 256QAM signals.
The frequency response requirements are also higher. At both the transmitter and the receiver end, constant signal power is required across a bandwidth that is four times higher than that with IEEE 802.11n. Deviations would lead to a higher EVM, which is an obstacle for transmission with higher-order modulation, such as in the case of 256QAM.
Digital predistortion compensates for nonlinear effects
In order to fulfill these requirements, components such as amplifiers and mixers must have low inherent noise and linear behavior across a wide frequency range. Although noise components cannot be removed from the signal, digital predistortion makes it possible to compensate for nonlinear effects.
Amplifier manufacturers must find a compromise between keeping the battery usage low on devices and limiting the nonlinear effects (and the associated signal quality degradations). In the upper power range, a high degree of energy efficiency is achieved. At the same time, the nonlinear effects increase in proximity to the 1dB gain compression point. It is possible, however, to digitally compensate for these effects by characterizing them. The signal is digitally predistorted before the amplifier opposite to the amplifier's distortion. After the amplifier, both the predistortion and distortion are canceled out by the amplifier. The result is a linearly amplified signal.