In part three of this series, we took an in-depth look at OFDMA, MU-MIMO and BSS Coloring. In this blog post, we’ll explore target wake time (TWT), 1024-QAM and Long OFDM Signal.
Target Wake Time (TWT) and Wi-Fi 6
Target wake time (TWT) is another mechanism introduced in the Wi-Fi 6 (802.11x) standard. Essentially, TWT allows devices to deterministically negotiate when and how often they wake up to send or receive data. TWT increases device sleep time and in turn, substantially improves battery life, a feature that is especially important for IoT devices. In addition to saving power on the client device side, TWT enables wireless access points (APs) and devices to negotiate and find specific times to access the medium. This helps optimize spectral efficiency by reducing contention and overlap between users.
1024-QAM & the Need for Speed
Although bolstering spectral efficiency is one of the defining features of Wi-Fi 6 (802.11ax), an additional speed boost facilitated by 1024-QAM is obviously a nice bonus. Quadrature amplitude modulation, or QAM, uses both phase and amplitude of an RF signal to represent data bits. As we mentioned above, Wi-Fi 6 (802.11ax) introduces 1024-QAM, along with new modulation and coding schemes (MCS). These define higher data rates that bolster throughput and enable 25% higher capacity with 10 bits per symbol versus 8 bits in 256-QAM, the latter of which is supported by Wi-Fi 5 (802.11ac). Put simply, more bits equal more data, making the (payload) delivery of data more efficient.
Wi-Fi 6 (802.11ax) also introduces two new modulation coding schemes: MCS 10 and MCS 11. Both will likely be optional. It should be noted that 1024-QAM can only be used with 242 subcarrier resource units (RUs) or larger. This means that at least a full 20 MHz channel will be required for 1024-QAM.
Long OFDM Signal & Outdoor APs
When indoor wireless devices transmit a signal, the RF signal reaches the destination receiver directly, or via rapid reflections of walls ceilings and other obstacles. This is referred to as multipath. The OFDM symbol was originally designed with indoor Wi-Fi in mind, with multipath reflected RF signals expected to reach the receiver very quickly. The original OFDM symbol was composed of guard intervals followed by a data portion, then another guard interval and then another data portion, area and so forth. The guard interval was either 0.4 or 0.8 microseconds – with the useful OFDM data portion set at 3.2 microseconds.
With outdoor Wi-Fi, the guard interval needs to be increased to compensate for extended or distant reflections. As such, Wi-Fi 6 introduces Long Signal OFDM, which allows up to a 3.2 microsecond guard interval with the data packet area being increased 4x, or up to 12.8 microseconds. This offers a much broader multipath tolerance, reduces overhead and bolsters throughput, thereby making outdoor Wi-Fi more reliable and dependable.
More GHz For the IoT
As we discussed in part one of this series, Wi-Fi 6 (802.11ax) will support both 2.4 GHz (for the IoT) and 5 GHz, as opposed to Wi-Fi 5 (802.11ac), which only supported the latter. Moreover, the FCC is slated to open the 6 GHz spectrum for Wi-Fi 6 in 2019, thereby creating more than one GHz of new unlicensed spectrum. This is an important development, as the amount of Wi-Fi spectrum in the United States has remained essentially unchanged for more than a decade. From our perspective, the combination of Wi-Fi 6 (802.11ax) and the newly opened 6 GHz spectrum has the potential to fuel a perfect storm of disruption for the wireless industry.
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