The 802.11n draft 2.00 document defines both 20 and 40 MHz spectral masks. A transmit spectrum mask is the power contained in a specified frequency bandwidth, and regulatory bodies such as the FCC (United States) regulate how much power can be emitted from a transmitter at the center frequency and at given frequency points (called offsets) on both sides of the center frequency.

- Introduction -

Transmit Beamforming (TxBF) is a method of using a set of separate antennas as a virtual array to form high-gain beams focused at client stations.  It is important that each transmitter (called the beamformer) understand the characteristics of the MIMO channel between itself and the receiver (called the beamformee) since transmissions will be tightly focused on a specific area.

The complexity of 802.11n rate adaptation has given birth to the concept of Modulation Coding Scheme (MCS).  MCS includes variables such as the number of spatial streams, modulation, and the data rate on each stream.  Radios establishing and maintaining a link must automatically negotiate the optimum MCS based on channel conditions and then continuously adjust the selection of MCS as conditions change due to interference, motion, fading, and other events.  There are 77 MCSs each for 20 MHz and 40 MHz channels in the current 802.11n draft, with eight of them (all for 20 MHz channels) being mandatory for 802.11n compliance (Section 20.6, Table n82).  Link Adaptation is the 802.11n methodology of a transmitter and receiver working together to help each other understand which modulation and code scheme (MCS) is optimal given the current environment.

Unscheduled PSMP (U-PSMP) extends the U-APSD (also known as WMM-Power Save or WMM-PS) mechanism.  

In a U-PSMP enabled WLAN, if there is no unscheduled service period (SP) in progress, the unscheduled SP begins when the AP receives a trigger frame from a station, which is a QoS data or QoS Null frame associated with an AC the station has configured to be trigger-enabled.  An unscheduled SP ends after the AP has attempted to transmit at least one MSDU, A-MSDU, or MMPDU associated with a delivery-enabled AC and destined for the station, but no more than the number indicated in the Max SP Length field of the QoS Info field in the trigger frame (if the field has a non-zero value).  The Max SP Length subfield is 2 bits in length and indicates the maximum number of total buffered MSDUs and MMPDUs the AP may deliver to a station during any SP triggered by the station.

- Introduction -

In the legacy power save polling method, the station awakes from dozing and sends a PS-Poll frame to the AP that has been queueing its data traffic while it dozed.  For each PS-Poll frame the station sends to the AP, the AP responds with one data frame.  Additionally, stations have to wait for the DTIM Beacon to know whether or not it has queued traffic.  It's a high-overhead, high-latency protocol that only allows stations to doze a moderate amount - not good enough for wVoIP.