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422 From GSM to LTE-Advanced Pro and 5G

PSMP downlink window
(PSMP-DTT)

Data Data Data
PSMP
AP dev.1 dev.2 dev.3
Devices Data Data Data
dev.1 dev.2 dev.3

PSMP uplink window
(PSMP-UTT)
Figure 6.19 A Power Save Multipoll (PSMP) window in which several clients transmit and receive data.

In PSMP mode, frames in both directions not only contain user data but also ACK
information for the received data frames. During a PSMP window, a device can transmit
and receive several frames. If these are sent individually, an SIFS gap has to be inserted
between the frames or, optionally, a shorter gap referred to as RIFS (Reduced Interframe
Space) is attached. Furthermore, data frames can also be aggregated by using the frame
aggregation extension described above to aggregate several frames into a single physical frame.
Figure 6.19 shows how a PSMP window can be used by several devices at the same
time. For this purpose, a PSMP frame is sent at the beginning of each interval and con-
tains information on which the device can transmit and receive data at which times
during the PSMP window. According to the standard, a PSMP window should be
inserted every 5–40 milliseconds, with a granularity of 5 milliseconds. For example, for
VoIP applications, a good interval is 20 milliseconds, as speech codecs usually compress
speech data over such a period and then transmit the result in a short data frame.
The PSMP windows and the transmission times for each client device are optimized
for periodic transmissions with a constant bandwidth requirement. This also optimizes
the use of the scarce air interface resources. In practice, however, a device might some-
times require additional bandwidth, for example, because of transmission errors or
because of an additional frame that has to be sent. Such transmissions cannot occur
during the normal transmission window as there is no space for unexpected transmis-
sions. In such cases, the client device can request additional uplink capacity via a flag in
the MAC header of a frame. This flag is similar to the HSUPA ‘happy’ bit (Chapter 3).
The AP can then schedule an additional PSMP window and announce this additional
transmission opportunity in the PSMP management frame that precedes the next
PSMP window. In case of an uplink transmission error, the AP returns a negative ACK
to a client during the next PSMP downlink window and also inserts an additional PSMP
window.
An additional PSMP functionality is the transmission of frames without the require-
ment of an ACK from the receiver. This is an interesting option for applications such as
VoIP clients, as voice transmissions are delay sensitive and it might thus be preferable to
ignore a faulty packet rather than to request a retransmission.
To make the use of MIMO as power efficient as possible, an additional PS functional-
ity has been introduced with 802.11n for MIMO‐capable devices. Even if no data is
transferred, such devices must keep both receivers activated, as the AP can transmit a