Chapter 1: Configuration
Configuring radio parameters
Page
1-209
Contention slots
Contention slots are symbols at the end of the uplink subframe that are reserved for random access
(network entry and bandwidth requests) and cannot be used for data transmission. These symbols form
the contention space.
The frame is 2.5 ms or 5 ms long, and it is divided into a downlink subframe (data transmitted from the
AP to the SM) and an uplink subframe (data transmitted from the SM to the AP).
Figure 29 Frame structure
The symbols in the uplink subframe can be scheduled or unscheduled. All scheduled symbols come
before all unscheduled symbols. The number of scheduled and unscheduled symbols changes frame by
frame depending on the amount of uplink requests received by the AP.
The contention slots number is selected by the operator and indicates the number of symbols that are
reserved in the unscheduled portion of the uplink. The total number of unscheduled symbols in each
frame is the sum of the contention slots and any additional symbol that was not used in uplink data
transmission. This means that the unscheduled portion of the uplink can be as small as the number of
contention slots, or as big as the whole uplink. This allows SMs in sectors with a small number of
contention slots configured to still successfully transmit bandwidth requests using unused data slots.
Random access
When an SM needs to send an unscheduled message (for network entry or a bandwidth request), it
randomly selects one symbol out of the unscheduled portion of the uplink subframe and uses that
symbol for transmission. The higher the number of unscheduled symbols, the lower the probability two
or more SMs will select the same symbol for transmission and their messages will collide. When two
messages collide at the AP receiver, most likely neither will be decoded correctly, and both SMs need to
start the random-access process one more time. If this happens frequently, the latency of the system
increases.
A higher number of contention slots give higher probability that an SM’s bandwidth request will be
correctly received when the system is heavily loaded, but with the tradeoff that sector capacity is
reduced, so there will be less capacity to handle the request. The sector capacity reduction is about 200
kbps for each contention slot configured in a 20 MHz channel at QPSK SISO modulation, for 2.5 ms
frame sizes. The reduction in sector capacity is proportionally higher at MIMO modulations, as shown in
the following table.
Summary of Contents for PMP 450 AP
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