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Slot (re)allocation among mobile devices can be performed as often as once every millisecond.
Different modulation schemes can also be used to change the transmission rate; see our earlier
discussion and dynamic selection of modulation schemes in Wi-Fi networks.

The particular allocation of time slots to mobile devices is not mandated by the LTE standard.
Instead, the decision of which mobile devices will be allowed to transmit in a given time slot on
a given frequency is determined by the scheduling algorithms provided by the LTE equipment
vendor and/or the network operator.

With opportunistic scheduling [Bender 2000; Kolding 2003; Kulkarni 2005], matching the
physical-layer protocol to the channel conditions between the sender and receiver and choosing
the receivers to which packets will be sent based on channel conditions allow the base station to
make best use of the wireless medium. In addition, user

priorities and contracted levels of service (e.g., silver, gold, or platinum) can be used in scheduling
downstream packet transmissions. In addition to the LTE capabilities described above, LTE-
Advanced allows for downstream bandwidths of hundreds of Mbps by allocating aggregated
channels to a mobile device [Akyildiz 2010]. 7.4.4 Additional LTE Functions: Network Attachment
and Power Management Let’s conclude or study of 4G LTE here by considering two additional
important LTE functions:
Figure 23: Twenty 0.5-ms slots organized into 10 ms frames at each
(i) the process with which a mobile device first attaches to the network and (ii) the techniques
used by the mobile device, in conjunction with core network elements, to manage its power use.

Network Attachment The process by which a mobile device attaches to the cellular carrier’s
network divides broadly into three phases:
• Attachment to a Base Station. This first phase of device attachment is similar in purpose to,
but quite different in practice from, the 802.11 association protocol that we studied in Section
7.31. A mobile device
wishing to attach to a cellular carrier network will begin a bootstrap process to learn about, and
then associate with, a nearby base station.

The mobile device initially searches all channels in all frequency bands for a primary
synchronization signal that is periodically broadcast every 5 MS by a base station. Once this signal
is found, the mobile device remains on this frequency and locates the secondary synchronization
signal.

With information found in this second signal, the device can locate (following several further
steps) additional information such as channel bandwidth, channel configurations, and the
cellular carrier information of that base station.

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