Establishing a Connection in LTE
1 Answer

In LTE, a connection establishment procedure consists of acquiring timing information, reception of BCH and System Information Blocks (SIBs).

i) Scanning and Synchronization

  • The very first thing an MS has to do is to acquire the timing of the signals in the cell it is in; this is done via the Synchonization Signals.
  • After that MS can learn about vital cell information (transmitted on the BCH), and perform the other functions it needs to do for communication.
  • After the acquisition of the timing and reception of the BCH, additional cell system information is communicated in the System Information Blocks (SIBs) transmitted via the DL-SCH, similar to the reception of payload data.

There are a number of different SIBs defined, depending on the type of information that is to be transmitted.

SIB 1: It contains information about the access to the cell, information about cell selection, etc. It also contains a System Indicator (SI) window length, which is needed for the reception of all other SIBs.

SIB 2: It contains configuration information valid for all MSs, like the configuration of common channels, pilot configuration, timers, etc.

SIB 3-8: They contain information related to intersystem, interfrequency, and intrafrequency handover.

SIB 9: It contains an identifier for the home BS.

SIB 10 and 11: They contain information for earthquake and tsunami warning systems. An SIB 1 is scheduled periodically every 80ms, with a fixed timing. The repetition period for the other SIBs can be configured by the network operator. The MS has to search for the corresponding information within a time window whose width is communicated in SIB 1.

ii) Random Access (RA)

If an MS wants to join the network, it has to inform the BS about its request. However, initially the MS does not have any resources assigned for communication; it must therefore make a contention-based access (also known as random access). It is always possible that its request might collide with requests from other MS. LTE defines a procedure for this access as follows:

a) MS transmits an RA preamble which allows the BS to compute the required timing advance. In this step only different PHY signaling is used. The subsequent steps are transmitted like normal data (except that HARQ is not used).

b) BS transmits an RA response: This contains

  • The index of the RA preamble for which the response is valid (note that at this time the BS does not know the ID of the MS yet, only the type of RA preamble that was used).
  • The timing advance to be used by the MS,
  • The resources to be used by the MS for the signaling in the subsequent step, and
  • A temporary ID.

Note that if several MSs used the same RA preamble, then the RA response is valid for all those MSs, and the resulting collision has to be resolved in the subsequent steps.

c) MS transmits Radio Resource Control (RRC) signaling information, which contains its ID. The details of the information depend on how much information the BS already has about this MS, e.g., whether the MS accesses network for the first time, or just reestablishes a link after the connection has been interrupted.

d) BS transmits a contention resolution message. As described above, ambiguities could have occurred if multiple MSs try to access the system with the same random access preamble. In the contention resolution message, the BS transmits explicitly the ID of the MS to which it assigns resources.

iii) Paging For paging, each MS is assigned (in the DL-SCH) a “paging interval,” and a certain subframe in which a paging message might be transmitted. Thus, the MS needs to wake up from a sleep state only once per paging interval and listen whether there are data for it. The paging interval can be configured, representing a tradeoff between energy saving and latency.

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