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c0mandir
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16.11.2012 19:22 |
Teкcт и гpaфичecкий мaтepиaл из книги "LTE–the UMTS long term evolution : from theory to practice / Stefania Sesia, Matthew Baker, and Issam Toufik."
7.1 Introduction
A UE wishing to access an LTE cell must first undertake a cell search procedure. This
consists of a series of synchronization stages by which the UE determines time and frequency
parameters that are necessary to demodulate the downlink and to transmit uplink signals with
the correct timing. The UE also acquires some critical system parameters.
Three major synchronization requirements can be identified in the LTE system: the first
is symbol timing acquisition, by which the correct symbol start position is determined, for
example to set the FFT window position; the second is carrier frequency synchronization,
which is required to reduce or eliminate the effect of frequency errors1 arising from a
mismatch of the local oscillators between the transmitter and the receiver, as well as the
Doppler shift caused by any UE motion; thirdly, sampling clock synchronization is necessary.
7.2 Synchronization Sequences and Cell Search in LTE
Two relevant cell search procedures exist in LTE:
• Initial synchronization, whereby the UE detects an LTE cell and decodes all the
information required to register to it. This would be required, for example, when the
UE is switched on, or when it has lost the connection to the serving cell.
• New cell identification, performed when a UE is already connected to an LTE cell
and is in the process of detecting a new neighbour cell. In this case, the UE reports
to the serving cell measurements related to the new cell, in preparation for handover.
In both scenarios, the synchronization procedure makes use of two specially designed
physical signals which are broadcast in each cell: the Primary Synchronization Signal (PSS)
and the Secondary Synchronization Signal (SSS). The detection of these two signals not only
enables time and frequency synchronization, but also provides the UE with the physical layer
identity of the cell and the cyclic prefix length, and informs the UE whether the cell uses
Frequency Division Duplex (FDD) or Time Division Duplex (TDD).
The cell search and synchronization procedure is summarized in Figure, showing the
information ascertained by the UE at each stage. The PSS and SSS structure is specifically
designed to facilitate this acquisition of information.
The PSS and SSS structure in time is shown in Figure for the FDD case and in
Figure for TDD: the synchronization signals are transmitted periodically, twice per 10 ms
radio frame. In an FDD cell, the PSS is always located in the last OFDM (Orthogonal
Frequency Division Multiplexing) symbol of the first and 11th slots of each radio frame, thus enabling the UE to acquire the slot boundary timing independently
of the Cyclic Prefix (CP) length. The SSS is located in the symbol immediately preceding
the PSS, a design choice enabling coherent detection of the SSS relative to the PSS, based on
the assumption that the channel coherence duration is significantly longer than one OFDM
symbol. In a TDD cell, the PSS is located in the third symbol of the 3rd and 13th slots, while
the SSS is located three symbols earlier; coherent detection can be used under the assumption
that the channel coherence time is significantly longer than four OFDM symbols.
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