LTE in Bullets - Sync Signals

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LONG TERM EVOLUTION (LTE)

5.1 SYNCHRONISATION SIGNAL The Primary Synchronisation Signal (PSS) is allocated the central 62 subcarriers belonging to the last symbol of slots 0

and 10 of every radio frame, i.e. the PSS is transmitted twice every 10 ms radio frame. Both transmissions are identical

The Primary Synchronisation Signal is used to:o achieve symbol, slot and subframe synchronizationo determine the Physical Layer Cell Identity (PCI) from within the Physical Layer Cell Identity Group. There are 3 PCI

within each group so the PSS is generated using 1 of 3 different sequences

The Secondary Synchronisation Signal (SSS) is allocated the central 62 subcarriers belonging to the second to last symbolof slots 0 and 10 of every radio frame, i.e. the SSS is also transmitted twice every 10 ms radio frame

The 2 SSS transmissions within each radio frame use difference sequences to allow the UE to differentiate between the 1stand 2nd transmission, i.e. allowing the UE to achieve frame synchronisation

The Secondary Synchronisation Signal is used to:o achieve frame synchronisationo determine the Physical Layer Cell Identity Group. There are 168 Physical Layer Cell Identity Groups so the SSS is

generated using 1 of 168 different pairs of sequences

The Primary and Secondary Synchronization Signals within the first half of a radio frame are illustrated in Figure 33Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.
















DTX DTX

Ref. DTX DTX Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

DTX DTX

DTX DTX

Ref. DTX DTX Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.S-Syn P-Syn

S-Syn P-Syn

Ref. S-Syn P-Syn Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

S-Syn P-Syn

S-Syn P-SynRef. S-Syn P-Syn Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

S-Syn P-Syn

S-Syn P-Syn


S-Syn P-SynS-Syn P-Syn

Ref. S-Syn P-Syn Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.S-Syn P-Syn

S-Syn P-SynRef. S-Syn P-Syn Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

S-Syn P-Syn

S-Syn P-Syn

Ref. S-Syn P-Syn Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.



S-Syn P-Syn

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S-Syn P-Syn

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S-Syn P-Syn

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Ref. S-Syn P-Syn Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.S-Syn P-Syn

S-Syn P-Syn


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S-Syn P-Syn

S-Syn P-Syn


DTX DTXDTX DTX

Ref. DTX DTX Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.DTX DTX


















DTX DTX

DTX DTX Ref.DTX DTX

DTX DTX

Ref. DTX DTXS-Syn P-Syn

S-Syn P-SynS-Syn P-Syn Ref.

S-Syn P-Syn

S-Syn P-Syn

Ref. S-Syn P-SynS-Syn P-Syn

S-Syn P-Syn

S-Syn P-Syn Ref.S-Syn P-Syn

S-Syn P-SynRef. S-Syn P-Syn

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S-Syn P-Syn


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S-Syn P-SynDTX DTX Ref.

DTX DTX

DTX DTXRef. DTX DTX

DTX DTX

Primary

Synchronisation Signal

Cell Specific

Reference Signal

Secondary


DTX

10 slots = 5 subframes = 0.5 radio frames = 5 ms

Central62subcarriers



















DTX DTX


DTX DTX

DTX DTX


S-Syn P-Syn


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DTX DTXDTX DTX





































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DTX DTXDTX DTX



















DTX DTX

DTX DTX Ref.DTX DTX

DTX DTX

Ref. DTX DTXS-Syn P-Syn


S-Syn P-Syn

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S-Syn P-Syn


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S-Syn P-Syn


S-Syn P-Syn


S-Syn P-SynDTX DTX Ref.

DTX DTX

DTX DTXRef. DTX DTX

DTX DTX

Primary


Cell Specific

Reference Signal

Secondary


DTX

10 slots = 5 subframes = 0.5 radio frames = 5 ms

Central62subcarriers

Figure 33 Synchronisation signals within slots 0 to 9 of a 3 MHz channel bandwidth

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IN BULLETS

The 5 resource elements above and below the synchronisation signals are not used for transmission. They represent periodof Discontinuous Transmission (DTX)

The set of resource elements allocated to the Synchronisation Signals is independent of the channel bandwidth. The UEdoes not require any knowledge of the channel bandwidth prior to detecting the Synchronisation Signals. The downlink

channel bandwidth is subsequently read from the Master Information Block (MIB) on the Physical Broadcast Channel

(PBCH)

Synchronisation Signals represent an overhead which reduces the number of resource elements available for user planedata

o overhead decreases for larger channel bandwidthso overhead increases when using the extended cyclic prefix

The overhead generated by the Synchronisation Signals is presented in Table 221.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Sync. Resource Elements

per 10 ms Radio Frame

2144 =

288

2144 =

288

2144 =

288

2144 =

288

2144 =

288

2144 =

288

Normal Cyclic Prefix

Total Resource Elements

per 10 ms Radio Frame10080 25200 42000 84000 126000 168000

Overhead 2.9 % 1.1 % 0.7 % 0.3 % 0.2 % 0.2 %

Extended Cyclic Prefix

Total Resource Elements

per 10 ms Radio Frame8640 21600 36000 72000 108000 144000

Overhead 3.3 % 1.3 % 0.8 % 0.4 % 0.3 % 0.2 %

Table 22 Overhead generated by Synchronisation Signals

3GPP References: TS 36.211

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LTE in Bullets - Sync Signals

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