Layer Traces and Resource Element Distribution (LTE)

Although the term "Layer" refers specifically to a collection of resource element values for PDSCHPhysical Downlink Shared Channel allocations, all downlink physical channels and signals are mapped onto Layer traces in some way.

For PDSCH allocations in a single-antenna LTELong Term Evolution signal, there is a one-to-one correspondence between the on-air subcarrier value and the subcarrier value shown on the Layer 0 trace. For PDSCH allocations in a multi-antenna signal (i.e. that uses transmit diversity or spatial multiplexing), the Layer traces show the subcarrier values after precoding has been removed and a subcarrier value shown on the traces has a one-to-many correspondence with the subcarriers on-air due to precoding between the layer data and the antenna port data. Control channels also undergo transmit diversity precoding in a multi-antenna signal, so they have the same issue with subcarriers values.

For PDSCH allocations that use spatial multiplexing, the values of the subcarrier values for each PDSCH layer are shown on the corresponding Layer traces. For PDSCH allocations that use transmit diversity, there is only one layer of data, and the LTE demodulator shows this data on the Layer 0 traces.

However, for most of the other signals and channels, with some exceptions, subcarrier values are copied to all the Layer traces. This is for convenience (for example when comparing relative powers when using markers). The following table shows the mapping between signal/channel and Layer traces.

Physical Layer Category	Name	Mapping Behavior
Reference Signals	C-RSCell-specific RS	Copied to all layer traces.
	CSI-RS	Ports 15/17/19/21 CSIchannel state information-RS mapped onto Layer 0 traces. Ports 16/18/20/22 CSI-RS mapped onto Layer 1 traces.
	MBSFN-RS	Copied to all layer traces.
	P-RSPositioning Reference Signal	Copied to all layer traces.
	UE-RS	Single-layer beamforming: Port 5, 7, or 8 UE-RS mapped onto Layer 0 traces. Multi-layer beamforming: Ports 7-14 UE-RS mapped onto Layer 0-7 traces, respectively.
Synchronization Signals	P-SSPrimary Synchronization Signal, S-SSSecondary Synchronization Signal	Copied to all layer traces.
Control Channels	PBCHPhysical Broadcast Channel	Copied to all layer traces.
	PCFICH
	PDCCHPhysical Downlink Control Channel
	PHICH
Data Channels	PDSCH (C-RS TxDiv)	Shown on Layer 0 traces.
	PDSCH (C-RS SpMux)	Shown on appropriate Layer traces.
	PDSCH (UE-RS)	Shown on appropriate Layer traces.
	PMCHPhysical Multicast Channel	Copied to all layer traces.