Layer Traces and Resource Element Distribution (LTE)

Although the term "Layer" refers specifically to a collection of resource element values for PDSCH Physical 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 LTE Long 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-RS Cell-specific RS	Copied to all layer traces.
	CSI-RS Channel State Information Reference Signal	Ports 15/17/19/21 CSI channel state information-RS mapped onto Layer 0 traces. Ports 16/18/20/22 CSI-RS mapped onto Layer 1 traces.
	MBSFN-RS Multicast-Broadcast Single Frequency Network Reference Signal	Copied to all layer traces.
	P-RS Positioning Reference Signal	Copied to all layer traces.
	UE-RS UE-specific Reference Signal	Single-layer beamforming: Port 5, 7, or 8 UE User Equipment (e.g. cell phone)-RS mapped onto Layer 0 traces. Multi-layer beamforming: Ports 7-14 UE-RS mapped onto Layer 0-7 traces, respectively.
Synchronization Signals	P-SS Primary Synchronization Signal, S-SS Secondary Synchronization Signal	Copied to all layer traces.
Control Channels	PBCH Physical Broadcast Channel	Copied to all layer traces.
	PCFICH Physical Control Format Indicator Channel
	PDCCH Physical Downlink Control Channel
	PHICH Physical Hybrid ARQ Indicator Channel
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.
	PMCH Physical Multicast Channel	Copied to all layer traces.