First of all, physical channel and a physical signal are all together different things. A channel can be considered as a medium through which some information is transmitted, where as a signal has a mathematical importance and it is, most of the times generated at the physical layer itself. So this article will speak about the different physical layer signals used at LTE downlink,
The downlink signals are broadly classified into two,
- Reference Signals
- Synchronization signals
The reference signal, as the name says, it is a reference to some one, who can predict certain other things based on the quality of these reference signals received. The LTE downlink reference signals are again classified into 2,
- Cell specific reference signals (CSRS)
- UE specific reference signals (UeSRS)
The CSRS is cell specific, which means that, these do not depend/change per user but remain same for all the users and entire system, once configured. These reference signals are used by the UE to estimate the downlink channel and do a relative equalization to remove the channel effect over the signal. Hence the UE will generate the CSRS on his side and do a comparison of the generated and received CSRS to get an estimate of channel effect. The CSRS is transmitted with some specific power, which the UE must know, to calculated the multipath effect and this power is conveyed to the UE using SIB messages. The CSRS is mapped onto symbol 0, 4, 7 ,11 of all downlink subframes in FDD. The CSRS is mapped to every sixth subcarrier in these symbols, the start index is determined by the physical cell ID using the below formula,
CSRS start position = Cell ID % 6
The below diagram shows 2 examples of CSRS mapping for 2 different cell ID 12 and 8. For Cell ID 12 since the above formula results in 0, the CSRS mapping starts at 0th subcarrier in 0th RB and continues to map every 6th subcarrier till end of the bandwidth. Similarly for the second case of cell ID 8, the formula results in 2 and the CSRS mapping starts at 3rd subcarrier (Since the subcarrier count starts from 0 and not 1) and continues to map.
LTE Downlink Reference Signals
The CSRS is a QPSK modulated sequence.
Similarly we also have UeSRS, which we will not discuss her, but this is UE specific and used only for beam forming.
There are 2 synchronization signals in LTE downlink,
- Primary synchronization signal – PSS
- Secondary synchronization signal – SSS
The PSS and SSS are both mapped always in Subframe 0 and 5 for FDD. The PSS is always mapped to the last symbol of first slot and SSS to the last but one symbol of first slot. Also these are always mapped to the central 6 RB of the bandwidth, irrespective of any system bandwidth or configuration. This enables the UE to decode these signals, even when it does not know the system bandwidth. The PSS/SSS detection is a very early procedure that the UE should do, to get the cell ID of the system.
The PSS is a Zadoff-Chu sequence, of length 62 and the SSS is a combination of 2 binary sequences of length 31. Since the PSS/SSS have only 62 subcarrier valid, the remaining 10 subcarrier are padded with zeros. 5 zero pads on each side of the sequence.
The PSS/SSS not only convey the cell ID but also the current subframe number, slot boundary, duplexing mode to the UE. There are 3 different sequences of PSS and 168 different sequences of SSS.
As part of later release of LTE (Rel 10) they have also introduced a new signal known as the Positioning Reference Signal, which assists in positioning detection.
We shall discuss the cell ID detection procedure by the UE in a separate article.