To be become a LTE Expert?

We have seen a lot of terms around when we discuss LTE data transmissions, especially when we get into some of the details of sending data over the PDSCH using multiple antenna techniques. We describe transport blocks as holding the data we're trying to send, but how do they relate to codewords? Assigning bits to different layers can be used to improve reliability or throughput, but are layers the same as antenna ports? Let's have a closer look at what's going on down in the Physical (PHY) Layer.

User data and signaling messages are processed by the PDCP, RLC and MAC layers before being passed down to the PHY layer to be sent over the air. A lot happens to a data packet before PHY gets it, but for the moment, let's just treat the MAC PDU (Protocol Data Unit) that PHY receives from MAC as "data". To PHY, it's just a string of bits anyway. This will be our transport block.

Transport Blocks to Codewords

Query: What does PHY do with a transport block? 

First, it converts the transport block into a codeword. There are a number of steps involved in this process, depending on the length of the transport block:

A codeword, then, is essentially a transport block with error protection. Note that a UE may be configured to receive one or two transport blocks (and hence one or two codewords) in a single transmission interval.

Codewords to Layers

PHY then converts each codeword into modulation symbols. For each codeword, PHY must:

In the case of a single transmit antenna, the last step is pretty simple: the contents of the codeword are mapped to a single layer. For transmit diversity, it's almost as easy: the symbols from the codeword are distributed evenly across the 2 or 4 layers in a round-robin fashion.

In spatial multiplexing situations, things get a little more complicated, since one or two codewords may be distributed across 1, 2, 3 or 4 layers. In brief, here's how the mapping is handled:

The number of layers used in any particular transmission depends (at least in part) on the Rank Indication (RI) feedback from the UE, which identifies how many layers the UE can discern.

Layers to Antenna Ports

The final steps apply any required precoding adjustments and assign the modulation symbols to the physical resources:

Note that the number of layers is always less than or equal to the number of antenna ports (transmit antennas). If there's only one antenna port, then it carries just a single layer. In multiple (2 or 4) antenna situations, though, each antenna port may end up carrying a complicated combination of the symbols from multiple layers. 

for more info you can check out spec 36.211, section 6.3.4 if you really want to dig into the details.

Conclude? One transport block -> one codeword -> one or two layers -> one or more antenna ports. Fortunately, the eNodeB and the UE always know what's going on, even if I have trouble keeping it all straight sometimes.