Chapter 6
Introduction to Downlink Physical Layer Design
6.1 Introduction
The LTE downlink transmissions from the eNodeB consist of user-plane and control-plane data from the higher layers in the protocol stack (as described in Chapters 3 and 4) multiplexed with physical layer signalling to support the data transmission. The multiplexing of these various parts of the downlink signal is facilitated by the Orthogonal Frequency Division Multiple Access (OFDMA) structure described in Chapter 5, which enables the downlink signal to be subdivided into small units of time and frequency.
This subdivided structure is introduced below, together with an outline of the general steps in forming the transmitted downlink signal in the physical layer.
6.2 Transmission Resource Structure
The downlink transmission resources in LTE possess dimensions of time, frequency and space. The spatial dimension, measured in ‘layers’, is accessed by means of multiple ‘antenna ports’ at the eNodeB; for each antenna port a Reference Signal (RS) is provided to enable the User Equipment (UE) to estimate the radio channel (see Section 8.2); the techniques for using multiple antenna ports to exploit multiple spatial layers are explained in Section 11.2.
The time-frequency resources for each transmit antenna port are subdivided according to the following structure: the largest unit of time is the 10 ms radio frame, which is subdivided into ten 1 ms subframes, each of which is split into two 0.5 ms slots. ...
