This question can take full pages and pages of discussion, or it can also take a brief discussion to answer, depending on how much depth the interviewer or client is asking for. I will explain paging briefly here.

Paging is used for network-initiated connection setup. An efficient paging procedure should allow the UE to sleep with no receiver processing most of the time and to briefly wake up at predefined time intervals to monitor paging information from the network.

For LTE, paging relies on the PDCCH channel. This channel is monitored at predefined time instants according to a Discontinuous Reception (DRX) cycle. This is illustrated in Figure below.

No separate physical-layer paging indicator channel is defined or used in LTE as the potential power savings are very small due to the short duration of PDCCH transmission, at most three OFDM symbols.

If the UE detects its paging group identity on the PDCCH, it demodulates and decodes the PDCCH and forwards the decoded data on the paging transport channel (PCH) to the MAC layer. The PCH transport block contains the exact identity of the terminal being paged. A UE which does not find its identity on the detected Paging Channel (PCH) will discard the information and sleep according to the DRX cycle.

Obviously, as the uplink is not necessarily time aligned when a terminal is paged, no ACK/NAK signaling can take place during paging, and consequently hybrid ARQ (HARQ) with soft combining cannot be used for paging messages.