Since I got a lot of requests to explain the LTE paging mechanism and paging capacity, so I decided to write a couple of short articles on these topics.
Paging is a mechanism to initiate services for UEs that are in idle mode. Idle mode transition is important to conserve battery of the UEs. If a UE is in connected mode and it has no data to send or receive, then the eNB waits for a specific timer (UE Inactivity Timer) and once that timer expires, the eNB sends the UE to idle mode. This is done by sending a RRC Release message to the UE.
However, a procedure has to be defined to access the users in idle mode if there is downlink data for them. This process is the paging mechanism. The main parameters that determine the paging capacity at the eNB are given below:
Default Paging Cycle: This is known as “T” and it is the value after which the UE wakes up from idle mode to read the paging messages. Usually it is set to 1280 ms or 128 frames. This means that if a UE woke up in frame # 3 then it will wake up again in frame # 131 (after 128 frames) and then at frame # 259 and so on. It will read the PDCCH and if the PDCCH directs it to a paging message, it will read the paging message. However, this only indicates that there is a paging message in this subframe but it might not be addressed to that UE as there can be multiple UEs using the same paging cycle. “T” is transmitted in SIB-2 but can also be shared by the MME and in that case the smaller value of the two is used. For instance, if T in SIB-2 is 1280ms and T shared by MME is 640ms then the system will use 640ms as the T’s value.
NB: This parameter depicts the number of paging subframes within a Default Paging Cycle. If NB is set equal to T, then every frame contains one Paging subframe. If NB is set to 2T, then every frame will contain 2 paging subframes. If NB is set to 1/2T then every second frame will have a paging subframe. So, in short, this parameter can impact the paging capacity but at the same time it can increase or decrease the paging overhead as well. This is also shared in SIB-2.
Paging Mechanism: These parameters are contained in the SIB-2 so the UE knows it’s paging subframe or paging occasion from the parameters in SIB-2 and from the function of it’s IMSI value. The UE will wake-up at the subframe where it expects it’s PO and will read the paging messages. The paging messages contain the TMSI value so if the UE does not find it’s TMSI inside the paging messages, it will assume that it is not paged, and it will go back to idle mode. However, if the UE finds it’s TMSI in one of the paging messages, it will understand that the page is addressed to it and it will initiate a RRC Connection.
From the core’s perspective, if there is downlink data, the SGW will inform the MME and the MME will send a paging message to the eNB over the S1 interface. The S1 paging message contains a UE ID value which is obtained from a function of the IMSI. Based on this number and the value of N (N=min(NB,T)), the eNB calculates the PO of the UE and it buffers the paging message until the PO. Since the UE already knows it’s IMSI and the eNB derives it from the UE ID in the S1 Paging message, both the UE and eNB are synchronized and they use the same PO subframe. Once, the UE receives the paging message addressed to itself, it initiates RACH process by sending a RACH preamble to the eNB. The eNB responds with a Random Access Response. The UE then sends the RRC Connection Request and the cause value of this request is set to mt-access which indicate that the UE is initiating the connection due to a paging message. The eNB responds with RRC Connection Setup and UE finally sends the RRC Connection Setup Complete to the eNB resulting in completion of the RRC Connection. The eNB sends a S1 Initial UE Message to the MME with the Service Request and this message is considered by the MME as the paging success.
Timer T3413 : The MME maintains a timer T3413 for paging messages. Once it sends the paging message to the UE, it starts the T3413 and it stops it after receiving the Service Request in S1 Initial UE Message. In case, the Service Request is not received and T3413 expires then the MME will resend the paging message to the UE.
However, this timer only works for PS pages and not for CS pages. In case of CS page, the page is retransmitted from the CS core.
Impact of Default Paging Cycle : The paging cycle or T as explained above tells the periodicity with which the UE wakes up to check the paging messages. The smaller the value of T, the UE will need to wake up more frequently. This means that a smaller value of T will lead to higher battery consumption or utilization and UEs will have to be charged more frequently. However, a larger value means that the latency for the UE will increase. For instance, if we use 1280ms for T, the incoming pages at the eNB will be buffered longer on average and that will increase delay. This is not significant for data services but it can be important for CS pages (CSFB) or for VoLTE and can increase call setup time. So, a compromise needs to be maintained between battery consumption and expected paging delay.
Paging Failures : Another important aspect is to ensure that paging messages are successfully reaching the destined users. The paging failures can be caused by decoding issues if the paging modulation scheme is aggressive or if the PDCCH allocation for paging is not using a robust aggregation layer. So, it is a good idea to keep the Paging MCS on the PDSCH much lower in order to ensure successful decoding and also to ensure that the PDCCH is using a conservative aggregation level like Aggregation Level 4 or 8.
Another issue that causes paging failures is frequent TAUs (Tracking Area Update). When the UE moves from one tracking area to another tracking area, it initiates a TAU and if during this process a paging message is received then the UE might not be able to read it as it is undergoing a TAU. The problem here is that when the UE is in idle mode, the MME knows it’s location at a TA or TAL (Tracking Area List) level. So, when the page comes for the UE, MME sends this paging message to all the eNBs in that TA or TAL. If the UE moves from one TA/TAL to another TA/TAL then the UE needs to do a TAU to tell the MME that the UE has moved to another TA/TAL and thus the MME will send the next paging messages for that UE to the new TA/TAL. If the UE is undergoing TAU and the TAU is not completed yet and a paging message comes for this UE, the MME will send it to the old TA/TAL and the UE will not be able to receive it. Thus, it is very important that the TAC planning is properly done to mitigate this issue. I will address the TAC dimensioning and paging capacity in the next short article.
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