Note : See "Idle Mode Procedure" section first for the big picture. (I wrote this section with more focus on LTE, but it is similar logic in UMTS as well). Followings are the topics to be described in this page.
"Cell Search" in this page means the collective term representing the combined procedure of Measurement, Evaluation, Detection process.
This is very tightly related to Cell Selection process because UE goes through this search process first before it goes through the cell selection.
Also this process influence greatly on engergy consumption of UE during the idle mode.
Following illustration shows a possible example of initial scan and cell search mechanism for WCDMA. It is not for LTE, but you may apply the similar logic in LTE case as well. Overall description for each step is :
When you first power on the device or your device got into out of coverage and try to detect/search a new cell,
UE does not have any idea on which frequency it has to try camp on.
so it may have to do some kind of blind search.
For example, let's assume that your device support WCDMA Band I.
The NodeB around your UE may use any frequency channel from 10562 to 10838.
There can be 276 possibilities of frequencies that eNB would use. Then how UE can detect/find the cell(NodeB) it would camp on ?
There can be many different algorithm to try.
These algorithms are not defined in 3GPP.. so it is all up to the implementation on UE side or chipset implementation.
One of the most likely algorithm can be as follows :
i) UE tune to each and every channel that it support and measure RSSI.
(RSSI is simply a measurement of whatever energy/power it can measure. This measurement does not require any channel coding process. So at this step, UE does not need to know anything about the network. At this step, UE does not try to decode PCPICH (in WCDMA) or Sync/Reference Signal (in LTE) to detect Physical Cell ID. It just measure the power of each channel.) As UE measure RSSI for each channel, it create a list of each channel numbers with the measured RSSI.
ii) Then UE go through the list from step i) and figure out all the channels which shows RSSI value greater than the threshold (this threshold is also up to UE/chipset implementation, not determined by 3GPP).
Then the question would be "Any frequency with Passing RSSI value can be the one that UE can camp on ?". The answer is "Not Necessarily". >
For find the more proper candidate to camp on, UE performs following steps.
iii) UE decode PCPICH and measure the power and detect physical cell ID from the each candidate from step ii).
(Some candidate give successful result but some would not. UE make the list of all the successful tries).
iv) From the list with successful result from step iii), UE decode MIBs for each and every candidate. With this procedure, now UE can make a list of frequency, Physical Cell ID (PSC in case of WCDMA) and PLMN.
v) Based on USIM information and the candidate table from the step iv), can figure out which cell is the real candidate cell to camp on and try decoding System Information and proceed to registration process.
If UE fail to find any Home PLMN cell in the step v) above and find only VPLMN cell, it would camp on the VPLMN cell. But once it gets into Idle mode in VPLMN cell, UE would try performing searching cell with HPLMN. This process may include all the steps described above or a little bit simplified process depending on UE implementation.
Usually these HPLM search process happens periodically as illustrated below. The search cycle (periodicity of HPLMN search is determined by a USIM parameter, but the detailed search algorithm is up to UE implementation). The period marked as 'backoff' is not defined by 3GPP.
If UE performs this periodic search in the area where there is no HPLMN, it would drain the battery too much.
So to save energy consumption, most of UE maker/chipset maker tend to implement a kind of 'backoff' method.
Overall logic would be as follows :
UE peforms the measurement for serving cell at every DRX cycle and check if it satisfy the cell selection criteria. If it successfully finds a cell meeting the criteria within a certain amount of trial, it stays at the cell but if it does not find the serving cell within a certain amount of trials (see the table shown below), it should initiate the measurement/evaluation for all the neighbour cells which is specified by the serving cell system information. (Refer to 36.133 section 184.108.40.206 for details)
36.304 section 220.127.116.11 (V8.10.0) describes the rule for measurement of non-serving cell as follows (depending on the version of this document, you would see a little bit different parameter name and way of description):
Regarding Intra Frequency Measurement : SIB3 and SIB4 are involved in this process
Regarding Inter Frequency Measurement : SIB3 and SIB5 are involved in this process
Regarding Inter RAT UTRA Measurement : SIB3 and SIB6 are involved in this process
Regarding Inter RAT GERAN Measurement : SIB3 and SIB6 are involved in this process
36.133 section 18.104.22.168 defines the intra frequency EUTRAN Measurement as follows :
Regarding Intra Frequency Measurement : ---------------
The major parameters mentioned in above statements came from SIB3 and 36.133 Table 22.214.171.124-1 as follows.
< 36.331 SystemInformationBlockType3 field descriptions >
Regarding Inter Frequency Measurement : --------------
Refer to 36.133 section 126.96.36.199 for the details.
Regarding Inter RAT UTRA_FDD Measurement : --------------
Refer to 36.133 section 188.8.131.52.1 for the details.
Regarding Inter RAT GSM Measurement : --------------
Refer to 36.133 section 184.108.40.206.3 for the details.