During RRC_CONNECTED mode, if the eNodeB decides that the UE needs to perform LTE inter-frequency and inter-RAT monitoring activities, it will provide the UE with a measurement configuration which includes a monitoring gap pattern sequence.
Similar mechanisms exist in UMTS (known as ‘Compressed Mode gaps’ and ‘FACH Measurement Occasions’ depending on the state and capabilities of the UE) and in GSM (known as GSM Idle frames in GSM Dedicated and Packet Transfer Mode states).
During the monitoring gaps, UE reception and transmission activities with the serving cell are interrupted.
The Main Reasons for Using Monitoring Gap Patterns Are as Follows:
• The same LTE receiver can be used both to perform both intra-frequency monitoring and to receive data when there is no transmission gap.
• The presence of monitoring gaps allows the design of UEs with a single, reconfigurable receiver. A reconfigurable receiver can be used to receive data and to perform inter- RAT activity, but typically not simultaneously.
• Even if a UE has multiple receivers to perform inter-RAT monitoring activity (e.g. one LTE receiver, one UMTS receiver and one GSM receiver) there are some band configurations for which monitoring gaps are still required in the uplink direction.
In particular, these are useful when the uplink carrier used for transmission is immediately adjacent to the frequency band which the UE needs to monitor.
There is always a significant power difference between the inter-RAT signal to be measured and the signal transmitted by the UE.
The amount of receive filtering which can be provided, within the cost and size limitations of a UE, is not sufficient to filter out the transmitted signal at the input of the receiver front end, so the transmit signal leaks into the receiver band creating interference which saturates the radio front end stages.
This interference desensitizes the radio receiver which is being used to detect inter-RAT cells. Rather than address each scenario (i.e. each pair of frequency bands) with a specific solution, uplink gaps in LTE are configured in the same way for all scenarios.
LTE monitoring gap patterns contain gaps every N LTE frames6 (i.e., the gap periodicity is the multiple of 10 ms) with a 6 ms duration for these gaps.
A single monitoring gap pattern is used to monitor all possible RATs (inter-frequency LTE FDD and TDD, UMTS FDD, GSM, TD-SCDMA, CDMA2000 1x and CDMA2000 HRPD).
Different gap periodicities are used to trade off between UE inter-frequency and inter- RAT monitoring performance, UE data throughput and efficient utilization of transmission resources.
In general, cell identification performance increases as the monitoring gap density increases, while the ability of the UE to transmit and receive data decreases as the monitoring gap density increases.
Most RATs (LTE, UMTS FDD, TD-SCDMA, CDMA2000) broadcast sufficient pilot and synchronization information to enable a UE to synchronize and perform measurements within a useful period slightly in excess of 5 ms. This is because most RATs transmit downlink synchronization signals with a periodicity no lower than 5 ms.
For Example, in LTE the PSS and SSS symbols are transmitted every 5 ms. Therefore a 6 ms gap provides sufficient additional headroom to retune the receiver to the inter-frequency LTE carrier and back to the serving LTE carrier and still to cope with the worst-case relative alignment between the gap and the cell to be identified.
 3GPP Technical Specification 36.133, ‘Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for Support of Radio Resource Management(Release 8)’, www.3gpp.org.
 3GPP Technical Specification 25.213, ‘Technical Specification Group Radio Access Network; Spreading and Modulation (FDD)’, www.3gpp.org.
 3GPP Technical Specification 36.214, ‘Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer – Measurements (Release 8)’, www.3gpp.org.