CLIAMS:None ,TagSPECI:FIELD OF THE INVENTION

The present invention generally relates to the field of cellular communication, and more particularly relates to method and system for evaluating and detecting Radio Link Failure (RLF) for a cellular system having asymmetric bandwidth serving cells.

BACKGROUND OF THE INVENTION

In existing approaches, Radio Link Failure (RLF) depends primarily on signal strength of serving cell/cells. This technique works well when a bandwidth of the serving cell/cells is of a same order. But when the bandwidth of the serving cells are asymmetric, declaring RLF merely based on the signal strength of one particular cell is sub-optimal. This is because a User Equipment (UE) having low signal strength in a higher bandwidth cell can be served at a lower code rate (higher redundancy and consequently, higher reliability) and yet satisfy the requirements on throughput due to higher available bandwidth.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 is a process flowchart illustrating a method of detecting a radio link failure (RLF) for two cells.

Figure 2 is a process flowchart illustrating a method of evaluating and detecting RLF using multiple cells transmission with JT CoMP having unequal bandwidth cells.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a method and system for evaluating and detecting Radio Link Failure (RLF) for a cellular system having different bandwidths serving cells. The method and system may be performed for the cellular system such as Long-Term Evolution (LTE) Heterogeneous Network with asymmetric carrier aggregation or a LTE macro cells with millimeter wave (5G) small cells, both of which have provision for carrier aggregation independently.

The present invention discloses a system using a single cell transmission wherein User Equipment (UE) maintains a list of k Cells and an available bandwidth for each, say B1, B2 …Bk. Without loss of generality, let B1 be a bandwidth of a corresponding cell. UE computes R1 = (B1+?1)log2(1 + SINR1 + ?1) where B1 is the available bandwidth. R1 is computed every t seconds for N1(B1) times, where R1 is an estimate of the achievable bitrate/throughput in bps or kbps, ?1 is an offset for the bandwidth estimate (either network configured or UE specific). Likewise, ?1 is a Signal to Interference plus Noise Ratio (SINR) offset.

In one implementation, if R1 is lesser than a threshold T1 for N1(B1) times the RLF entry is detected. The available bandwidth B1 can be any one of the following three such as the total available bandwidth, function of a scheduled bandwidth for the UE in the past t scheduling intervals or foreseeable bandwidth allocation for the UE as communicated to the UE by the Base Station (BS).

In another implementation, for an explicit purpose of monitoring RLF, the available bandwidth is assumed to be communicated to the UE by the BS either periodically or, if the available bandwidth is equal to system bandwidth, it is assumed to be known to the UE.

In yet another implementation, the threshold T1 can be any one of the following such as fixed apriori for all transmissions, changed periodically by the BS and communicated to the UE. For an example, during night time or low load periods, the threshold will be decreased as more system bandwidth can be given to the UE.

Figure 1 is a process flowchart illustrating a method of detecting a radio link failure (RLF) for two cells. Consider a scenario where K cells transmit to a UE. The UE has to decide whether the transmission from the current CoMP set will lead to a RLF condition.

In one implementation, UE computes Ri = (Bi+?1)log2(1 + SINRi + ?i) where Bi is the available bandwidth from the ith cell. Ri is computed every ti (Bi, K) seconds for N1(B1, K) times. If Ri is lesser than a threshold Ti (Bi, K) for N1(B1, K) times, RLF entry is detected for the ith cell. It should be noted that N is a function of not only the bandwidth but also the number of cells in the set for joint transmission. Ti is function of number of serving cells and/or bandwidth.

In another implementation, based on the computed RLF condition R, UE shall detect and declare RLF for a particular radio-link and this radio condition can be conveyed to the BS from which the UE does not have RLF such that BTS may modify the transmission set ensuring reliable reception of Acknowledgement (ACK)/Negative Acknowledgement (NACK) messages.

The method and system discloses the present invention using multiple cells transmission with Probable RLF state wherein even before detection of RLF with a particular radio-link, i.e., after (ti – ti) seconds or after (Ni -ni) consecutive out-of-sync-indications, the UE can indicate a radio link condition to N/W over other established radio-links. By using the RLF condition state, the N/W may remove the cell from the transmission set or modify the transmission set cells.

Figure 2 is a process flowchart illustrating a method of evaluating and detecting RLF using multiple cells transmission with JT CoMP having unequal bandwidth cells. Consider a scenario where K cells transmit to a UE. UE has to decide whether the transmission from the current CoMP set will lead to a RLF condition. In one implementation, UE computes R = min(B1 , B2 , …,BK)*log2(1 + f(SINR1, SINR2 , …SINRK)) where Bi is the “available bandwidth” of the ith cell and f is a function that yields the post-processing SINR. R is computed every t seconds for N(min(B1 , B2 , …,BK), K) times. It should be noted that N is a function of not only the bandwidth but also the number of cells in the set for joint transmission. If R is lesser than a threshold T1 for N(min(B1 , B2 , …,BK), K) times, RLF entry is detected.

The method and system discloses the present invention using multiple cells transmission with CoMP having unequal bandwidth cells & single cell transmission such as dynamic point selection. UE computes Ri = Bilog2(1 + SINRi) for i = 1,…K BS. UE compares Ri for the serving BS with a threshold T(B1,2,…K, K, SINR1,2,…K) that is a function of the bandwidth of other cells in DPS, SINR of other cells and number of cells for a time period t(B1,2,…K, K, ) and maintaining counters N(Bi, K).

In one embodiment, the threshold is inversely proportional to the number of cells. This is because of the fact that as the number of cells increases, the UE has additional cells to which it can fall back upon. However, since the UE can fall back to other cells only if their signal condition is good, the threshold is a function of SINR of other cells in the list of BS that are supporting DPS for this UE. Further, as the bandwidth increases, the UE has additional opportunities and so RLF has to be initiated earlier so that the UE can be served better by cells with better resources for this UE. In other words, RLF is initiated not just because the serving signal is weak, but also due to the prospect of the non-serving cells having better resources and radio condition with respect to UE.

In accordance with the above description, Throughput threshold (T) can be inversely proportional to the summation of the bandwidths of the cells in the transmission set. Likewise, RLF timers and constants can also be inversely proportional to the summation of the bandwidths of the cells in the transmission set.