What do you mean by a bts “picking up” the signal from a neighbouring cell ?
Each cell is defined with a list of neighbour BCCH’s. For example, in your case, let’s assume a cell with 5 neighbours. In the OMCR you will declare 5 neighbours for this cell. And the system, internally, will interpret this list of cells as a list of BCCH :
BCCH ARFCN of the neighbours = 47, 51, 52, 56, 61
Then the MS in this cell will only measure those ARFCNs and consider only those as potential target cells for reselection or handovers.
Reading your post, it seems to me that you didn’t declare the right neighborus in the cells. Since you are talking about inter-OMCR handovers, the problem probably lies in the declaration of the external cells :
-> in the Huawei OMCR, you must declare a list of external cells (= nortel cells), with their proper CI, LAC, BCCH and BSIC.
Then, include the relevant external cells in the “list of neighbours” of the huawei cells.
-> in the nortel OMCR, declare the external huawei cells, and use them as neighbours.