Let Vji( n) ∈ CN×Qj denote the sth Qj-dimensional subspace transmitted bytransmitter i which participates in the chain that originates from transmitter j.Consider one alignment chain originating from transmitter 1, where V1( 1) is nulledat receiver 2 but causes an interference dimension at receiver 3. The second signal,V12( 1) from transmitter 2, should be aligned with V1( 1) on receiver 3, so that nomore interference dimension is generated on receiver 3. Then, if V12( 1) can be nulledat receiver 1, the chain is finished. Otherwise, transmitter 3 should send a vector,V13( 1) , which is aligned with V12( 1) on receiver 1. The chain will keep going until thezero-forcing can be achieved. Mathematically, it can be expressed as follows,
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