Cross-Layer Interactions in UMTS Evolved UTRAN (E-UTRAN)

In this paper, we describe possible interactions between UMTS Layer 2 and Layer 1 protocols to gain throughput performance improvements. The effect of transmission errors in hybrid ARQ (HARQ) control signalling is shown along with the improvements that are possible by tighter coupling of medium access control (MAC) layer HARQ and radio link layer control (RLC) layer ARQ level retransmissions. It is shown how the number of RLC segments in the MAC SDU affects the design of the required reliability of the ACK and signalling. MAC HARQ "NACK to ACK" errors can cause considerable throughput reductions, which can be substantially alleviated by the use of HARQ error recovery techniques.