Adaptive transmission of VoIP packets using TTI bundling in LTE uplink

In long term evolution (LTE) uplink, transmission time interval (TTI) bundling technique is employed for the voice over internet protocol (VoIP) service to improve the cell coverage performance. In frequency-division duplex (FDD) mode, LTE uses 4 TTIs or subframes as a bundle for transmission with a round trip time (RTT) 16ms. Thus the maximum number of transmission bundles is only three if the delay budget is strictly limited to 50ms in VoIP service. Recently, a coverage-enhanced scheme can transmit up to five bundles, but lacks time for feedback processing. Once the unnecessary retransmissions occur, the user equipment (UE) transmit power can be wasted for extra transmission. To overcome this problem, we propose an adaptive scheme using block error rate (BLER) versus signal-to-noise ratio (SNR) mapping table, which enables UE predetermine the maximum number of transmission bundles appropriately. Instead of using five transmission bundles, our proposed scheme requires no more than five transmission bundles according to SNR variation scale of the channel. The conducted simulation results show that our proposed scheme can achieve higher throughput than coverage-enhanced scheme by reducing unnecessary retransmissions, while it still satisfies the BLER requirements by employing a tolerance factor.