While doing some background research on LTE voice capacity I noticed there's a feature I haven't yet seen called Uplink TTI Bundling. At first the feature name reminded me of the packet bundling that is done in Wi-Fi, which concatenates several packets into one transmission burst thereby saving a lot of air interface overhead. TTI (Transmit Time Interval) Bundling, however, is quite the opposite as I found out when I had a closer look.
The purpose of TTI Bundling is to improve cell edge coverage and in-house reception for voice. When the base station detects that the mobile can't increase it's transmission power and reception is getting worse it can instruct the device to activate TTI bundling and send the same packet but with different error detection and correction bits in 2, 3 or even 4 consecutive transmit time intervals. The advantage over sending the packet in a single TTI and then detecting that it wasn't received correctly which in turn would lead to one or more retransmissions is that it saves a lot of signaling overhead. Latency is also reduced as no waiting time is required between the retransmissions. In case the bundle is not received correctly, it is repeated in the same way as an ordinary transmission of a packet. Holma and Toskala anticipate a 4dB cell edge gain for VoIP with this feature which is quite a lot. For details how the feature is implemented have a look at 3GPP TS 36.321.