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Figure 5.2. Example of Semi-Persistent Scheduling.<br />
TTI bundling is another feature in Rel-8 that optimizes the UL coverage for VoLTE. LTE defined 1 ms<br />
subframes as the Transmission Time Interval (TTI) which means scheduling occurs every 1 ms. Small<br />
TTIs are good for reducing round trip latency, but do introduce challenges for UL VoIP coverage. This is<br />
because on the UL, the maximum coverage is realized when a user sends a single PRB spanning 180<br />
kHz of tones. By using a single 180 kHz wide PRB on the UL, the user transmit power/Hz is maximized.<br />
This is critical on the UL since the user transmit power is limited, so maximizing the power/Hz improves<br />
coverage. The issue is that since the HARQ interlace time is 8 ms, the subframe utilization is very low<br />
(1/8). In other words, 7/8 of the time the user is not transmitting. Therefore, users in poor coverage areas<br />
could be transmitting more power when a concept termed TTI bundling (explained in the next paragraph)<br />
is deployed.<br />
While it’s true that one fix to the problem is to just initiate several parallel HARQ processes to fill in more<br />
of the 7/8 idle time, this approach adds significant IP overhead since each HARQ process requires its<br />
own IP header. Therefore, TTI bundling was introduced in Rel-8 which combined four subframes<br />
spanning 4 ms. This allowed for a single IP header over a bundled 4 ms TTI that greatly improved the<br />
subframe utilization (from 1/8 to 1/2) and thus the coverage (by more than 3 dB).<br />
This section has provided a high level overview of the key features and capabilities introduced in 3GPP<br />
Rel-8 and Rel-9. The focus of the remainder of this paper is on Rel-10 and beyond; updated status and<br />
significant details of Rel-8 can be found in the February 2010 white paper by 3G <strong>Americas</strong>, 3GPP Mobile<br />
Broadband Innovation Path to <strong>4G</strong>: Release 9, Release 10 and Beyond: HSPA+, LTE/SAE and LTE-<br />
Advanced, 100 while updated status and significant details on Rel-9 can be found in Appendix B of this<br />
paper.<br />
100<br />
3GPP Mobile Broadband Innovation Path to <strong>4G</strong>: Release 9, Release 10 and Beyond: HSPA+, LTE/SAE and LTE-Advanced, 3G<br />
<strong>Americas</strong>, February 2010.<br />
www.4gamericas.org February 2011 Page 45
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