We are never really happy living in the present and always love to discover or innovate ‘the next big thing.’ That’s how we have advanced from 2G to 3G, and now have got to LTE and many of its advanced features to fulfill our continuously increasing high-speed data greed, better user experience and high quality. High-speed data demands are leading to use of higher MIMO configuration and higher modulation schemes. Although these advanced communication technologies are meant for providing high data throughput, there are certain scenarios where low data transmission can provide better quality and increased user satisfaction. Maintaining quality becomes more challenging when user equipment (UE) has to do hand-in/hand-outs between different cells or channel quality estimated by UE is not very good. There’s where the problem of packet loss and degraded quality creeps in. In this blog, I will try to describe one problem scenario and an approach for a solution to support my views.
While implementing the Aricent LTE Femto solution in fields, we came across similar challenges and observed that in many cases users experience the voice to be in mute during an ongoing voice call. This happens when mobile device hands in from Macro to Femto system which was configured as Open Loop MIMO using 2 antennas configuration. The reason for mute voice is due to loss of many voice packets when cell switching happens. When analyzed, it has been found that during this transition time, there will be many accumulated packets (termed forwarded packets) which need transmission. During handover state, when UE is aligning with eNodeB (eNB), it may not be able to decode high number of data transport bits leading to negative HARQ acknowledgements which causes both delay and loss of these packets.
As part of the solution, we then decided to take two approaches to reduce these negative acknowledgements and prevent packet loss:
- The first part was to transmit packets, during this transition state, in single transport block on two antenna/layer using Transmit (tx) diversity. As during handover procedure, UE takes time to report rank indicator, Femto system shouldn’t use two transport blocks for data transmission. Data transmission should happen on single transport block unless UE reports rank indicator. With this approach, UE will have better chances to decode these packets when transmitted in single TB using tx diversity. Using tx diversity can also help to improve voice quality even if UE is stable. Voice packets are generated in small chunks with a regular intervals of 20 milli seconds, so make a small data to be transmitted at a given time. These packets can be transmitted anytime using tx diversity even if UE reports high rank giving more probability in single transmission.
- The second part was to work on a low modulation scheme such as the (Quadrature Phase Shift Keying) (QPSK) during handover or for application which generates very low data. Applications like voice and videos usually generate small packets at regular intervals. Transmitting these packets with QPSK modulation will increase chances for UE to decode them at the first attempt without retransmissions leading to less delay and high reliability. As voice packets can be easily identified from QCI of DRB, it is easy to use low QPSK for these packets.
After applying this solution to our system, we saw a considerable improvement in the voice call quality. Also, other applications also indicate less packet drop during handover time.
There are certain scenarios and use cases where data transmission using lower MIMO configuration and low modulation schemes yield better results. Although we are using TTI bundling to achieve better voice quality, the quality of these applications can be improved by using a few simple methods. So transferring voice packets on lower MIMO configuration and low modulation schemes and during handover or during low channel quality time can help immensely.