VoIP Episode 4: VoIP over UMTS high speed channels

In the first episode we discussed how the various implementation options compare in terms of performance and complexity. We say that there are various necessary changes to the high speed channels to make them an attractive medium to carry VoIP traffic. Some of these changes are also necessary to reach VoIP capacities that exceed conventional circuit switched capacity. In this post I will discuss some of the suggested changes to the high speed channels to make them suitable to carry VoIP.

• Scheduler changes:
  3GPP defined the structures of the high speed shared channels such that they are optimal to carry large chunks of data. To make the system reactive to users' channel conditions, an intelligent scheduling entity that usually resides in the RNC is moved to base station. Thus if a user happens to be in good channel conditions, the scheduler allocates resources and send them to the user before the favorable channel conditions disappear. The problem with VoIP traffic is that the packets have to be delivered to the user pretty frequently in order to maintain the perception of a good quality connection. The packets can not be queued forever waiting until the user has good channel conditions. Therefore the scheduler algorithms, which were originally developed to be optimal to carry discontinuous chunks of data, have to be adapted to work well with VoIP traffic side-by-side. One of the concepts to enable this is called “priority queuing" which as the name implies, gives priority to delay intolerant traffic such as VoIP. Most of what I describe above applies to HSDPA (downlink). In the uplink, 3GPP included enough functionality in the HSUPA requirements to make it versatile and able to cope with VoIP traffic using a scheme called “Non-Scheduled Transmission”.
• HARQ Retransmission Reduction:
  The HSDPA channel uses also a concept called "HARQ retransmission", where lost packets are retransmitted (partially or completely). The idea is to optimize the air interface efficiency, so instead of sending few highly protected bytes that will be definitely received correctly, the transmitter sends a big chunk of bytes and hope for the best. If part of the data is received incorrectly, then the transmitter can retransmit the packet. This approach is one of the enablers of high speeds achieved on this type of shared channels (see note 1 below). It turns out that the HARQ retransmission concept is a challenge to smooth VoIP operation. This is because the VoIP connection is intolerant to delay, and obviously packet retransmission will increase the round-trip delay, which degrades the call quality. Packet retransmissions also increase the severity of packet jitter at the receiving end: one packet may arrive almost immediately and the next packet may be transmitted several times before successful reception happens. If you eliminate re-transmission altogether the channel efficiency drops and you end up with a reduced cell capacity. (Check note 1 below) .
• Reduce the possibility of code limitation (Fractional DPCH):
  Every user connected to the network using a shared channel requires a number of small dedicated channels to carry signaling and control information. These channels carry very little traffic, but they are numerous. If there are many users in a cell using the HSDPA channel, a large number of dedicated channel is required to support them, and it could happen that the system runs out of channelization codes (if you don’t understand the concept of channelization codes, check this). Therefore, 3GPP included a new feature called Fractional DPCH, which essentially multiplexes the control information of multiple users on one physical channel.
• Continuous Packet Connectivity (CPC):
  This is a huge 3GPP work item which includes many aspects that, although not necessary for VoIP to work, but will benefit its operation and increase the cell VoIP capacity. The most noteworthy of these aspects are:
  
  1. DPCCH gating: The control channels have a discontinuous pattern, which reduces the amount of interference and increases battery life.
  
  2. Modified HS-SCCH operation. HS-SCCH is a channel that carries HSDPA signaling in the downlink. This channel typically consumes large amount of power which eats up a part of the cell capacity. Therefore 3GPP came with various suggestions to reduce its effects by restricting it, or eliminate it altogether.
  
  3. CQI reporting reduction: CQI stands for Channel Quality Indicator, which is a message sent by the terminal to the bas station reporting the channel quality so that the base station can decide what to send to the terminal. As we discussed in the scheduler section above, successful VoIP operation is more concerned with the continuity of the traffic instead of mapping transmission to channel conditions, therefore reducing the rate of CQI signaling will reduce overheads without affecting VoIP quality. This means that more bandwidth is available for VoIP packets instead of overhead.
  
  All in all, CPC is expected to increase VoIP cell capacity by anything from 15% to 40%.
  

Note 1: There is an optimal packet loss rate that will cause the system to operate at the maximum possible capacity. On one hand, high packet loss rates mean that many packets are being retransmitted many times and hence the air interface resource is being wasted on these retransmissions. On the other hand, low packet loss rates mean that the transmitter is not taking chances and is protecting data more than necessary. This will lead to a lower cell capacity.

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