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Saturday, March 12, 2011

HSPA Packet scheduling: An Insight

One of the most important features of HSPA is packet scheduling. The main goal of packet scheduling is to maximize the system throughput while satisfying the QoS requirements of the users.
In this article I provide an insight into how this feature is performed in HSPA. The packet scheduler determines which user the shared channel transmission should be assigned to at a given time. In HSDPA, the packet scheduler can exploit the short-term variations in the radio conditions of different users by selecting those with favorable instantaneous channel conditions
for transmission, as illustrated in the figure below;

This idea is based on the fact that good channel conditions allow for higher data rates (R) by using a higher order modulation and coding schemes which results in increasing the system throughput. In order to quickly obtain up-to-date information on the channel conditions of different users, the functionality of the packet scheduler has been moved from the radio network controller (RNC) in UMTS to the medium access control high-speed (MAC-hs) sublayer at the NodeB, as shown in the figure below;

The MAC-hs is a new sublayer that is added to the MAC layer at the NodeB in HSDPA in order to execute the packet scheduling algorithm. In addition, the time transmission interval (TTI) (i.e., the time between two convective transmissions) has been reduced from 10 ms in UMTS Release 99 to 2 ms in Release 5 that includes HSDPA. This is because it allows the packet scheduler to better exploit the varying channel conditions of different users in its scheduling decisions and to increase the granularity in the scheduling process. It should be noted that favoring users with good channel conditions may prevent those with bad channel conditions from being served and may, therefore, result in starvation. A good design of a scheduling algorithm should take into account not only maximization of the system throughput through service differentiation, but also being fair to users who use the same service and pay the same amount of money. That is, scheduling algorithms should balance the trade-off between maximizing throughput and fairness.

The packet scheduler for HSDPA implemented at the MAC-hs layer of NodeB works as follows; 
Every TTI, each user regularly informs the NodeB of his channel quality condition by sending a report known as a channel quality indicator (CQI) in the uplink to the NodeB. The CQI contains information about the instantaneous channel quality of the user. This information includes the size of the transport block that the NodeB should send to the user, the number of simultaneous channel codes, and the type of modulation and coding schemes that the user can support. NodeB then would select the appropriate mobile user according to the adopted scheduling discipline and send data to the selected user at the specified rates. The user is able to measure his current channel conditions by measuring the power of the received signal from the NodeB and then using a set of models, determine his current supportable data rates (i.e., the rates that he can receive data from the NodeB given his current channel condition). Therefore, users with good channel conditions will enjoy potentially higher supportable data rates by using higher modulation and coding rates, whereas users with bad channel conditions will experience lower data rates instead of adjusting their transmission power.

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