On Flow Control and Scheduling in Time-Shared Wireless Packet Data Channels Patrick Hosein and Rath Vannithamby Ericsson Inc. 5012 Wateridge Vista Dr., San Diego, CA 92121, US Email: {patrick.hosein, rath.vannithamby}@ericsson.com Abstract— In high-speed wireless packet data networks such as 1xEV-DV, 1xEV-DO and HSDPA, the mobiles periodically (on the order of 1-2 ms) report channel quality information to the Base Station which then uses this information to schedule mobiles on the forward link over a slotted time-shared channel. The rate at which a mobile is served varies widely (by orders of magnitude) from slot to slot. Since each time a mobile switches to a new cell all data queued at the old cell must be forwarded on to the new cell for transmission it is desirable to keep only a limited amount of queued data at the cell. Therefore, due to the fluctuating servicing of data, the fluctuating arrival of data and the desire to maintain a small queue at the Base Station, flow control between the Base Station Controller and the Base Station is necessary. In this paper we consider this flow control problem and its interaction with the scheduling mechanism at the Base Station. We show that, for best effort traffic, instead of controlling the flow for each individual user one can maintain good performance by controlling the flow to a shared buffer. This results in reduced signaling overhead. We also show that for QoS traffic, minimal flow control is necessary. I. I NTRODUCTION High-speed wireless packet data support on the forward link is included in all third generation (3G) standards such as IS- 2000 (also called 1xEV-DV [1]), IS-856 (also called 1xEV- DO [2]) and HSDPA [3]. This support was included because of the increase in data usage and the increasing demands of new data applications being introduced by operators. However, increasing capacity alone is insufficient. Traditional wireless networks supported circuit switched services over fixed rate (power controlled) channels. Since the arrival rate and service rate were constant, buffer management was not necessary. In 3G forward link packet data channels, the arrival rate of packets (e.g. for best effort services) as well as the service rate provided to each user both fluctuate widely and hence proper buffer management is critical for optimal performance. Best effort traffic such as web browsing tends to be bursty by nature. In addition, TCP flow control works by increasing the packet rate until packets are lost which is taken as a sign of congestion. TCP flow control then backs-off and the process is repeated providing the typical saw-tooth throughput. Therefore the packet arrival rate for a mobile user tends to be bursty. In addition, the forward packet data channel of 3G networks is time-shared and users are typically served when in good radio conditions. Therefore the intra-service time of a user is stochastic. Furthermore, when a user is served the rate at which it is served varies by several orders of magnitude. The PDSN BSC BSC BTS BTS MS MS MS MS MS Scheduler Scheduler Flow Control RLP H-ARQ Fig. 1. C-RAN Packet Data Network Architecture throughput of a user can therefore be highly variable. Note however, that appropriate scheduling of users can reduce this variation and we will see that such schedulers are required for QoS guaranteed services. In a typical 3G network (e.g. see Figure 1 for the case of a CDMA2000 based network), data enters the CDMA Radio Access Network (C-RAN) at a Packet Data Switching Node then to the Base Station Controller (BSC), then to the Base Station (BS) and finally to the mobile over the radio link. If no flow control is performed on any of the connecting (wired) links then data will be queued at the BS for transmission over the radio link since this typically forms the bottleneck due to the fact that it is a limited resource (the capacity of the wired links can always be increased but the radio link is limited by the spectrum assigned). During the lifetime of a mobile, multiple cells may be in- volved in providing its service. Whenever the mobile changes its forward link serving cell to a neighboring cell all data that was queued at the source cell must be re-transmitted (by the BSC) to the destination cell. This requires bandwidth on the inter-BSC link and the process introduces a pause in service to the mobile. In order to reduce the bandwidth overhead and the delay introduced by cell switching, the amount of data queued at the BS should therefore be kept small. Controlling flow between the BSC and BS to maintain small queues at the BS and instead buffering the data at the BSC can accomplish