HSDPA (High Speed Downlink Packet Access)

HSDPA is is an advanced protocol for mobile telephone data transmission. HSDPA is an evolved form of W-CDMA (Wideband Code Division Multiple Access) technology. HSDPA promises to provide download speeds equivalent to an ADSL (Asymmetric Digital Subscriber Line) line on your mobile phone. The technology standard used in HSDPA is called 3.5G.

The main goal of HSDPA is to allow instantaneous bit rates upto 10Mbps for best-effort packet data services with certain bounds on delay and capacity. The HSDPA channel is a resource that is shared among several users in the mobile communication system. By using a fast scheduler located at the base station, the HSDPA channel can be assigned to the user with the currently best channel, i.e., the user that can transmit with the highest data rate. The idea behind this is that all users shall only use the HSDPA channel when their own downlink is good, and let other users utilize the channel when it is anyway bad. Some of the basic principles used in HSDPA are fast link adaptation, fast scheduling and fast retransmissions of erroneously received packets.

In the HS-DCH channel, the variable spreading factor and fast power control of the WCDMA channels are done away with. The new technology uses Adaptive Modulation and Coding (AMC), fast packet scheduling at the Base Station and fast retransmissions from the Base Station. The retransmissions are done through Hybrid Automatic Repeat Request (HARQ), where the concept of incremental redundancy comes into play. Here, the retransmissions have different codes of the data against the codes of the original transmission. In other words, the data received at the base station is ‘processed’ to manage any corruption the data may have gone through at the time of the original transmission. The net result of this process is that the user device can receive error free packets, thus ensuring better quality to the video and sound.

The success of HSDPA will also depend on many other factors. There are other technologies like CDMA2000, 1xEV-DO and WiMAX that promise high data transmission speeds. Further, HSDPA is an improvement upon the W-CDMA standard, so it can gain acceptance only in places where W-CDMA has been deployed successfully.

HSDPA Features Explanation

Adaptive Modulation and Coding (AMC): One of the major techniques introduced in WCDMA is power control. The idea is to increase the transmission power when the quality of the received signal is poor and decrease the transmission power when the quality of the received signal exceeds a given threshold. This results in reliable communication between the transmitter and the receiver. Also, since the power control technique reduces the unnecessary intercell and intracell interference caused by excessive transmit power, the overall system capacity is increased.

Hybrid ARQ (HARQ): In order to make sure that the data reaches the terminal error free, two basic error-control strategies used in any data communication are forward error correction and Automatic repeat request.

Scheduler: The scheduler for HSDPA is referred to as being fast due to the fact that, compared with Release 99 specifications; the scheduler is moved from RNC to node Bs to reduce delays so faster scheduling decisions can be made. In addition to other functionalities, such as the choice of redundancy version and modulation and coding scheme, a fundamental task of the scheduler for HSDPA is to schedule the transmission for users. The data to be transmitted to users are placed in different queues in a buffer and the scheduler needs to determine the sequential order in which the data streams are sent.