High-Speed Downlink Packet Access (HSDPA) is a new mobile telephony protocol that provides a smooth evolutionary path for UMTS networks to higher data rates and higher capacities, in the same way as EDGE does in the GSM world.
Sometimes referred to as a 3.5G technology, HSDPA is an evolution of the WCDMA standard, designed to support data transmission rates of 4 to 5 times that of current generation 3G networks and 15 times faster than GPRS. While this equates to a theoretical maximum data transfer speed of between 10 and 14Mbit/s, real life end-user speeds will be in the range of 2 to 3 Mbit/s. Shared among users in an adequately covered area, this will provide each user with a 300K to 1Mbit/s downlink, comparable to prevailing wireless LAN standards and domestic fixed line broadband. Uplinks will be 128Kbit/s – double that of current UMTS systems.
In a UMTS network, the base station (known in a UMTS network as a Node B) is a transmission and reception station that acts as the access point for the user to the network and handles network traffic; the radio network controller (RNC) has overall control of the resources in the Node Bs and is also responsible for handovers in the network; the serving and gateway nodes handle and route packet switched data traffic while the mobile switching centre handles circuit switched traffic (e.g. voice or video conferencing).
HSDPA is primarily implemented in the Node B and the RNC. The technology minimises transmission delays in the network by placing key processing at the base station and thus closer to the air interface and the user. The new technology also introduces an additional transport channel called the highspeed downlink shared channel (HS-DSCH). Up to 15 of these can operate in the 5 MHz WCDMA radio channel. Each uses a fixed spreading factor of 16. User transmissions are assigned to one or more of these channels for a short transmission time interval of 2ms, significantly less than the interval of 10 to 20ms used in WCDMA. The network can then readjust which users are assigned which HS-DSCH every 2ms. The result is that resources are assigned in both time (the TTI interval) and code domains (the HS-DSCH channels).
In addition to Quadrature Phase Shift Keying (QPSK), the modulation used in WCDMA, under good radio conditions, HSDPA also uses an advanced modulation scheme, 16 Quadrature Amplitude Modulation (16 QAM). The benefit of 16 QAM is that four bits of data are transmitted in each radio symbol compared to two with QPSK. So while use of 16 QAM enables increased data throughput when conditions permit, QPSK allows for reliable transmission under adverse conditions.
HSDPA also employs adaptive modulation and coding techniques. This means that the base station schedules the transmission of data packets to a user by matching the user’s priority and estimated channel operating environment with the appropriate coding and modulation scheme. This maximises capacity and ensures all users benefit from the best possible data rate. Another aspect of these techniques is the use of different levels of forward error correction (channel coding), depending on the condition of the radio channel. For example, a three quarter coding rate means that three quarters of the bits transmitted are user bits and one quarter are error correcting bits.
The following table shows the different throughput rates achieved based on the modulation, the coding rate, and the number of HS-DSCH codes in use. Note that the peak rate of 14.4Mbit/s occurs with a coding rate of 4/4, 16 QAM and all 15 codes in use.
| Modulation | Coding Rate | Throughput
with 5 codes |
Throughput with 10 codes |
Throughput
with 15 codes |
|---|---|---|---|---|
| QPSK | 1/4 | 600kbit/s | 1.2Mbit/s | 1.8Mbit/s |
| 2/4 | 1.2Mbit/s | 2.4Mbit/s | 3.6Mbit/s | |
| 3/4 | 1.8Mbit/s | 3.6Mbit/s | 5.4Mbit/s | |
| 16 QAM | 2/4 | 2.4Mbit/s | 4.8Mbit/s | 7.2Mbit/s |
| 3/4 | 3.6Mbit/s | 7.2Mbit/s | 10.7Mbit/s |
Finally, HSDPA provides re-transmission mechanisms for faster error correction. In a spread spectrum network, it’s the role of the user device to both acknowledge the receipt of data and communicate key information relating to issues like channel condition and power control back to the Node B. With HSDPA, when a base station despatches a data packet to a handset, the Node B then waits for an acknowledgement. If it does not receive one within a prescribed time, it assumes that the data packet was lost and retransmits it.
The first phase of HSDPA has been specified in 3GPP release 5. The second phase, specified in 3GPP release 6, will introduce antenna array processing technologies to enhance the peak data rate to around 30 Mbit/s:
- smart antenna using beamforming techniques for mobiles with one antenna
- MIMO technologies for mobiles with from two to four antennas.
Finally, phase 3 will see the introduction of a new air interface to HSPDA to increase the average bitrate towards 50 Mbit/s:
- an Orthogonal Frequency Division Multiplexing (OFDM) physical layer in combination with higher modulation schemes and array processing
- shared medium access control (MAC) with fast scheduling to optimise performance by selecting dedicated sets of subcarriers for each mobile according to the quality of the air interface
- multi-standard MAC as a control entity to realise fast switching between Orthogonal Frequency Division Multiple Access (OFDMA) and Code Division Multiple Access (CDMA) channels.
It is unclear which of HSDPA and WiMAX will win out in the long run. In the early stages, it is expected that HSDPA will focus on mobility and data and voice from cellular platforms, and WiMAX will be more about delivering broadband data to the enterprise and to under-served rural areas. Eventually though, the two technologies will intersect as HSDPA gets faster and WiMAX’s mobility improves.