Mobile way to the future

HSDPA will empower UMTS networks by providing higher data rates to end-users. In addition, HSDPA provides lower latency with Round Trip Delays of 70 ms, enabling greater interactive applications like multi-user gaming. The use of HSDPA optimises the investment in the network, as some traffic, normally transported on a dedicated channel, can be supported by HSDPA more efficiently, saving capacity that can be allocated to new users. At least twice as many subscribers per cell should be supported with HSDPA.

HSDPA innovations compared to today's UMTS include increasing range of applications available to the end user, enabling access to broader content because of high-speed downlink transmission, and the increase of data users per cell due to the better spectral efficiency. This will boost the use of applications by the end users, which will generate more revenues for wireless operators. HSDPA will change wireless communications by delivering broadband through wireless access. This is the next big technological advancement needed to increase usage. It will boost usage in business sectors by providing a virtual office environment anywhere with wireless access and it will also trigger usage by the consumer market by offering a greater broadband experience for the end users. HSDPA allows for broadband to be truly mobile for the very first time without the inconvenience of looking for hot spots or wireless access points.

HSDPA will provide the seamless access to all applications already used at home for entertainment like music and video downloads, multiplayer gaming and TV. HSDPA has a great opportunity to enter the triple play market by addressing residential access with a bundle offer for TV, Internet access and voice and mobile services. HSDPA allows up to 3.6 Mbps peak data rate for a Category 6 Mobile per user with a classical Rake receiver and up to 14.4 Mbps peak data rate for a Category 10 mobile per user with advanced receiver solutions.

HSDPA does not require radical changes in the core network except for minor changes due to the higher bandwidth access. For instance, in the 3GPP Rel'5, the maximum throughput set into the signalling protocol has been increased from 2 Mbps to 16 Mbps in order to support the theoretical maximum limit of HSDPA data rate, which is 14.4 Mbps. This is why signalling between the UTRAN, SGSN, and GGSN need to be changed in order to support newly expanded QoS parameters.

Basically, HSDPA introduces a new common High Speed Downlink Shared Channel (HS-DSCH) shared by several users. In addition, it introduces enablers for the high-speed transmission at the physical layer like the use of a shorter TTl (2 ms), the use of adaptive modulation and coding, and the use of fast retransmission based on hybrid ARQ (HARQ) techniques. These key mechanisms are located within the UMTS BTS.

Basically, the downlink HS-DSCH channel is shared in a number of SF 16 codes and time. Within each 2 ms TTl, a constant spreading factor of 16 is used with a maximum of 15 parallel channels for the HS-DSCH. These channels may all be assigned to one user during the TTl, or may be split among several HSDPA users. There is no more power control with HSDPA and the High Speed Downlink Shared Channel is transmitted at a constant power, while the modulation, the coding and the number of codes are changed to adapt to the variations of radio conditions. According to the real time knowledge (every 2 ms) of the radio conditions for each user, HSDPA matches the exact throughput to the radio bandwidth available for every user during the communication, which means higher average throughput and higher spectrum efficiency.

By taking the best of the radio spectrum in a real-time process, the adaptive modulation and coding enables "bursty" traffic, hence higher average throughput. This end-user experience will depend on the number of HSDPA users in the cell, but three to five times higher throughput is expected on the field with HSDPA. It also reduces the interference variation due to link adaptation based on variations in the modulation/coding scheme instead of variations of the transmit power.

It is this use of the 16 QAM modulation and the use of five SF 16 codes that enables the 3.6 Mbps of throughput, which has the capability of a Category 6 Mobile. This modulation enables higher data rates as four bits are transmitted per symbol.

In the indoor environment, the small cell size, the very good and controlled coverage, and low mobility lead to a very high spectrum efficiency and very high data rate per user. Even if the 16 QAM modulation is very sensitive to the radio conditions, this modulation will be used most of the time in an indoor environment. In addition, there is a very low impact on PA power for HSDPA operation, which means the downlink throughput is not significantly affected by the minimum power required for the signalling HS-SCCH channel. However, when dealing with outdoor configurations, the broadband performances are much more challenging due to higher interference at the cell edge and larger cell size compared to indoor coverage for WLAN-type services. Practically, there is a significant impact on PA power for HSDPA operation, i.e., lower downlink throughput due to required power for HS-SCCH. Therefore, HS-SCCH power control is required to reduce impact on HSDPA throughput. HSDPA with MIMO implementation increase the throughput. MIMO increases the capacity due to the multi-stream transmission and code reuse with multiple antennas on both the transmitter and receiver sides.

WiMAX is convenient for long haul data communication and it eliminates the need of expensive leased lines. Aside from this, WiMAX is capable of covering huge geographical areas with theoretical speed of about 70 Mbps. In developing countries, where rural/remote areas remain outside the broadband coverage, can get advantage from WiMAX deployment. Although the speed of HSDPA is not high as WiMAX, it ensures a smoother and quicker deployment than WiMAX. HSDPA utilises the existing cellular network, whereas WiMAX requires completely new infrastructure to initiate the service. In essence, HSDPA is truly mobile than WiMAX.
Information of this article were collected from the following sources:

Nortel HSDPA white paper , www.iec.org

The author is a computer engineer