Surfing, soaring from Web pages to music and video. Browsing Web sites without barriers. Choosing a high-quality song and having it ready for listening in the blink of an eye. Downloading e-mails with vacation videos and watching them a few seconds later. All this is taken for granted by people who surf the net from their personal computers (PCs) on asynchronous digital subscriber line (ADSL) connections. Still a dream for people in parks, cars, or at tram stops with their mobile phones in their hands … until now. Today it is possible.

High-speed downlink packet access (HSDPA) is the name of the technology that unleashes high-speed connections on mobile phones and portable computers: nearly 2 MBps at the outset, double that within the year, up to more than 7 MBps in 2007. These are theoretical peak speeds, obtainable under optimal conditions, but the predicted standard of an average of 2 MBps is certainly nothing to scoff at. In addition to speed for individual users, HSDPA also increases the number of users in a given cell at one time.

HSDPA is the first technology to offer multiple megabytes of bandwidth per second in full mobility, i.e., even in the car or on the train. It is a development of Universal Mobile Telecommunications System (UMTS) technology, which means it can also rely on existing networks as a fallback: in areas without HSDPA coverage, terminals will be able to connect seamlessly, i.e., without disrupting the connection, to UMTS, enhanced data rates for Global System for Mobile Communications evolution (EDGE), and general packet radio service (GPRS) networks. This will help guarantee continuity of service throughout all areas of Italy already covered by 2.5G and 3G networks.

Upgrading the UMTS network will not take long: Telecom Italia Mobility (TIM) plans to extend HSDPA coverage to nearly the whole UMTS network by 2008, covering 86 percent of the Italian population. As early as this summer, TIM customers will have access to a HSDPA network that will be extended to cover all the main Italian cities by the end of the year. The upgrade requires a commitment of energy and resources to all components of the network: radio access and core equipment must be brought into line with the new services and connections must be capable of supporting the increased bandwidth offered to users on the radio segment.

HSDPA requires the use of new terminals with greater processing speeds and more memory. The first 1.8 MBps PC cards are currently on the market, and the first Samsung, LG, and Huawei mobile phones will arrive shortly; 3.6 MBps PC cards and terminals are slated to hit the market before the year is out. The new terminals will combine HSDPA features with higher-quality graphics, audio, and video, enabling better services and content to be offered.

The world's largest mobile operators, including Cingular Wireless in the United States, SK Telecom in South Korea, and T-Mobile in Germany, are all implementing the new technology along with TIM.

Super High-Speed Services
High bit rates are most beneficial to Web surfing and downloading content, reducing the average time needed to download images, music, and video by 65 percent compared to UMTS networks. This means that the time it takes to download a 1 MB e-mail message could be cut down to 10 to 15 seconds by the end of the year, and eventually to the predicted standard of five seconds. Mobile phones could then boast the same audio quality as an iPod, downloading even 3 MB or 4 MB songs in no time at all; users could watch mobile TV at 300 KBps even when many other users are connected to the same cell.

Greater bandwidth is not the only factor in speeding up services: latency reduction, i.e., a decrease in the time the network takes to respond, also works in favor of higher speed. Instant messaging (IM) and push-to-talk over cellular (PoC) messages will be faster, and users will even be able to play Doom or other highly interactive multiplayer games on-line with friends.

HSDPA enhances the channel that leads from the network to the terminal: at the outset the return channel, from the terminal to the network, will use UMTS at 64 KBps, soon to become 384 KBps. In 2008 high-speed uplink packet access (HSUPA) will become available, expanding the bandwidth of the uplink segment and enabling symmetric broadband services to be offered, even between several users: Push-to-view, for example, may be used to exchange video in walkie-talkie style, and Turbo Conference may be used to share music or video during a voice conference.

HSDPA and Other Wireless Broadband Technology
HSDPA is only one of the wireless broadband technologies currently being launched or developed: digital video broadcast - handheld (DVB-H) and wireless broadband (WiBro) will also provide high-bandwidth wireless, but with significantly different characteristics.

DVB-H is the mobile version of digital terrestrial television, using its radio frequencies to transmit classic television programming to mobile phones at quality comparable to that of home TV. That's not all: it also enables a variety of new services such as interactive TV to be offered, which will allow users to make purchases on-line, consult Web pages with further information, and download from a range of offered content.

DVB-H technology is best suited for broadcast-style services (in which a radio station sends the same content to all users) with coverage that will reach 75 percent of the population by 2006 and will give users fully mobile access to services, even in environments such as airports and shopping centers, as well as in cars and buses. The system's total capacity is 11 MBps, which may be used to provide up to 20 high-quality broadcast television channels.

WiBro is Samsung's wireless interoperability for microwave access (WiMAX) solution. It is an offshoot of the Wi-Fi family, adding mobility and giving users peak speeds of around 20 MBps, enabling a vast array of Internet protocol (IP)-based services, such as mobile office, Web browsing, content download, video telephony, and videoconferencing to be offered.

HSDPA Technology
The system is based on applying advanced radio broadcasting techniques that allow the following:

1. Downlink transmission capacity to be increased (i.e., from the base radio station to the mobile terminal) in order to manage more users per cell or provide higher speed to a limited number of users; the system's theoretical maximum speed is 14 MBps, compared to the 2 MBps provided by UMTS.
2. The access network's transmission delays and latency times to be reduced: round-trip time is in fact cut from 200 ms with UMTS to around 100 ms with HSDPA.

Figure 1: Radio access; The Segment of the Network on which HSDPA Is Implemented; Enhancement of Transmission Links to Support Increased Bandwidth; Adaptation of Core Network Equipment; PSTN ISDN Networks; Data Packet Networks

The main results that HSDPA achieves from a technological point of view are as follows:

1. Increased spectrum efficiency, i.e., the number of bits (amount of information) the radio channel can handle.
2. Improved transmission of high-speed peak bursts. Dedicated channels provided by UMTS are not an ideal solution for intermittent sources. HSDPA will be introduced along with high-speed downlink shared channel (HS-DSCH), which is shared by several users and has faster mechanisms for assigning and freeing up resources.

Figure 2: Portion of Capacity Set Aside for HSDPA; Cell's Total Capacity

The HS-DSCH shared channel can be obtained by setting aside part of the cell's downlink capacity. Consequently, the speed that individual users can reach depends on the characteristics of the user's mobile terminal and the network configuration, i.e., the resources set aside for HSDPA.

The total increase in bandwidth that may be obtained within a cell using HSDPA is based on the following radio techniques:

• Adaptive modulation and encoding — This technique varies the type of modulation (up to 16-quadrature amplitude modulation [QAM]) and error correction based on the quality of the radio channel. In other words, the varying quality of the radio channel is exploited by transmitting to a particular user when the channel quality is good. This allows a greater number of bits to be sent over the same band by using 16-QAM and less burdensome error-correction codes.
• Hybrid adaptive response request (ARQ) — Packet transmission involves re-transmitting packets that are found to be corrupt after decoding and error control. Conventional ARQ systems discard corrupt information units and request re-transmission. H-ARQ combines the error-protection bits from the first transmission of the packet with those from later re-transmissions, thereby increasing the probability that errors will be corrected.
• Packet scheduling — The scheduler for the shared channel (HS-DSCH) is not located in the RNC as is the case for dedicated channels; instead it is located in node B, i.e., closer to the terminal. This ensures greater speed and efficiency of channel management and error correction systems.