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HSDPA boosts 3G network's capabilities

Posted: 01 Mar 2006 ?? ?Print Version ?Bookmark and Share

Keywords:rudolf tanner? nick hallam-baker? aeroflex wireless? hsdpa? 3g?

As 3G networks roll out across the world, cellular operators are already thinking about how to offer even higher data rates and enhanced services. While 3G networks now provide 384Kbps services and theoretical data rates as high as 2Mbps when close to the base station, operators still feel compelled to satisfy users' unquenchable thirst for new applications and faster services.

In 2003, the 3GPP finalized high-speed downlink packet access (HSDPA) as part of Release 5 specifications. Over the past year, it has undergone a number of high-profile field trials with cellular operators. Full commercial services are expected to be available soon. Designed as an upgrade to existing 3G networks, HSDPA offers theoretical data rates of up to 14Mbps and reduced latencies to support real-time applications.

HSDPA doesn't just provide users with improved services. The technology introduces techniques that let network operators use and manage their valuable radio spectrum more efficiently. While established cellular systems attempt to transmit data independently of channel conditions by using brute-force techniques based on more power to overcome deep fades, HSDPA applies some intelligence to the problem. In fact, by considering multi-user diversity, HSDPA targets the highest data rates towards users with the best instantaneous channel quality.

How it works
HSDPA is a packet-based technology that allows data to be transmitted to the mobile handset or user equipment (UE) in short (2ms) packets. The amount of data sent in each packet depends on the state of the propagation channel. The base station, known as node B, learns about the quality of the downlink channel using channel quality indicator (CQI) reports from each HSDPA-capable UE in the cell. These CQI reports are made periodically by the UE, and are based on signal-to-interference ratio (SIR) measurements that the UE has made on the downlink pilot channel. Depending on the environment, channel conditions may change rapidly or remain largely static. For example, a user driving through a busy urban environment is likely to experience greater and more rapid changes in channel quality than a stationary user in an open space.

User data is transported over the high-speed downlink shared channel (HS-DSCH). The HS-DSCH is shared among all HSDPA users in the cell so that resources can be either shared equally among several users or allocated to one user. The process of sharing the downlink resource requires intelligent coordination by node B. In addition, HSDPA incorporates adaptive modulation and coding (AMC) techniques, allowing the downlink data rate to each user to be adapted to the user's channel conditions. For example, if interference is low, then node B can transmit a large packet using 16QAM modulation and reduce the number of parity bits. In poor signal conditions, the packet will contain less user data with the HS-DSCH restricted to QPSK modulation, allowing more parity bits for forward error correction.

However, HSDPA must recognize the fact that some packets transmitted by the node B won't be received correctly by the UE. A mechanism known as hybrid automatic repeat request (HARQ) is used to retransmit these erroneous packets. By providing this retransmission functionality in the node B, erroneous packets can be retransmitted quickly without the need for higher protocol layers. This in turn reduces the average packet latency through the system, making popular real-time services a reality.

Purpose of scheduler
While acting as the resource allocation manager, the scheduler determines the timing and quantity of data to be transmitted to each HSDPA user. The scheduling algorithm resides in node B and is responsible for scheduling data for each cell. Locating the scheduler in node B allows this function to quickly react to changing environmental conditions.

Scheduling algorithms can be designed to maximize cell capacity or give each user fair access to the downlink capacity. An optimized algorithm may attempt to meet both of these objectives. An efficient scheduling algorithm will need to consider many different parameters, which may include information about channel quality and its variations such as the CQI, power control and reported packet error rate. Also to be considered are UE capabilities, which determine how much data a UE can process in one time slot, and QoS parameters associated with the data. In addition, a cellular operator may want to prioritize data transmission in favor of the heaviest data users to maximize revenue.

Using an optimized scheduling algorithm can pay big dividends to a cellular operator. Effectively managing a spectrum can increase cell capacity and lower the effective data transmission costs. Indeed, estimates indicate that HSDPA can increase cell capacity by 30 percent and reduce the effective cost per megabyte by 50 percent.

Obviously, turning all this real-time information into a packet-transmission plan is a complex undertaking that needs thorough testing. Unfortunately, even advanced Monte-Carlo simulations can't account for all possible artifacts encountered in a real-world system. To make matters worse, there's a lack of commercial HSDPA handsetsthose that are available support only the lower data rates. The TM500 Test Mobile product range resolves these issues by providing specialized test equipment to ensure successful network roll-outs.

Scheduler testing
The ideal HSDPA scheduler test solution consists of large numbers of UEs, each experiencing and reporting its own independent downlink fading channel conditions. A key requirement for scheduler testing is to consider the effects of the variations in the downlink channel conditions experienced by each UE.

Drive testing is one method of verifying scheduler performance against real fading conditions. However, drive tests are difficult to coordinate, expensive to run and, most important, don't guarantee repeatable channel conditions. The traditional lab-based solution lies in the realm of expensive fading-channel simulators. Clearly, the use of many fading-channel simulators in a test system spells configuration problems and higher costs. A more cost-efficient solution would be to incorporate fading channel simulation into the UE processing. Using the proprietary "mobility model," the TM500 HSDPA multi-UE incorporates the behavior of up to 32 mobiles, each with its own independent fading channel.

Using the mobility model, users can quickly configure realistic fading-channel conditions, including the ITU PA-3 and VA-30 channels specified by the 3GPP for HSPDA conformance. Most importantly, feedback information from each mobile in the multi-UE, including CQI, HARQ response and power-control information, is calculated based on the instantaneous fading channel. From the node B's perspective, the multi-UE provides the same information as real UEs in real fading environments, but with repeatable and deterministic scenarios.

Optimizing scheduler
Basic schedulers take the form of simple algorithms, such as the "round-robin" or "proportionally fair." However, careful mathematical analysis of the problem can reveal areas for optimization. By understanding the complexity of HSDPA, one sees the SNR needed to receive an HSDPA packet with a particular transport format resource index (TFRI) and HS-PDSCH codes. The TFRI corresponds to the HSPDA packet's size and is defined in the 3GPP specifications. Index 1 to 15 represent the number of HS-PDSCH codes for QPSK modulation, while index 16 to 31 (number of codes + 15) represents 16QAM transmissions. Note that the SNR needed to receive very large packets doesn't increase linearly. Also, the surface is not smooth, particularly for large packet sizes. In practice, this means that careful selection of the transmission parameters allows more user data to be transmitted in poorer signal conditions.

Looking ahead, as services and networks become more IP-centric, the scheduler's task will become even more critical. The introduction of high-speed uplink packet access in the 3GPP release 6 specifications will again require node B developers to create and design new scheduling algorithms to manage uplink capacity among many users.

Rudolf Tanner, System and Technology Manager
Nick Hallam-Baker, Product Manager
Aeroflex Inc.

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