There's no doubt that wideband CDMA (W-CDMA) networks will make the delivery of data to end users a more feasible option for today's wireless carriers. But, to make a real impact for end users, carriers need technologies that will allow them to increase downlink throughput and, in turn, offer advanced quality of service (QoS) capabilities and speeds that rival those delivered by cable modem and DSL service providers.
Fortunately an answer is on the way. The 3G Partnership Project (3GPP) group has built a specification called the high-speed downlink packet data access (HSDPA) protocol that allows carriers to increase downlink throughput over W-CDMA links.
3GPP-standardized HSDPA provides a two-fold improvement in network capacity as well as data speeds up to five times (over 10 Mbit/s) higher than those in even the most advanced 3G networks. This new technology also shortens the round-trip time between network and terminal, while reducing variances in downlink transmission delay. Those performance advances translate directly into improved network performance and higher subscriber satisfaction.
And because HSDPA is an extension of the GSM family, it builds directly on the economies of scale offered by the world's most popular mobile technology. To fully appreciate the importance of HSDPA, let's take a detailed look at how this new concept will fit in the network of tomorrow.
HSDPA: What's it All About
HSDPA is based on W-CDMA evolution and is standardized as an element of the 3GPP Release 5 WCDMA specification, and is in fact the key new feature in this latest release.
HSDPA is built on a distributed architecture that achieves low delay link adaptation by placing key processing at the base station and thus closer to the air interface (Figure 1). HSDPA leverages methods that are well established within existing GSM/EDGE standards, including fast physical layer (L1) retransmission combining and link adaptation techniques, to deliver significantly improved packet data throughput performance.
Figure 1: Diagram illustrating how HSDPA-enabled wireless networks will operate.
HSDPA employs several important new technological advances. These include: scheduling for the downlink packet data operation at the base station; higher-order modulation; adaptive modulation and coding; hybrid automatic repeat request (HARQ); physical layer feedback of the momentary channel condition; and a new transport channel type known as high-speed downlink shared channel (HS-DSCH) that allows several users to share the air interface channel.
The capacity, quality and cost/performance advantages of HSDPA yield measurable benefits for network operators, and, in turn, their subscribers. For operators, this backwards-compatible upgrade to current W-CDMA networks is a logical and cost-efficient next step in network evolution. When deployed, HSDPA can co-exist on the same carrier as the current Release'99 WCDMA services, allowing operators to introduce greater capacity and higher data speeds into existing WCDMA networks.
Operators can leverage this solution to support considerably higher number of high data rate users on a single radio carrier. HSDPA makes true mass-market mobile IP multimedia possible and will drive the consumption of data-heavy services while at the same time reducing the cost-per-bit of service delivery, thus boosting both revenue and bottom-line network profits.
For data-hungry mobile subscribers, the performance advantages of HSDPA translate into shorter service response times, less delay and faster perceived connections. Users can also download packet-data over HSDPA while conducting a simultaneous speech call.
Key HSDPA Technologies
HSDPA replaces to basic features of WCDMAthe variable spreading factor and fast power controlwith adaptive modulation and coding, extensive multi-code operation, and a fast and spectrally efficient retransmission strategies. Let's look at each of these in more detail.
1. Adaptive modulation and coding
In current-generation W-CDMA networks, power control dynamics are on the order of 20 dB in the downlink and 70 dB in the uplink. W-CDMA downlink power control dynamics are limited by potential interference between users on parallel code channels and by the nature of W-CDMA base station implementations. For W-CDMA users close to the base station the power control cannot reduce power optimally, and that reducing power beyond the 20 dB dynamics would have only a marginal impact on capacity.
HSDPA utilizes advanced link adaptation and adaptive modulation and coding to ensure all users enjoy the highest possible data rate. This upgrade technology adapts the modulation scheme and coding to the quality of the appropriate radio link.
While the spreading factor is fixed, the coding rate can vary between 1/4 and 3/4, and the HSDPA specification supports the use of five, 10 or 15 multicodes. This more robust coding, fast HARQ, and multi-code operation eliminates the need for variable spreading factor. This approach also allows users with good signal quality (higher coding rate) typically close to the base station, and those at the more distant edge of the cell (lower coding rate) to each receive an optimum available data rate.
2. Fast Scheduling
While current W-CDMA technology schedules data traffic at the radio controller level, HSDPA moves these decisions to the base station, and thus closer to the air interface. HSDPA uses information about channel quality, terminal capabilities, QoS, and power/code availability to achieve more efficient scheduling of data packet transmissions.
By moving these intelligent network operations to the base station, this approach allows the system to take full advantage of short-term variations, and thus to speed and simplify the critical transmission scheduling process. The HSDPA approach can, for example, manage scheduling to track the fast fading of the users and when conditions are favorable to allocate most of the cell capacity to a single user for a very short period of time.
3 Fast PHY Re-Transmissions
When a link error happens, due to interference or other causes, a mobile terminal immediately requests the retransmission of the data packets. While current-generation W-CDMA networks handle those retransmission requests by the radio network controller, in HSDPA those retransmission requests are managed in the base station.
Using this approach, in packets are combined at the physical (PHY) layer, the terminals stores the received data packets in soft memory. If decoding has failed, the new transmission is combined with the old transmission before channel decoding. The HSDPA approach allows previously transmitted bits from the original transmission to be combined with the retransmission. This combining strategy provides improved decoding efficiencies and diversity gains while minimizing the need for additional repeat requests.
This operation is denoted as a hybrid automatic repeat request (HARQ), which is an operation designed to reduce the delay and increase the efficiency of re-transmitting data. Layer 1 HARQ control is situated in the Node B, or base station, thus removing retransmission-related scheduling and storing from the radio network controller, as illustrated in Figure 2. This HARQ approach avoids lub delay and measurably reduces the resulting retransmission delay.
Figure 2: HSDPA principle with Node B-based HARQ.
4. Channel Quality Feedback
At the base station, HSDPA gathers and utilizes estimates of the channel quality of each active user. This feedback provides current information on a wide range of channel variable physical layer conditions, including power control, ACK/NACK ratio, QoS, and HSDPA-specific user feedback.
5. High-Speed Downlink Shared Channel (HS-DSCH)
While Release '99/Release 4 contain the support for the downlink shared channel (DSCH), the HSDPA operation is carried on the high-speed downlink shared channel (HS-DSCH). This higher-speed approach uses a 2-ms frame length, compared to frame lengths of 10, 20, 40 or 80 ms with DSCH. While DSCH utilizes a spreading factor that may vary from 256 and 4, the HS-DSCH uses a fixed spreading factor of 16 with a maximum of 15 codes.
HS-DSCH supports 16-level quadrature amplitude modulation (16-QAM), link adaptation, and the combining of retransmissions at the physical layer with HARQ. HSDPA also leverages a high-speed shared control channel (HS-SCCH) to carry the required modulation and retransmission information. An uplink high-speed dedicated physical control channel (HS-DPCCH) carries ARQ acknowledgements, downlink quality feedback and other necessary control information on the uplink.
HSDPA provides a number of significant performance improvements when compared to previous or alternative technologies. Let's look at some of these benefits, starting with bit-rate improvements.
HSDPA extends the W-CDMA bit rates up to 10 Mbps, achieving higher theoretical peak rates with higher-order modulation (16-QAM) and with adaptive coding and modulation schemes. The maximum bit rate with 5.3 Mbit/s with QPSK modulation and 10.7 Mbit/s with 16-QAM. Theoretical bit rates of up to 14.4 Mbit/s could be achieved with no channel coding. The terminal capability classes start from 900 kbit/s and 1.8 Mbit/s with QPSK only modulation and 3.6 Mbit/s with 16-QAM modulation. The highest capability class supports the maximum theoretical bit rate of 14.4 Mbit/s.
In practical deployments, the HSDPA approach delivers peak user bit rates over 100% higher than possible using Release'99-based design. With bit rates that are comparable to DSL modem rates, HS-DSCH can deliver user bit rates in large macro cell environment exceeding 1 Mbit/s, and rates in small micro cells up to 5 Mbit/s. The HSDPA approach supports both non-real-time UMTS QoS classes and real-time UMTS QoS classes with guaranteed bit rates.
HSDPA's shorter 2-ms frame length supports a significantly reduced round trip time, which enables shorter network latency and better response times when user together with fast Layer 1 retransmissions to ensure minimum delay variations.
Cell throughput, as defined by the total number of bits per second transmitted to the users through a single cell, increases 100% with HSDPA when compared to the Release '99. This is because HSDPA's use of HARQ combines packet retransmission with the earlier transmission, and thus no transmissions are wasted. The 16-QAM modulation available in HSDPA provides higher bit rates than QPSK of Release'99 with the same usage of orthogonal codes. The highest throughput is obtained with low inter-path interference and low inter-cell interference conditions.
In microcell designs, the HS-DSCH can support up to 5 Mbit/s per sector per carrier, or 1 bit/s/Hz/cell. Example results are shown in Figure 3 for the macrocell and microcell environments, with more details and results in different environments.1
Figure 3: Example HSDPA performance in macrocell and microcell environments.
As subscriber counts and data traffic volumes grow in W-CDMA networks, operators will seek to improve peak data rates, QoS, and spectral efficiency for downlink asymmetrical and bursty packet data services. HSDPA promises to achieve those objectives by building on and extending the capabilities of Release '99 specifications.
As described here, HSDPA offers breakthrough advances in W-CDMA network packet data capacity, enhanced spectral and RAN hardware efficiencies, and streamlined network implementations. Those improvements translate directly into lower delivery cost-per-bit, faster and more available services, and a network that is positioned to compete more effectively in the data-centric markets of the future.
1. H.Holma and A.Toskala., WCDMA for UMTS, Wiley 2002.
About the Author
Antti Toskala is the HSDPA chief architect at Nokia Networks. He holds an M.Sc in engineering and has acted as the chair of 3GPP TSG RAN WG1. Antti can be reached at firstname.lastname@example.org.