Glossary 350

HSDPA: Shifting Gears Into 3.5G

Incorporating HSDPA in Release 5 of the 3GPP W-CDMA specification is the most significant change on the RF side since Release 99 five years ago.

Just when the quest for bandwidth is accelerating competition among wireless technologies, W-CDMA appears to have hit a speed bump. W-CDMA technology, which provides the radio interface in the 3G UMTS mobile system defined by the 3GPP, theoretically can deliver peak data rates up to 2.4 Mb/s. In actual networks, though, the average data throughput rate reportedly doesn�t go much beyond 384 kb/s.

Release 5 of the 3GPP W-CDMA specification adds HSDPA technology in an effort to make the system more efficient for bandwidth-intensive data applications. A W-CDMA network upgraded to HSDPA will support downlink data rates well over 2 Mb/s, up to a theoretical 14 Mb/s. Because the new technology is backwards compatible with 3GPP Release 99, voice and data applications developed for W-CDMA still can be run on the upgraded networks, and the same radio channel will support W-CDMA and HSDPA services simultaneously.

Although industry predictions regarding the ultimate performance of HSDPA vary, it likely will increase W-CDMA downlink speeds by a factor of five, double the network capacity, and support a greater number of users on the network. With these significant improvements for data, W-CDMA systems will be able to shift gears and move ahead to 3.5G�the enhanced performance enabled by this latest inter-generational mobile communication technology.

Mobile carriers committed to W-CDMA are pushing for quick development of the HSDPA network and UE to keep them competitive with 1�EV-DO- and 1�EV-DV-based rivals. That means design and test engineers must gain a thorough understanding of the changes in channel coding and physical parameters introduced by HSDPA and the dynamic nature of the technology.

What Is New With HSDPA?
To improve W-CDMA system performance, HSDPA makes a number of changes to the radio interface that mainly affect the physical and transport layers:

� Shorter radio frame.

� New high-speed downlink channels.

� Use of 16 QAM in addition to QPSK modulation.

� Code multiplexing combined with time multiplexing.

� A new uplink control channel.

� Fast link adaptation using AMC.

� Use of HARQ.

� MAC scheduling function moved to Node-B.

The HSDPA radio frame, actually a subframe in the W-CDMA architecture, is 2 ms in length, equivalent to three of the currently defined W-CDMA slots. There are five HSDPA subframes in a 10-ms W-CDMA frame. User data transmissions can be assigned to one or more physical channels for a shorter duration, allowing the network to readjust its resource allocation in time as well as in the code domain.

HSDPA introduces new physical channels and a new transport channel. Two new physical channel types are added to the downlink: the HS-PDSCH that handles the payload data and the HS-SCCH that carries the UE identity and channel parameters of the associated HS-PDSCH. Also added in the downlink is a new transport channel, the HS-DSCH.

HSDPA adds one uplink physical channel, the HS-DPCCH, for carrying the HARQ ACK and CQI information.

With these enhancements, layer 2 (the MAC layer) can map existing logical channels DCCH and DTCH onto the HS-DSCH. Layer 1, in turn, maps the transport channel, HS-DSCH, onto up to 15 channels (HS-PDSCH). The physical layer then creates the HS-SCCH and HS-DPCCH to control and assist with HS-DSCH transmission.

Downlink Transport Channel Coding
The HS-DSCH is evolved from the DSCH introduced in W-CDMA Release 99 to enable time-multiplexing different user transmissions. To obtain higher data rates and greater spectral efficiency, the fast power control and variable spreading factor of the DSCH are replaced in Release 5 by short packet size, multicode operation, and techniques such as AMC and HARQ on the HS-DSCH.

The channel coding always is 1/3 rate (for every bit that goes into the coder, three bits come out), based on the Release 99 1/3 turbo encoder. The effective code rate varies, however, depending on the parameters applied during the two-stage HARQ rate-matching process.

During this process, the number of bits at the output of the channel coder is matched to the total number of bits of the HS-PDSCH set to which the HS-DSCH is mapped. The HARQ functionality is controlled by the RV parameters. The exact set of bits at the output depends on the number of input bits, the number of output bits, and the RV parameters.

Physical channel segmentation divides the bits among the different physical channels when more than one HS-PDSCH is used. Interleaving is done separately for each physical channel.

HSDPA uses QPSK modulation and, when radio conditions are good, 16QAM. Constellation rearrangement applies only to 16QAM modulation, in which two of the four bits in a symbol have a higher probability of error than the other two bits. The rearrangement occurs during retransmission and disperses the error probability equally among all the bits in the average, after the retransmission combining.

An example HS-DSCH channel coding is shown in Figure 1. The coding corresponds to a fixed reference channel, FRC H-Set 4, that is used for testing the UE receiver. The first rate-matching stage matches the number of input bits to a virtual IR buffer. The second rate-matching stage matches the resulting number of bits to the number of physical channel bits available in the HS-PDSCH set during the TTI. This stage is controlled by the RV parameters.

Figure 1. Example of Channel Coding for an HS-DSCH

The number of HS-PDSCHs and the modulation format define the number of physical channel bits after RV selection (960 bits for QPSK modulation � 5 = 4,800 bits). The turbo-encoding code rate is fixed at 1/3, but the effective code rate is the combination of turbo-encoding and the rate matching stages. The effective code rate for any HS-DSCH configuration can be calculated if the transport block size, the number of HS-PDSCHs, and the modulation format are known. In this case, the effective code rate is 0.67 or [(3,202 + 24) bits/(960 bits � 5)].

Downlink Physical Channel Structure
Of the three slots within a 2-ms subframe, the first slot carries critical information for HS-PDSCH reception, such as the channelization code set and the modulation scheme. After receiving the first slot, the UE has just one time slot for decoding the information and preparing to receive the HS-PDSCH.

The number of HS-PDSCHs or code channels that map onto a single HS-DSCH can vary dynamically between 1 and 15. OVSF codes are used. The number of multicodes and the corresponding offset for the HS-PDSCHs mapped from a given HS-DSCH are signaled on the HS-SCCH. The multicodes (P) at offset (O) are allocated as follows:

Cch, 16, O � Cch, 16, O+P-1

The second and third slots carry the HS-DSCH channel coding information, such as the transport block size, HARQ information, the RV and constellation versions, and the new data indicator. The data of the three slots is covered with the 16-bit UE identity.

Uplink Physical Channel Structure
The HS-DPCCH carries uplink feedback signalling related to the downlink HS-DSCH transmission. This signalling consists of HARQ-ACK and CQI shown in Figure 2. Each 2-ms subframe, like those of the downlink physical channels, consists of three slots, each with 2,560 chips. The HARQ-ACK is carried in the first slot of the HS-DPCCH subframe and the CQI in the second and third slots.

Figure 2. High-Speed Dedicated Physical Control Channel That Carries the Uplink

There is at most one HS-DPCCH on each radio link. The HS-DPCCH can exist only in association with a W-CDMA uplink DPCCH.

Two different paths are used for HARQ-ACK and CQI coding. The HARQ-ACK information is coded to 10 bits, with ACK coded as 1 and NACK coded as 0. The CQI information is coded using a 20,5 code. The coded bits are mapped directly to the HS-DPCCH.

The feedback cycle of the CQI can be set as a network parameter in predefined steps from 2 ms to infinity. An active HS-DPCCH may have slots in which no HARQ-ACK or CQI information is transmitted. The HS-DPCCH can be a bursted channel.

Adaptive Modulation and Coding
Link adaptation is one important way in which HSDPA improves data throughput. The technique used, AMC, matches the system�s modulation-coding scheme to the average channel conditions during each user transmission. The power of the transmitted signal is held constant over a subframe interval, and the modulation and coding format are changed to match the current received signal quality or channel conditions.

In this scenario, users close to the BTS typically are assigned higher order modulation with higher code rates, such as 16QAM with an effective code rate of 0.89, but the modulation order and code rate will decrease as the distance from the BTS increases. The 1/3 rate turbo coding is used, and different effective code rates are obtained through various rate-matching parameters.

In HSDPA, the UE reports the channel conditions to the BTS via the uplink channel CQI field in the HS-DPCCH. The CQI value can be 0 to 30, with a value of 0 indicating out of range. Each CQI value corresponds to a certain transport block size, the number of HS-PDSCHs, and the modulation format for a certain UE category. These parameters are used by the BTS in combination with other parameters to determine the appropriate TF and effective code rate.

For example, the largest transport block size is 27,952 bits, which corresponds to the highest data rate of 13.976 Mb/s (27,952 bits/2 ms = 13.976 Mb/s). This data rate is obtained by using 16QAM, an effective code rate of 0.9714, and 15 HS-PDSCHs.

Hybrid ARQ
HARQ is a technique combining FEC and ARQ methods that save information from previous failed attempts to be used in future decoding. HARQ is an implicit link-adaptation technique. AMC uses explicit C/I or similar measurements to set the modulation and coding format; HARQ utilizes link-layer acknowledgements (ACK/NACK) for retransmission decisions. Put another way, AMC provides the coarse data-rate selection; HARQ accommodates fine data-rate adjustment based on channel conditions.

For a retransmission, HARQ uses the same transport-block set and consequently the same number of information bits as were used in the initial transmission. However, it may use a different modulation scheme�channelization-code set including the size of the channelization-code set�or transmission power. As a result, the number of channel bits available for a retransmission may differ from that of the initial transmission. Channel bits are the bits actually transmitted over the air. Even if the number of channel bits is the same, the channel-bit set may be different.

To minimize the number of additional retransmission requests, HARQ uses one of two soft-combining schemes to ensure proper message decoding. CC involves sending an identical version of an erroneously detected packet. Received copies are combined by the decoder prior to decoding. IR involves sending a different set of bits incrementally to be combined with the original set, increasing the amount of redundant data and the likelihood of recovering from errors introduced on the air.

Use of Incremental Redundancy
Figure 3 illustrates how the IR scheme works. For simplicity, an IR buffer size of 10 bits/process and a single process are assumed in this example. The original data (4 bits) corresponds to the data block after the CRC has been added. The data is encoded at the 1/3 rate, and it then gets punctured as part of the first rate-matching stage. At this stage, the number of output bits is matched to the IR buffer size, 10 bits in this case.

Figure 3. Demodulation of the HS-DSCH Using Incremental Redundancy

The second rate-matching stage punctures the data again. The data can be punctured into different data sets, each corresponding to a different RV. These are indicated here as three different colors: red, green, and orange. Only one of these data sets will be sent in any given transmission.

The five red bits are sent OTA, resulting in an effective code rate of 4/5. That is, for every original data bit, 1 + 1/4 bits are transmitted OTA. The data arrives at the UE and is demodulated, padded with dummy bits, and stuffed into the IR buffer. The data then is decoded, with some possibility of error, to provide the four blue bits. This block is checked against the CRC, and if found to be in error, is stored, and a NACK requests a retransmission.

When the retransmission is sent, it uses a different RV or puncture scheme and sends the five green bits OTA. At the UE, the green bits are recombined with the original transmission�s red bits to provide an effective code rate of 2/5. Now for every data bit there are 2� bits available for decoding, which increases the likelihood of success. However, when the results are checked against the CRC, if the block is still in error, the retransmission process begins again.

Yet another RV or puncture scheme is used, appearing now as the orange bits that are sent OTA and recombined at the UE with the red and green bits from the first and second transmission. The new RV provides additional redundant data even if some or all of the encoded bits are repetitions of encoded bits sent earlier.

After the third transmission, the effective code rate is 4/15; for every data bit, there now are 3 + 3/4 bits. At last, the data is correctly decoded, and an ACK is sent back. If the block were still in error, a NACK would be sent, and still more RVs could be transmitted, depending on the maximum number of transmissions allowed for a block.

In the case of 16QAM formats, the different RVs not only may correspond to different puncturing schemes, but also to different constellation versions or rearrangements.

HARQ Processes
The HSDPA system does not retransmit a data block until it receives an ACK or NACK for that data. To avoid wasting time between transmission of the data block and reception of the ACK/NACK response, which would result in wasted throughput, multiple independent HARQ processes can be run in parallel.

Five subframes are needed to receive the ACK/NACK for a transmitted data block. Since the ACK/NACK is required before data transmission for a specific process can continue, the minimum interval between TTIs must be at least six for a single HARQ process. Six HARQ processes running simultaneously will completely fill every subframe with data to specific UE.

UE must support a minimum inter-TTI interval of receiving data every subframe, every other subframe, or every third subframe. The minimum interval value they support depends on the HS-DSCH category.

Packet Scheduling Functionality
In addition to the channel coding and physical and transport layer changes, HSDPA implements another change to support fast packet transfer. It relocates the packet scheduling functionality from the network controller to the MAC layer in the Node B.

The packet-scheduling algorithm takes into account the radio channel conditions, based on the CQI from all the UE involved, and the amount of data to be transmitted to the different users. Throughput gains can be maximized by serving the UE that is experiencing the best radio channel conditions, but obviously some degree of fairness in scheduling is required.

In addition, there are other factors that the scheduling algorithm could take into account, such as quality of service. The actual throughput also will depend heavily on the packet-scheduling algorithm used.

Scheduling, modulation and coding adaptation, and HARQ retransmissions in HSDPA are fast because they are performed as close to the air interface as possible and because a short frame length is used. Fast scheduling makes it possible to track the fast-channel variations.

What Comes Next?
HSDPA technology is incorporated in W-CDMA Release 5 to increase data throughput and improve the efficiency of the system for downlink data traffic. The main changes introduced by HSDPA are new high-speed data channels, the combination of time-division multiplexing with code-division multiplexing, the use of AMC and HARQ techniques, and the relocation of MAC layer scheduling to the Node-B. With a thorough understanding of these changes, design and test engineers can begin to successfully implement HSDPA into network and UE.

Looking forward to Release 6, the content of which is being finalized at this time, the most significant feature targeted for the radio interface is the EUDCH. This feature will introduce techniques similar to HSDPA to improve coverage, increase throughput, and reduce delay on the uplink this time. Release 7 will likely include MIMO antennas, which support higher data rates and are considered an enhancement to HSDPA.

About the Author
Marta Iglesias is a wireless industry marketing engineer at Agilent Technologies. She holds a B.S.E.E. from the Universitat Politecnica de Catalunya, Spain. Agilent Technologies, 395 Page Mill Rd., Palo Alto, CA 94303, 800-829-4444, e-mail: [email protected]

on the 3GPP Project Plan

February 2005

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