Introduction to High speed downlink packet access (HSDPA)

By Devendra Sharma
Last Updated: 11/05/06

Introduction

High speed downlink packet access (HSDPA) is the new technology which is introduced in 3GPP Release 5. As the name itself suggests, this will enable the user to achieve high data rates in the downlink while on the move.

Motivation behind HSDPA

  • Sophisticated UE applications need higher bit rates,
  • Primary target of HSDPA is to enhance system throughput with minimum changes in network architecture,
  • HSDPA id Release 5 an extension to WCDMA Release '("99")

Transition from Present UMTS system

  • Current WCDMA network can be upgraded to support HSDPA
  • To implement HSDPA it does not required to change network architecture dramatically
  • In a general scenario there can be HSDPA supported cells and regular cells. This mean that a particular time there can be some users in a particular cell who are experiencing high data rate because of HSDPA as compared to other users who are in a cell where HSDPA is not supported.
  • If HSDPA is not accessible at UE location it will use normal Release 99 (DCH) to communicate at regular service speeds

The Release 99 or current UMTS system provide data rates of 384Kbps to 2Mbps. HSDPA will increase peak data rates up to 14Mbps.

How HSDPA differs from current UMTS system

Various methods for packet data transmission in WCDMA downlink already exist in Release'99. The three different channels in Release'99/ Release 4 WCDMA specifications that can be used for downlink packet data are:

  • Dedicated Channel (DCH)
  • Downlink-shared Channel (DSCH)
  • Forward Access Channel (FACH).

The basic requirements for HSDPA are to carry high data rate in the downlink. The HSDPA technology will:

  • Increase the UTRAN network capacity
  • Reduce the round trip delay
  • Increase the peak data rates up to 14 Mbps

In order to achieve this few architectural changes have been made in the R99 architecture.

The transport channel carrying the user data with HSDPA operation is denoted as the High-speed Downlink-shared Channel (HS-DSCH) known as downlink "fat pipe".

As discussed above the primary motivation behind HSDPA was to achieve high data rates by not disturbing to the current UMTS architecture too much. Thus it's clear that by implementing the HSDPA the current UMTS architecture is maintained and some other features or functionalities are added on top of the existing architecture.

So the question arises is that to implement HSDPA (Release 5) which new features comes in, what goes out from the existing UMTS (Release 99) and what is added onto it.

In HSDPA (Release 5) three new transport channels are introduced. They are:

  • HS-DSCH (High Speed Down link Shared Channel)

TO support the HS-DSCH Operation Two Control Channels are added
  • HS-SCCH (High Speed Shared Control Channel)

DL channel
  • HS- DPCCH (High Speed Dedicated Physical Control Channel)

UL Channel

With HSDPA two fundamental features of WCDMA are disabled which is:

  • Variable SF
  • Fast Power Control

These two features are replaced by

  • Adaptive Modulation and Coding (AMC)
  • Fast retransmission strategy (HARQ)
  • Scheduling Algorithm

Thus the comparison for the DSCH (UMTS) and HS_DSCH (HSDPA) can be made as:
FeatureDSCHHS_DSCH
Variable spreading factorYesNo
Fast power controlYesNo
Adaptive modulation and coding (AMC)NoYes
Multi-code operationYesYes, Extended
Fast L1 HARQNoYes

Note: HARQ: Hybrid Automatic Repeat request

HSDPA Features Explanation

Adaptive Modulation and Coding (AMC)

One of the major techniques introduced in WCDMA is power control. The idea is to increase the transmission power when the quality of the received signal is poor and decrease the transmission power when the quality of the received signal exceeds a given threshold. This results in reliable communication between the transmitter and the receiver. Also, since the power control technique reduces the unnecessary intercell and intracell interference caused by excessive transmit power, the overall system capacity is increased.

An alternative technique to the power control in dealing with the time varying effects of the wireless channel is to “ride” the fading profile of the channel. Instead of trying to keep the signal quality at the receiver constant, once can change the modulation and the coding scheme of the transmitted signal in such a way that more information bearing bits are transmitted when the channel condition is good, and the less information bearing bits are transmitted when the channel condition deteriorates. This technique is known as adaptive modulation and coding (AMC), or link adaptation. Compared to conventional power control technique, AMC can lead to much higher system capacity for packet radio systems.

The AMC is aimed at changing the modulation and coding format in accordance with variations in the channel conditions. The channel conditions can be estimated based on either on the feedback from the receiver or from the transmission power of the other downlink channels under power control.

Thus according to the above principle, in system with AMC, users in the favourable positions, such as close to the cell site or at the peak of a fading profile, are typically assigned a higher-order modulation with higher code rates, such as 64QAM/16QAM and ¾ rate turbo codes. Whereas users in the unfavourable position, such as ones close to the cell boundary or at the lower peak of a fading profile, are assigned a lower-order modulation with lower coding rates, such as QPSK with ½ rate turbo codes.

If there are large number of users in cell and the channel condition of the different users vary with time, which is normally the case, the base station can choose to serve users in favourable condition and use high modulation scheme and the coding rates most of the time, and the system capacity would be greatly improved. Another advantage of AMC is that since the transmitted power is fixed (no fast power control is used), the interference to the other users is significantly reduced.

Thus the main benefits of the AMC can be summarised as follows:

  • Higher data rates can be achieved for users in favourable conditions, which in turn increase the average throughput of the cell.
  • Interference is reduced due to link adaptation based on the variation in the modulation and coding scheme instead of variation in the transmit power.

Hybrid ARQ (HARQ)

In order to make sure that the data reaches the terminal error free, two basic error-control strategies used in any data communication are forward error correction and Automatic repeat request:

  • Forward error correction (FEC): In this information bearing bits are protected by some coding scheme employing redundancy bits, and most errors caused by the channel impairment can be recovered at the receiver by the decoder. The main advantage of FEC is that no feedback channel is required and so the delay is only contained in the decoding process. The disadvantage of the FEC is that the decoded data is always delivered to the user regardless of whether it is correct or incorrect.
  • Automatic repeat request (ARQ): In this scheme a high-rate detection codes are normally used and a transmission is requested if the received data is found to be erroneous. The functionalities of the ARQ include the following:
    1. Error detection: The receiver detects error in the so called PDU.
    2. Positive acknowledgement: The receiver returns a positive ACK to the successfully received error free PDUs.
    3. Retransmission after timeout: The transmitter retransmits the PDU that has not been acknowledged after a predetermined amount of time.
    4. Negative acknowledgement and retransmission: The receiver return a negative acknowledgement (NACK) to the erroneous PDUS and transmitter retransmit them.

There are three types of basic ARQ schemes:

  • stop-and-wait (SAW): In this system the transmitter sends off a packet and then waits for an acknowledgment. A positive ACK will encourage sender to send he next PDU in the queue whereas the negative ACK will prompt the sender to retransmit the PDU. Stop-and-wait is the simplest scheme but inherently inefficient because of the idle time spent waiting for the acknowledgment of each transmitted packet before the transmitter starting the next transmission.
  • Selective Repeat (SR): is the most theoretically efficient ARQ scheme among the three basic ones and it has been employed by many systems, including the Release 99 UTRAN radio link control (RLC) layer. SR is theoretically insensitive to delay and has the favourable property of repeating only those blocks that have been received in error. To accomplish this task, however, the SR ARQ transmitter must employ a sequence number to identify each block that it sends. SR may fully utilize the available channel capacity by ensuring that the maximum block sequence number (MBSN) exceeds the number of blocks transmitted in one round-trip feedback delay. The greater the feedback delay, the larger the maximum sequence number must be. Unfortunately, there is a major problem in the implementation of selective repeat ARQ in HSDPA - high memory requirement in the mobile terminal. This is due to the fact that the mobile terminal must store soft samples for each transmission of a block.

A hybrid ARQ (HARQ) system consists of an FEC subsystem contained in an ARQ system. The function of the FEC system is to reduce the frequency of retransmission by correcting the error patterns that occur most frequently, thus ensuring a high system throughput. When a less frequent error pattern occurs and is detected, the receiver requests a retransmission instead of passing the erroneous data to the user. This increases the system reliability. There are three type of HARQ

  • HARQ Type I: In this the same protocol data unit (PDU) is retransmitted until the receiver accepts it as error-free or until the maximum number of allowed retransmission attempts is reached. HARQ Type I scheme is best suited for communications systems in which a fairly constant level of noise and interference is anticipated in the channel. In this case, an adequate amount of redundancy can be built into the system to correct the vast majority of errors so the number of retransmissions can be kept to a minimum. For non stationary channels, such as the ones encountered in mobile communications, the HARQ Type I can be very inefficient.
  • HARQ Type II: This is particularly suited for time varying channels. In this scheme, the concept of incremental redundancy is employed and the received PDUs are concatenated to form corrupted code words from increasingly longer and lower rate codes. In the first transmission, the PDU may be coded with a high-rate code (low redundancy) for error detection and correction. If the receiver detects the presence of errors in the PDU, it saves the erroneous PDU in a buffer and at the same time requests a retransmission. Unlike HARQ Type I, what is retransmitted in HARQ Type II is not the original PDU but a block of new data. The new data are formed based on the original PDU and the error correcting code used. When the new PDU is received, it is used to correct the errors in the erroneous PDU stored previously in the buffer. If the second attempt fails again, the receiver will request a further retransmission and this process continues until satisfactory results are achieved.
  • HARQ Type III: also belongs to the class of incremental redundancy ARQ schemes. With HARQ Type III, however, each retransmission is self-decodable. Chase combining (also called HARQ Type III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the signal-to-noise ratio (SNR) of the received signal for each attempt.

The performance of different type of HARQs depends on the condition of the radio channel. Considering all factors, including spectral efficiency, implementation complexity, and robustness, it appears that Chase Combining offers a very attractive compromise. In UTRAN, it is the scheduler that determines the redundancy version parameters for the HARQ functional entities in node Bs.

Combining the two complementary techniques, AMC and HARQ, leads to an integrated robust and high-performance solution, in which AMC provides the coarse data rate selection, whereas the HARQ provides for fine data rate adjustment based on channel conditions.

Scheduler

The scheduler for HSDPA is referred to as being fast due to the fact that, compared with Release 99 specifications; the scheduler is moved from RNC to node Bs to reduce delays so faster scheduling decisions can be made. In addition to other functionalities, such as the choice of redundancy version and modulation and coding scheme, a fundamental task of the scheduler for HSDPA is to schedule the transmission for users. The data to be transmitted to users are placed in different queues in a buffer and the scheduler needs to determine the sequential order in which the data streams are sent. The scheduling algorithms are:

  • round-robin method: This algorithm selects the user packets in a round robin fashion. In this method, the number of time slots allocated to each user can be chosen to be inversely proportional to the users’ data rates, so the same number of bits is transmitted for every user in a cycle. Obviously, this method is the “fairest” in the sense that the average delay and throughput would be the same for all users. However, there are two disadvantages associated with the round-robin method. The first is that it disregards the conditions of the radio channel for each user, so users in poor radio conditions may experience low data rates, whereas users in good channel conditions may not even receive any data until the channel conditions turn poor again. This is obviously against the spirit of the HSDPA and it would lead to the lowest system throughput. The second disadvantage of the round-robin scheduler is that there is no differentiation in the quality of services for different classes of users.
  • Maximum C/I (carrier-to-interface) ratio method: In this method, the scheduler attempts to take advantage of the variations in the radio channel conditions for different users to the maximum, and always chooses to serve the user experiencing the best channel condition, that is, the one with maximum carrier-to-interference ratio. Apparently, the max C/I scheduler leads to the maximum system throughput but is the most unfair, as users in poor radio conditions may never get served or suffer from unacceptable delays.
  • Proportional fairness or R[n]/Rav Method: This method takes into account both the short-term variation of the radio channel conditions and the long-term throughput of each user. In this method, the user with the largest R[n]/Rav is served first, where R[n] is the data rate in the current time slot n and Rav is the average data rate for the user in the past average window. The size of the average window determines the maximum duration that a user can be starved from data, and as such it reflects the compromise between the maximum tolerable delay and the cell throughput. According to this scheduling scheme, if a user is enjoying a very high average throughput, its R[n]/Rav will probably not be the highest. Then it may give way to other users with poor average throughput and therefore high R[n]/Rav in the next time slots, so the average throughput of the latter can be improved. On the other hand, if the average throughput of a user is low, the R[n]/Rav could be high and it might be granted the right of transmission even if its current channel condition is not the best.

The figure below illustrates the performance of different scheduling algorithm

Fast scheduling and AMC, in conjunction with HARQ, is a way of maximizing the instantaneous use of the fading radio channel in order to realize maximum throughput. The HSDPA technology enables higher-rate data transmission through a higher-modulation and coding rate and limited retransmissions, while keeping the power allocated to HS-DSCH channel in a cell constant. Notwithstanding, the slow power control is still needed to adjust the power sharing among terminals and between different channel types.

HSDPA Impact on Radio Access Network And UE Architecture

All Release’99 transport channels presented earlier in this document are terminated at the RNC. Hence, the retransmission procedure for the packet data is located in the serving RNC, which also handles the connection for the particular user to the core network. With the introduction of HS-DSCH, additional intelligence in the form of an HSDPA Medium Access Control (MAC) layer is installed in the Node B. This way, retransmissions can be controlled directly by the Node B, leading to faster retransmission and thus shorter delay with packet data operation when retransmissions are needed. With HSDPA, the Iub interface between Node B and RNC requires a flow control mechanism to ensure that Node B buffers are used properly and that there is no data loss due to Node B buffer overflow.

Although there is a new MAC functionality added in the Node B, the RNC still retains the Release’99/Release 4 functionalities of the Radio Link Control (RLC), such as taking care of the retransmission in case the HS-DSCH transmission from the Node N would fail after, for instance, exceeding the maximum number of physical layer retransmissions.

The key functionality of the new Node B MAC functionality (MAC-hs) is to handle the Automatic Repeat Request (ARQ) functionality and scheduling as well as priority handling. Ciphering is done in any case in the RLC layer to ensure that the ciphering mask stays identical for each retransmission to enable physical layer combining of retransmissions.

Similar to Node B a new MAC entity, MAC-hs is added in the UE architecture. The functionality of the MAC-hs is same as on the Node B side.

Node B protocol stack in R99

Node B Protocol stack in R-5

UE protocol stack in R-5

Transport and Control Channel in HSDPA

High Speed Downlink Shared Channel (HS-DSCH)

The HS-DSCH is allocated to users mainly on the basis of the transmission time interval (TTI), in which users are allocated within different TTIs.

HS-DSCH has the following features:

  • TTI = 2ms (3 time slots): This is to achieve short round trip delay for the operation between the terminal and the Node B for retransmissions. TTI in R99 is 10ms
  • Adding higher order modulation scheme, 16 QAM, as well as lower encoding redundancy has increased the instantaneous peak data rate. In the code domain perspective, the SF is fixed; it is always 16, and multi-code transmission as well as code multiplexing of different users can take place.

The maximum number of codes that can be allocated is 15, but depending on the terminal (UE) capability, individual terminals may receive a maximum of 5, 10 or 15 codes.

The physical channel carrying HS-DSCH transport traffic is termed as HSPDSCH, and each HS-PDSCH is identified by its specific channelization code. Therefore, there can be up to 15 HS-PDSCHs in a cell. This means that for HS-PDSCH, both QPSK and 16QAM modulation schemes can be used and these modulation schemes lead to different slot formats. With 16QAM, a single HS-PDSCH channel can achieve a data rate of 960 kbps. Using 15 HS-PDSCH channels (codes), HSDPA can produce the headline data rate of 14.4 Mbps.


The HSDPA specification does permit simultaneous transmissions. For instance, two to four users can be supported within the same TTI by using different subset of the channelization codes allocated to HS-DSCH.

High Speed Shared Control Channel (HS-SCCH)

Besides user data, the node B must also transmit associated control signalling to user terminals, so terminals scheduled for the upcoming HS-DSCH TTI can be notified. Similarly, additional lower-layer control information such as the transport format, including the modulation and coding schemes to be used, and hybrid ARQ related information must be transmitted. This control information applies only to the user equipment that is receiving data on the HS-DSCH and is transmitted on a shared control channel, HS-SCCH.

The UTRAN needs to allocate a number of HS-SCCHs that correspond to the maximum number of users that will be code-multiplexed. If there is no data on the HS-DSCH, then there is no need to transmit the HS-SCCH either. From the network point of view, there may be a high number of HS-SCCHs allocated, but each terminal will only need to consider a maximum of four HS-SCCHs at a given time. The HS-SCCHs that are to be considered are signalled to the terminal by the network. In reality, the need for more than four HS-SCCHs is very unlikely. However, more than one HS-SCCH may be needed to better match the available codes to the terminals with limited HSDPA capability.

Each HS-SCCH block has a three-slot duration that is divided into two functional parts.

  • The first slot (first part) carries the time-critical information that is needed to start the demodulation process in due time to avoid chip level buffering.
  • The next two slots (second part) contain less time-critical parameters including Cyclic Redundancy Check (CRC) to check the validity of the HS-SCCH information and HARQ process information.

For protection, both HS-SCCH parts employ terminal-specific masking to allow the terminal to decide whether the detected control channel is actually intended for the particular terminal.

The HS-SCCH is a fixed rate (60kbps) DL channel and uses SF=128 that can accommodate 40 bits per slot (after channel encoding) because there are no pilot or Transmit Power Control TPC bits on HS-SCCH.


The HS-SCCH used half-rate convolution coding with both parts encoded separately from each other because the time-critical information is required to be available immediately after the first slot and thus cannot be interleaved together with Part 2.

The HS-SCCH Part 1 parameters indicate the following:

  • Codes to despread. This also relates to the terminal capability in which each terminal category indicates whether the current terminal can despread a maximum of 5, 10 or 15 codes.
  • Modulation to indicate if QPSK or 16 QAM is used.

The HS-SCCH Part 2 parameters indicate the following:

  • Redundancy version information to allow proper decoding and combining with the possible earlier transmissions.
  • ARQ process number to show which ARQ process the data belongs to.
  • First transmission or retransmission indicator to indicate whether the transmission is to be combined with the existing data in the buffer (if not successfully decoded earlier) or whether the buffer should be flushed and filled with new data.

Parameters such as actual channel coding rate are not signalled but can be derived from the transport block size and other transport format parameters.

The terminal has single slot duration to determine which codes to despread from the HS-DSCH. The use of terminal-specific masking allows the terminal to check whether data was intended for it. The total number of HS-SCCHs that a single terminal monitors (the Part 1 of each channel) is at a maximum of 4, but in case there is data for the terminal in consecutive TTIs, then the HS-SCCH shall be the same for that terminal between TTIs to increase signalling reliability. This kind of approach is also necessary not only to avoid the terminal having to buffer data not necessarily intended for it but also as there could be more codes in use than supported by the terminal capability. The downlink DCH timing is not tied to the HS-SCCH (or consequently HS-DSCH) timing.

Uplink High Speed Dedicated Physical Control Channel (HS-DPCCH)

The uplink direction has to carry both ACK/NACK information for the physical layer retransmissions as well as the quality feedback information to be used in the Node B scheduler to determine to which terminal to transmit and at which data rate. It was required to ensure operation in soft handover in the case that not all Node Bs have been upgraded to support HSDPA. Thus, it was concluded to leave existing uplink channel structure unchanged and add the needed new information elements on a parallel code channel that is named the Uplink High Speed Dedicated Physical Control Channel (HS-DPCCH). There is one HS-DPCCH for each active terminal using HSDPA services.

The HS-DPCCH is divided into two parts as shown in Figure below and carries the following information:

    ACK/NACK transmission, to reflect the results of the CRC check after the packet decoding and combining. Downlink Channel Quality Indicator (CQI) to indicate which estimated transport block size, modulation type and number of parallel codes could be received correctly (with reasonable BLER) in the downlink direction.


CQI

  • Information is Based on (Cpich) Common Pilot Channel
  • Is used to estimate the following:
    • Transport Block Size
    • Modulation Type
    • No. of Channelisation Codes (that can be supported at a given reliability level)
  • The feedback of CQI can be set as Network parameter in a Predefined steps of 2ms
  • When longer feedback cycles are used, the (PDCH) Power Control Command can be used to update the channel quality estimation
  • CQI (0-30) => UEs estimates highest value with probability of TB error < 10% for the next transmission slot, and with various power assumptions

DL-DPCH & UL-DPCH

  • The DL-DPCH carries HSDPA Indicator (HI)
  • HI indicates which HS-SCCH subset the terminal should monitor in order to obtain the signalling information,
  • The UL_DPCH carries power control signals for the DL-DPCH,
  • The UL-DPCH power control signal information and feedback information from HS-DPCCH can be used by the network to estimate the channel quality.

HSDPA Architecture

The figure below illustrates the HSDPA architecture for both UE and the network

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MAC- hs

Functions, such as adaptive modulation and coding and fast scheduling, are placed in node B. In contrast, in the Release 99 UTRAN architecture, the scheduling and transport-format selections are performed in the radio network controller (RNC). For HSDPA, it is advantageous to move parts of the functionality from RNC to node B, thus forming a new Node B entity, MAC-hs. The MAC-hs is responsible for handling scheduling, HARQ, and transmit format (TF) selection. Apparently, some upgrading is needed in the node B to enable the MAC-hs functionalities. The consensus among the 3G network vendors is to implement MAC-hs in the channel coding card.

There is one MAC-hs entity in the UTRAN for each cell supporting HS-DSCH. The MAC-hs is responsible for handling the data transmitted on the HS-DSCH. Furthermore, it is responsible for managing the physical resources allocated to HSDPA. MAC-hs receive configuration parameters from the higher layers.

The functional entities included in MAC-hs are shown in the figure below


  • Flow control: This is the companion flow control function to the flow control function for existing dedicated, common, and shared channels in RNC. This function is employed to limit layer 2 signalling latency and reduce discarded and retransmitted data as a result of HS-DSCH congestion.
  • Scheduling/Priority handling: This function manages HS-DSCH resources between HARQ entities and data flows according to their priority. There is one priority queue for each MAC-d protocol data unit (PDU) in the MAC-hs. Based on status reports from associated uplink signalling in HS-DCCH, either new transmission or retransmission is determined. A new transmission can be initiated instead of a pending retransmission at any time to support the priority handling.
  • HARQ: One HARQ entity handles the hybrid ARQ functionality for one user. One HARQ entity is capable of supporting up to eight HARQ processes of stop-and-wait HARQ protocols. There is one HARQ process per HS-DSCH per TTI.
  • TFRC selection: This is to select an appropriate transport format and resource combination (TFRC) for the data to be transmitted on HSDSCH.

To summarize, the MAC-hs needs to perform the following tasks when dealing with HSDPA traffic:

  • Decode the higher-layer information regarding UE capability and required QoS for initial connection.
  • Decode the ACK/NAK and CQI transmitted in the uplink and check the power level of the downlink dedicated physical channel.
  • Make a scheduling decision on which terminal is due to receive data among the terminals having data in the transmission buffer.

Set the transport format combination indicator (TFCI) and HARQ parameters in the downlink shared control channel HS-SCCH

In the following section the function of two major functional entities in MAC-hs, the scheduler and the HARQ unit are explained further.

Scheduler

The scheduler is one of the most important functional entities in determining the QoS and data rate of HSDPA services, as it controls when and how to transmit data streams dedicated at each terminal. For each terminal, the information available to the scheduler includes the estimate of channel quality (CQI) received on the HS-DPCCH, the knowledge of priority queues, and the HARQ processes and terminal capability. Based on the information, the scheduler performs the following functions:

  • Schedules all HSDPA users within a cell.
  • Services priority queues.
  • Schedules MAC-hs PDUs based on information from the HS-DSCH frame protocol. One terminal may be associated with one or more MAC-d flows. Each MAC-d flow contains HS-DSCH MAC-d PDUs for one or more priority queues.
  • Determines the HARQ entity and the queue to be serviced.
  • Indicates the queue ID and the transmit sequence number (TSN) to the HARQ entity for each MAC-hs PDU to be transmitted.
  • Schedules new transmissions and retransmissions.
  • Based on the status reports from HARQ processes, the scheduler determines if either a new transmission or a retransmission should be made. A new transmission can, however, be initiated on a HARQ process at any time.
  • Determines a suitable redundancy version and modulation scheme for each transmitted and retransmitted MAC-hs PDU and indicates the redundancy version to lower layers.

HARQ

The HARQ unit is responsible for handling the HARQ functionalities of all mobile terminals. There is one HARQ functional entity per mobile terminal in UTRAN. Each functional entity can manage up to eight parallel stop-and-wait HARQ processes. As the input, the HARQ entity receives the acknowledgment (ACK/NAK) from the mobile terminal. The HARQ entity sets the queue ID in transmitted MAC-hs PDUs based on the identity of the queue being serviced. The HARQ entity sets the transmission sequence number (TSN) in transmitted MAC-hs PDUs. The TSN is set to value 0 for the first MAC-hs PDU transmitted for one HS-DSCH and queue ID and it is increased by one for each subsequent transmitted MAC-hs PDU. The HARQ entity determines a suitable HARQ process to service the MAC-hs PDU and sets the HARQ process identifier accordingly.

The HARQ process sets the new data indicator in the transmitted MAC-hs PDUs. It sets the new data indicator to value “0” for the first MAC-hs PDU transmitted by a HARQ process and then increases the new data indicator with one for each transmitted MAC-hs PDU containing new data. The HARQ processes received status messages. UTRAN delivers received status messages to the scheduler.

Mobility Procedures

Once a terminal is in the so-called CELL_DCH state when dedicated channels have been set up, it can be allocated with one or more HS-PDSCH(s), thus allowing it to receive data on the HS-DSCH. For dedicated channels, it is advantageous to employ the so-called soft handover technique, which is to transmit the same data from a number of Node Bs simultaneously to the terminal, as this provides diversity gain. Owing to the nature of packet transmission, however, synchronized transmission of the same packets from different cells is very difficult to achieve, so only hard handover is employed for HS-PDSCH.

This is referred to HS-DSCH cell change, and the terminal can have only one serving HS-DSCH cell at a time. A serving HS-DSCH cell change message facilitates the transfer of the role of serving HS-DSCH radio link from one belonging to the source HS-DSCH cell to another belonging to the target HS-DSCH cell. In theory, the serving HS-DSCH cell change can be decided either by the mobile terminal or by the network. In UTRAN Release 5, however, only network-controlled serving HS-DSCH cell changes are supported and the decision can be based on UE measurement reports and other information available to the RNC. A network-controlled HS-DSCH cell change is performed based on the existing handover procedures in CELL_DCH state.

Since the HSDPA radio channel is associated with dedicated physical channels in both the downlink and uplink, there are two possible scenarios in changing a serving HS-DSCH cell: (1) only changing the serving HS-DSCH cell and keeping the dedicated physical channel configuration and the active set for handover intact; or (2) changing the serving HS-DSCH cell in connection with an establishment, release, and/or reconfiguration of dedicated physical channels and the active set.

Although an unsynchronized serving HS-DSCH cell change is permissible, a synchronized one is obviously preferable for ease of traffic management. In that case, the start and stop of the HS-DSCH transmission and reception are performed at a given time. This is convenient especially when an intranode B serving HS-DSCH cell change is performed, in which case both the source and target HS-DSCH cells are controlled by the same node B and the change happens between either frequencies or sectors.

If an internode B serving HS-DSCH cell change is needed, the serving HS-DSCH Node B relocation procedure needs to be performed in the UTRAN. During the serving HS-DSCH node B relocation process, the HARQ entities located in the source HS-DSCH node B belonging to the specific mobile terminal are deleted and new HARQ entities in the target HS-DSCH node B are established. In this scenario, different controlling RNCs may control the source and target HS-DSCH node Bs, respectively.

Intranode B Serving HS-DSCH Cell Change

Figure below illustrates an intranode B serving HS-DSCH cell change while keeping the dedicated physical channel configuration and the active set, using the physical channel reconfiguration procedures. The transition from source to target HS-DSCH cells is performed in a synchronized fashion, that is, at a given activation time. For clarity, only the layers directly involved in the process are shown and the sequence of the events starts from the top and finishes at the bottom.


In this scenario, the terminal transmits a measurement report message containing intrafrequency measurement triggered by the event change of best cell. When the decision to perform handover is made at the serving RNC (SRNC), the node B is prepared for the serving HS-DSCH cell change at an activation time indicated by CPHY-RL-Commit-REQ primitive. The serving RNC then sends a physical channel reconfiguration message, which indicates the target HS-DSCH cell and the activation time to the UE. Since the same node B controls both the source and target HS-DSCH cells, it is not necessary to reset the MAC-hs entities. Once the terminal has completed the serving HS-DSCH cell change, it transmits a physical channel reconfiguration complete message to the network.

It should be pointed out that, in this particular case, it is assumed that HS-DSCH transport channel and radio bearer parameters do not change. If transport channel or radio bearer parameters are changed, the serving HS-DSCH cell change would need to be executed by a transport channel reconfiguration procedure or a radio bearer reconfiguration procedure, respectively.

Internode B Serving HS-DSCH Cell Change

For terminals on the move, what happens more often than the intra-node B serving HS-DSCH cell change is the so-called internode B serving HS-DSCH cell change. For synchronized case, the reconfiguration is performed in two steps within UTRAN.

To begin with, the terminal transmits a measurement report message containing measurement triggered by the event change of best cell. The serving RNC determines the need for hard handover based on received measurement report and/or load control algorithms. As the first step, the serving RNC establishes a new radio link in the target node B. After this, the target Node B starts transmission and reception on dedicated channels. In the second step, this newly created radio link is prepared for a synchronized reconfiguration to be executed at a given activation time indicated in the CPHY-RL-Commit-REQ primitive, at which the transmission of HS-DSCH will be started in the target HSDSCH node B and stopped in the source HS-DSCH node B.

The serving RNC then sends a transport channel reconfiguration message on the old configuration. This message indicates the configuration after handover, both for DCH and HS-DSCH. The transport channel reconfiguration message includes a flag indicating that the MAC-hs entity in the terminal should be reset. The message also includes an update of transport channel-related parameters for the HS-DSCH in the target HS-DSCH cell.

After physical synchronization is established, the terminal sends a transport channel reconfiguration complete message. The serving RNC then terminates reception and transmission on the old radio link for dedicated channels and releases all resources allocated to the UE. The process of internode B handover for HS-DSCH is shown in Figure below.


It should be noted that in the case of internode B handover, the radio link control (RLC) for transmission/reception on HS-DSCH may be stopped at both the UTRAN and the terminal sides prior to reconfiguration and continued when the reconfiguration is completed, which could result in data loss during the handover period. Furthermore, the transport channel reconfiguration message indicates to the terminal that the MAC-hs entity should be reset and a status report for each RLC entity associated with the HS-DSCH should be generated. However, a reset of the MAC-hs entity in the terminal does not require flushing the reordering buffers but delivering the content to higher layers.


REFERENCES:

[1] 3GPP TR 25.855 - HSDPA; Overall UTRAN Description; Release 5

[2] 3GPP TR 25.858 - HSDPA; Physical Layer Aspects; Release 5

[3] 3GPP TR 29.950 - UTRA High Speed Downlink Packet Access; Release 4

[4] UMTS Evolution to High Speed Downlink Packet Access - Douglas McCarthy

[5] UMTS - High Speed Downlink Packet Access - Rohde & Schwarz

[6] 3GPP TR 25.877 - HSDPA: Iub/Iur Protocol Aspects

[7] 3GPP TR 25.899 - HSDPA Enhancements; Release 5

[8] Nokia: HSDPA Solution White Paper

[9] 3GPP TS 25.211 - Physical Channels and mappin of Transport Channels onto Physical Channels; Release 5

[10] HSDPA/HSUPA for UMTS: High Speed Radio Access for Mobile Communications - Harri Holma, Antti Toskala

[11] Advances in Mobile Radio Access Networks - Y.Jay Guo



































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