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
Transition from Present UMTS system
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:
The basic requirements for HSDPA are to carry high data rate in the downlink. The HSDPA technology will:
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:
TO support the HS-DSCH Operation Two Control Channels are added
With HSDPA two fundamental features of WCDMA are disabled which is:
These two features are replaced by
Thus the comparison for the DSCH (UMTS) and HS_DSCH (HSDPA) can be made as:
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:
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:
There are three types of basic ARQ schemes:
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
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.
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:
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:
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.
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:
The HS-SCCH Part 2 parameters indicate the following:
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:
DL-DPCH & UL-DPCH
The figure below illustrates the HSDPA architecture for both UE and the network
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
To summarize, the MAC-hs needs to perform the following tasks when dealing with HSDPA traffic:
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.
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:
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.
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.