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www.huawei.com Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved. Principles of HSDPA ISSUE 1.0
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Owj200501 Wcdma Hsdpa Principles

Oct 03, 2015

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Raghava Raghava

WCDMA HSDPA Principles
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  • Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission WCDMA EvolutionWCDMA evolved from GSM/GPRS, inheriting much of the upper layer functionality directly from those systems. The first commercial deployments of WCDMA are based on a version of the standards called Release 99.Enhanced Data rates for GSM Evolution (EDGE) is another system in the GSM/GPRS family that some operators have deployed as an intermediate step before deploying WCDMA.HSDPA was introduced in WCDMA Release 5 to offer higher speed Downlink data services.Release 6 introduces the Enhanced Uplink (EUL) that will provide faster data services for the Uplink.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Release 99 Packet DataThere are different techniques defined in the Release 99 specification to enable Downlink packet data. Most commonly, data transmission is supported using either the Dedicated Channel (DCH) or the Forward Access Channel (FACH).The DCH is the primary means of supporting packet data services. Each user is assigned a unique Orthogonal Variable Spreading Factor (OVSF) code dependent on the required data rate. Fast closed loop Power Control is employed to ensure that a target Signal to Interference Ratio (SIR) is maintained in order to control the block error rate (BLER). Macro Diversity is supported using soft handover.Data transfer can also be supported on the FACH. This common channel employs a fixed OVSF code. As it needs to be received by all UEs, higher data rates are generally not supported. Macro Diversity is also not supported and the channel operates with a fixed (or slow changing) power allocation. Each data block contains a unique UE identifier that allows a given UE to keep itsown data and discard that belonging to other UEs.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Release 99 Downlink LimitationsAlthough WCDMA Release 99 standard allows for maximum data rates of up to 2.0 Mbps, it has only been widely implemented with a maximum data rate of 384 kbps. This data rate is achieved by allocating a dedicated channel to each user. The use of dedicated resources can be a limitation, especially for data applications with bursty characteristics. Each dedicated channel uses an OVSF code. Shorter codes are used for higher data rates and longer codes for lower data rates. When an OVSF of a particular length is used, all longer OVSF codes derived from that code become unavailable. This limits the number of simultaneous high speed data users in a given cell. The Release 99 standards provide support for a Secondary Scrambling Code, which eases this limitation, but it has not been widely implemented in commercial systems and will likely be removed from future versions of the specification. The data rate of a dedicated channel can be adjusted to accommodate varying requirements of a data service application, but the procedure for doing so is slow and thus inefficient. Capacity is controlled both by the maximum amount of PA power that is available and by the power requirement of each data service. In dedicated mode, fast power control is used so that a target Eb/No is achieved on the Downlink. However, the required Eb/No set point changes at a much slower rate. This can result in wasted resources whereby a better than required Eb/No is achieved for the required BLER.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission High Speed Downlink Packet Access (HSDPA)In HSDPA, the Node B allocates a set of high speed channels. These channels are assigned to a user using a fast scheduling algorithm that allocates the channels every 2 ms. All or part of the channels may be assigned to a given user during any 2 ms period.The rapid scheduling of HSDPA is well-suited to the bursty nature of packet data. During periods of high activity, a given user may get a larger percentage of the channel bandwidth, while it gets little or no bandwidth during periods of low activity.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HSDPA Basic ConceptsIn HSDPA a common channel with fixed power is employed for data transfer. Users are separated in both the time and code domains. A fixed spreading factor is employed but multi codes operation is possible for increased data rates. Adaptive Modulation and Coding (AMC) replaces the role of power control so that the modulation and coding rate are changed depending on the channel condition. This is accomplished by locating the scheduling algorithm for channel allocation at the Node B instead of the RNC in Release 99.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Comparison SummaryDCH and FACH are the two Release 99 channels typically used for packet switched data in practice. The advantages and disadvantages of each approach are apparent. Whereas DCH is suited for medium high data rates (with a maximum rate of 384 kbps), rate switching is slow, making it unsuitable and inefficient for bursty data such as a Web browsing application. By contrast, FACH provides good support for bursty data but is a common channel without power control or other mechanism to account for channel conditions. This makes it unsuitable for higher data rates. Switching from DCH to FACH is slow and inefficient, due in part to the typical timer values used to detect inactivity HSDPA is suitable to high date rates for a bursty application, though we will see that the absence of soft handover makes it more suitable for stationary or low-mobility users than for highly mobile users. HSDPA typically operates at a fixed power, but feedback from the UE can instruct the Node B to use lower power when the UE is in good channel conditions. Link adaptation is used to adjust data rate, coding, and modulation to quickly respond to changing channel conditions.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission CQI Mapping TableThe CQI table consists of 30 entries, where each entry indicates a different TFRC. Transport Format Resource Combination (TFRC) points to the combination of number of HS-PDSCH channelization codes, modulation scheme, and the HS-DSCH transport block size. The 5-bit CQI reported by a UE is an index into this table containing all possible TFRC combinations for that UE category. The TFRC combinations are different for UEs with different HS-DSCH UE categories because of the differences in the UE capabilities. Along with TFRC, CQI may also indicate a power offset relative to the current HS-PDSCH power. The CQI table shown in the slide is for UE categories supporting up to 15 HS-PDSCH codes (HSDPA terminal category 10).

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission UE CategoriesHSDPA is advertised with data rates up to 14 Mbps. However, the actual HS-DSCH peak data rate depends on the UEs HS-DSCH category. As shown in the table, only a category 10 UE can achieve the maximum HSDPA throughput of 14 Mbps when using all 15 HS-PDSCHs simultaneously. Factors that decide the UEs HS-DSCH category are:HS-PDSCH codes Determines the number of simultaneous HS-PDSCH channels that can be decoded by a UE.Inter-TTI interval Determines the minimum interval (in terms of HS-DSCH TTI) between two successive HS-PDSCH assignments. The more HARQ processes a UE supports, the shorter the inter-TTI interval. A minimum inter-TTI of 1 requires at least 6 simultaneous HARQ processes.Transport Block size Determines the maximum size of transport block that can be sent on HS-DSCH in a TTI. It is dependent on the number of HS-PDSCH codes and the modulation scheme.IR buffer size Determines the maximum number of soft bits that can be buffered by a UE across all simultaneously running HARQ processes.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HARQ is a technique that transmitter sends new set of parity bits if the previous transmission failed (NACK) and receiver buffer the failed decodes for soft combining with later retransmission.The RV parameter indicates different bit transmit in IR buffer. RV parameter covers 3 parameters, i.e. s,r,bs identifies the HARQ combining scheme (CC or IR)r identifies redundancy versionb identifies the 16QAM constellation versionConfidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission The maximum retransmission count can be set on the NodeB LMT. Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission The basic idea of fast scheduling is to transmit at the fading peaks of the channel in order to increase the throughput and to use resource more efficiently. But this might lead to large variations in data rate of the users. The trade-off is between the cell throughput and fairness against users. There are a number of scheduling algorithms that take into consideration the trade-off between throughput and fairness:Round Robin (RR): radio resource are allocated to communication links on a sequential basis, not taking into account the instantaneous radio channel conditions experienced by each link.Max C/I: for maximum cell throughput ,the radio resource should be as much as possible be allocated to communication links with the best instantaneous channel condition. Proportional Fair (PF): allocates the channel to the user with relatively best channel quality. Enhanced Proportional Fair (EPF): allocates the channel to the user according to relatively best channel quality, fairness, guarantee bit rate requirment.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Shared channel transmission implies that a certain amount of radio resource of a cell (code and power) is seem as a common resource that is dynamically shared between users.Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission WCDMA R99 uses QPSK data modulation for downlink transmission. To support higher data rate, higher order data modulation, such as 16QAM can be used.Compared to QPSK modulation, higher order modulation is more bandwidth efficient i.e. can carry more bits per HertzConfidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Release 99 ChannelsThis diagram shows possible mappings of logical, transport, and physical channels in the control and user planes for UMTS Release 99.Some channels exist only in Physical Layer (CPICH, SCH, DPCCH, AICH, PICH). These channels carry no upper layer signaling or user data.Transport channels carry the following types of information:Broadcast Control Channel (BCH) Broadcast information that defines overall system configuration.Paging Channel (PCH) Paging notification messages. A Paging Indicator Channel (PICH) is associated with a PCH to allow a UE to quickly determine whether it needs to read the PCH during its assigned paging occasion.Forward Access Channel (FACH) Common Downlink signaling messages. Also carries dedicated Downlink signaling and user information to a UE operating in Cell_FACH state. Random Access Channel (RACH) Common Uplink signaling messages. Also carries dedicated Uplink signaling and user information to a UE operating in Cell_FACH state.Dedicated Channel (DCH) Dedicated signaling and user information for a UE operating in the Cell_DCH state. DCH is mapped to a Dedicated Physical Data Channel (DPDCH). An associated Dedicated Physical Control Channel (DPCCH) carries Physical Layer control information, such as power control commands.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HSDPA introduces three new Downlink channels and one new Uplink channel:High Speed Downlink Shared Channel (HS-DSCH) A Downlink transport channel shared by several UEs. The HS-DSCH is associated with one or several Shared Control Channels (HS-SCCH). It operates on a 2 ms Transmission Time Interval (TTI).High Speed Shared Control Channel (HS-SCCH) A Downlink physical channel used to carry Downlink control information related to HS-DSCH transmission. The UE monitors this channel continuously to determine when to read its data from the HS-DSCH, and the modulation scheme used on the assigned physical channel.High Speed Physical Downlink Shared Channel (HS-PDSCH) A Downlink physical channel shared by several UEs. It supports Quadrature Phase Shift Keying (QPSK) and 16-Quadrature Amplitude Modulation (16-QAM) and multi-code transmission. It is allocated to a user at 2 ms intervals.High Speed Dedicated Physical Control Channel (HS-DPCCH) An Uplink physical channel that carries feedback from the UE to assist the Node Bs scheduling algorithm. The feedback includes a Channel Quality Indicator (CQI) and a positive or negative acknowledgement (ACK/NAK) of a previous HS-DSCH transmission.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HSDPA Channels (continued)Only dedicated logical user data channels may be mapped to HS-DSCH. When DTCH is mapped to HS-DSCH, only Unacknowledged Mode (UM) and Acknowledged Mode (AM) channels may be used.A UE operating in HSDPA mode also has at least one Release 99 dedicated channel (DCH/DPDCH) allocated, to ensure that RRC and NAS signaling can always be sent, even if the UE is not able to receive the high speed channels.The HS-DPCCH is a Physical Layer control channel. It carries no upper layer information, and therefore has no logical or transport channel mapping.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HS-PDSCHWhen the UE decodes the HS-SCCH and determines that there is an HS-DSCH assignment in the next TTI, it decodes the assigned HS-PDSCHs. Each HS-PDSCH uses an OVSF of length 16. If multiple HS-PDSCHs are assigned simultaneously to one UE, they must use consecutive OVSF codes. The HS-SCCH indicates the first OVSF code and the number of codes for each assignment.A UE is a member of one of 12 categories, as a function of its hardware capabilities. Each category represents different values of the following parameters:Number of simultaneous HS-PDSCH codes (5, 10, or 15)Maximum transport block sizeInter-TTI interval minimum time between consecutive assignments.Incremental redundancy buffer size used to soft-combine symbols from retransmissions.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Xue [16bits]:UE identity, Multiple UEs may be monitoring the same set of HS-SCCHs. Each UE has an assigned identity called the H-RNTI. Xccs [7bits]:channelization code set, The HS-SCCH indicates which of the OVSF codes allocated to the HS-PDSCHs will be used. HS-PDSCH uses multi-code transmission, which means that multiple OVSF codes may be assigned to one UE at the same timeXms [1bit]:modulation scheme, HS-PDSCH uses either QPSK or 16-QAM modulation. This can change from one assignment to the next, and HS-SCCH indicates which method will be used.Xrv[3bits]:redundancy version, The HARQ protocol supports retransmissions and incremental redundancy. These parameters allow the UE to differentiate new transmissions from retransmissions.Xtbs [6bits]:transport block size, The HS-SCCH indicates how much data will be sent during the next assignmentXhap [3bits]:HARQ process numberXnd [1bit]:new data indicatorConfidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HS-DPCCHWhenever the UE is operating in HSDPA mode, it uses the HS-DPCCH to give feedback to the serving Node B. This feedback consist of two parts:ACK/NAK The UE sends a positive or negative acknowledgement for each HS-DSCH assignment. UTRAN may configure the UE to repeat the ACK/NAK, up to a maximum of 4 transmissions. The first ACK/NAK for a given HS-DSCH assignment is sent 5 ms (7.5 slots) after the end of the HS-DSCH transmission.Channel Quality Indicator (CQI) The UE measures the channel quality of the Downlink CPICH and computes a CQI value. The value is an index into a table, and corresponds to the maximum data rate that the UE can decode with an error rate of less than 10%, assuming the channel conditions dont change. UTRAN may configure the UE to repeat the CQI, up to a maximum of 4 transmissions. UTRAN may also configure the periodicity of CQI reporting, ranging from 2 ms to 160 ms.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission When a DL RAB is mapped onto the HS-DSCH, UL DCH is set up regardless of the existence of UL data. UL DCH transmits the UL signaling, UL RLC acknowledgement message and possible UL service data. DL DCH is set up to transmit the DL signaling. These DCHs are called associated DCHs.When the UE is in soft handover or softer handover, the HS-DSCH data can be transmitted only in the HSDPA serving cell while the DCH data can be transmitted in all the cells in the active set. Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Theoretical HSDPA Maximum Data RateThe theoretical maximum data rate is 14.4 Mbps. The following techniques are used to achieve this data rate:Multi-code transmission Up to 15 HS-PDSCH channels may be assigned to a single UE during one 2 ms TTI.Consecutive assignments The HARQ procedure allows the Node B to send back-to-back assignments at 2 ms intervals.Lower Coding Gain Higher data rates can be achieved by puncturing more bits for a higher effective code rate (and thus lower coding gain).16-QAM This modulation scheme increases the data rate over QPSK by a factor of 2.3.84Mcps/16=Vsymbol, chip rate / SF = symbol rateVsymbol4Vbit for one channel, symbol rate modulation factor = bit rate on one channelVbit15= max. rate for one UE, bit rate on one channel max. channel number = max. bit rate for one UE

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission HSDPA Protocol StackIn a Release 99 PS network, the NAS layer protocols are terminated at the SGSN. RRC, RLC,and MAC protocols are terminated at the RNC. The Physical Layer protocol is terminated at the Node B.The Release 5 specifications define a new sublayer of MAC called MAC-hs, which implements the MAC protocols and procedures for HSDPA. This sublayer operates at the Node B and the UE.UTRAN MAC-hs is responsible for fast scheduling of the HS-PDSCHs. The scheduler determines:To which UEs the channels are assigned.How much data to send.Which modulation scheme to use.Whether to send new data or retransmitted data.Which redundancy version to send.UE MAC-hs is responsible for:Sending ACK or NAK after decoding a block.Re-ordering data blocks before submitting to upper layers, if retransmissions caused data to be received out of order.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission UTRAN MAC ArchitectureThe UTRAN MAC protocol consists of three entities:MAC-hs Responsible for the high speed HSDPA channels and the only entity of MAC that resides in the Node B. When a UE operates in HSDPA mode, MAC-hs maps user data and signaling from DCCH and DTCH onto the shared HS-DSCH transport channels.MAC-c/sh Responsible for common and shared logical (PCCH, BCCH, CCCH, and CTCH) and transport (PCH, BCH, RACH, FACH) channels. MAC-c/sh resides in the RNC, and there is one MAC-c/sh entity per RNC. When a UE operates in Cell_FACH state, MAC-c/sh maps user data and signaling from its DCCH and DTCH onto the common FACH and RACH transport channels.MAC-d Responsible for mapping data from dedicated logical channels (DCCH and DTCH) onto dedicated transport channels (DCH). MAC-d resides in the RNC, and there is one MAC-d entity for each UE to which dedicated logical channels have been assigned.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission UTRAN MAC-hs ArchitectureData enters the UTRAN MAC-hs from a set of MAC-d flows. The data is routed to a set of priority queues with the following properties:Up to 8 priority queues and 8 MAC-d flows are allowed per UE.The queue distribution entity maps each MAC-d flow onto one or more priority queues.The mapping is configured when the HSDPA operation begins.Each priority queue is mapped to only one MAC-d flow.When data is removed from a priority queue for transmission, it is assigned to a HARQ process.There are a minimum of 6 and a maximum of 8 HARQ processes per UE. The HARQ process tracks the ACK/NAK signaling for the data block and determines when retransmission is necessary.In response to CQI and ACK/NAK signaling on HS-DPCCH, the scheduler decides:To which UEs the HSDPA channels will be assigned.For each scheduled UE, whether to send new data from a priority queue or a retransmission from a HARQ process.Signaling on HS-SCCH indicates the scheduling decision to the UEs operating in HSDPA mode.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission UE MAC-hs ArchitectureWhen the UE Physical Layer decodes a data block addressed to it, the associated HARQ process determines whether to ACK or NACK the block. If an ACK is sent, the data block is passed to the assigned re-ordering queue.Re-ordering of MAC-hs PDUs is necessary because up to 8 HARQ processes can be operating on sequentially transmitted data. MAC-hs PDUs can be received out of order when a HARQ process sends a NACK.The re-ordering queue passes the block up to the disassembly entity when it receives consecutive data blocks. The disassembly entity takes apart the MAC-hs PDU into its constituent MAC-d PDUs and passes them up to the appropriate MAC-d flow for processing by the MAC-d layer.

    Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission Confidential Information of Huawei. No Spreading Without Permission