high speed downlink packet access (hsdpa): higher...

Seminar on Topics in Communications Engineering Master of Science in Communications Engineering Munich University of Technology

High Speed Downlink Packet Access (HSDPA): Higher Data Rates for UMTS

Author: Naveen Shankpal Advisor: Dr. Christian Hartmann Date: 03.12.2004

Institute for Communications Engineering (LNT) Institute of Communication Networks (LKN)

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Naveen Shankpal, Advisor: Christian Hartmann

Abstract – High Speed Downlink Packet Access is the first step in the evolution of WCDMA (UMTS) networks. The second step will be to enhance the uplink data rates, improved uplink capacity and reduced uplink delay. The maximum data rate to a user in present WCDMA networks achieved in ideal conditions is limited to 2MB/ s. The users hunger for higher data rates gave an impetus to find new ways to get higher data rates beyond 2MB/ s. The introduction of HSDPA to the UMTS specification will potentially offer up to 10MB/ s data rates while backward compatible with the existing user equipment. The present report will explore the changes required to support HSDPA. Further the different H-ARQ techniques will be seen in depth which is required to ensure that the necessary retransmission is executed quickly and efficiently.

I. INTRODUCTION High Speed Downlink Packet Access (HSDPA) is a concept included in WCDMA 3GPP (Third Generation Partnership Project) Release 5 specifications. The main target is to increase the user peak data rates and quality of service (QoS), and in general improve the spectral efficiency for downlink asymmetrical and bursty packet data services.

When HSDPA will be implemented, it can coexist on the same carrier as the current Release’99 WCDMA services. This will enable a smooth and cost-efficient introduction of HSDPA into the existing WCDMA networks. The driving force for high data rates are greater speed, shorter delays when downloading audio, video and large files which will be used in PDA’s, smart phones etc. Further a user can download packet data over HSDPA, while at the same time having a speech call. HSDPA offers theoretical peak rates of up to 10MB/ s and in practice more than 2MB/ s. The technical aspects behind the HSDPA concept include the following:

Shared channel transmission

Adaptive Modulation and Coding (AMC)

Fast Hybrid Automatic Repeat Request (H-ARQ)

Fair and fast scheduling at Node B

Fast cell site selection (FCSS)

Short transmission time interval (TTI)

II. MAIN PART

A. Features of HSDPA Shared channel transmission The HSDPA concept introduced few additional physical channels. They are High Speed Physical Downlink Shared Channel (HS-PDSCH) and a dedicated HS-Physical Control Channel (HS-DPCCH). HS-PDSCH: This channel is both time and code shared between users attached to a Node-B. It is the transport mechanism for additional logical channels; they are HS-Downlink Shared Channel (HS-DSCH) and HS-Shared Control Channel (HS-SCCH). The HS-DSCH code resources consist of one or more canalization codes with a fixed spreading factor (SF) of 16. At the most 15 such codes can be allocated leaving sufficient room for other required control and data carriers. The available code resources are primarily shared in time domain but it is

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possible to share the code resources using code multiplexing. When it is both time and code shared, two to four users can share the code resources with the same TTI.

Figure1: HS-DSCH code and time shared [8] HS-DPCCH: This channel is an uplink channel used to carry the acknowledgement signals to the Node-B for each block. It is also used to indicate the Channel Quality (CQI) which is used for Adaptive Modulation and Coding. Adaptive Modulation and Coding (AMC) In present WCDMA networks fast power control is used for radio link adaptation. This power control is done per slot in WCDMA. Basically link adaptation is required because, in cellular communication systems the SINR of the received signal at the UE (Mobile Equipment is called User Equipment for 3rd generation mobile systems) varies over time by as much as 30-40 dB due to fast fading and geographic location in a particular cell. In order to over come this fading effect and improve the system capacity and peak data rates, the transmitted signal to a particular UE is modified in accordance with the signal variations through a process called link adaptation. In HSDPA the transmission power is kept constant over the TTI (length of the frame is referred to as Transmit Time Interval) and uses adaptive modulation and coding (AMC) as an alternative method to power control in order to improve the spectral efficiency. HSDPA uses higher order modulation schemes like 16-quadrature amplitude modulation (16QAM) besides QPSK. The modulation to be used is adapted according to the radio channel conditions. QPSK can support 2 bits/ symbol where as 16QAM can support 4 bits/ symbol, and hence twice the peak rate capability as compared to QPSK, using the channel bandwidth more efficiently. Different code rates used are 1/4, 1/2, 5/8, 3/4. The Node-B (Base Station) receives the Channel Quality indicator (CQI) report and power measurements on the associated channels. Based on these information it determines the transmission data rate. In HSDPA, users close to the Node-B are generally assigned higher modulation with higher code rates (e.g. 16QAM and 3/4 code rate), and both decreases as the distance between UE and Node-B increases. Example: 1TTI = 3 slots = 2ms = 2560*3 chips = 7680 chips = 480 symbols @ SF=16 480 symbols = 960 bits @ QPSK and 480 symbols = 1920 bits @ 16QAM

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Fast Hybrid Automatic Repeat Request (H-ARQ) The H-ARQ protocol used for HSDPA is stop and wait (SAW). In SAW the transmitter sends a block of TTI (3 slots) and waits until acknowledge or negative acknowledge is received from the UE. In order to utilize the time when it waits for the acknowledgements, N parallel SAW-ARQ processes may be set for a UE, so different processes transmit in separate TTI’s. The value of N is explicitly signaled using 3 bits, hence at the most N can be 8. UE1 packet1

UE1 packet2

UE1 packet3

UE1 packet4

UE2 packet1

UE1 packet5

UE1 packet2

UE1 packet6

UE1 packet4

UE1 packet7

Figure 2: N-channel HARQ

UE packet Packet transmitted and received with acknowledge UE packet Packet transmitted and received with negative-acknowledge UE packet Packet re-transmission The UE requests the retransmission of erroneous data received earlier. Ones the UE receives the 2nd transmission, it combines the information from the original transmission with that of the 2nd transmission before trying to decode the message. Further H-ARQ methods will be discussed in the report later. Fast and fair scheduling at Node B Typically in WCDMA networks the packet scheduling is done at the RNC (radio network connection), but in HSDPA the packet scheduler (medium access layer-hs) is shifted to the Node-B. This makes the packet scheduling decisions almost instantaneous. In addition to this, the TTI length is shortened to 2ms. Hence the scheduling is done very fast as its done every TTI. A first approach for fair scheduling can be Round-Robin method where every user is served in a sequential manner so all the users get the same average allocation time. However, the requirement of high scheduling rate along with the large AMC availability with the HSDPA concept, where the channel is allocated according to the instantaneous channel conditions. Another popular packet scheduling is proportional fair packet scheduling. Here, the order of service is determined by the highest instantaneous relative channel quality. Since the selection is based on relative conditions, still every user gets approximately the same amount of allocation time depending on its channel condition. Fast cell site selection (FCSS) Typically on an average 20-30% of the MS’s are in soft or softer handover condition. Soft handover is a handover between two Node-B’s where as softer hand over is between sectors of a Node-B. So it’s very important to track the active set of Node-B’s connected to a UE for communication. FCSS allows a UE to select the Node-B with the best current transmission characteristics [UMTS evolution to HSDPA]. The advantage of this system is that higher data rates can be achieved at most of the time.

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Short transmission time interval (TTI) The length of the frame is referred to as Transmission Time Interval (TTI). The HS-DSCH which is added in the HSDPA standard uses this TTI of 2ms than the Release’99 transport channel TTI. This is done to reduce the round trip time, increases the granularity in the scheduling process and for better tracking of the time varying radio channel. Actually the length of the frame is variable and is selected based on traffic supported and the number of supported users. A typical value is 2ms.

B. H-ARQ The AMC uses an appropriate modulation and coding scheme according to the channel conditions. Even after AMC, we may land up with errors in the received packets due to the fact that the channel may vary during the packet is on the fly. An automatic repeat request (ARQ) scheme can be used to recover from these link adaptation errors. When the transmitted packet is received erroneous then the receiver requests the transmitter for the retransmission of that erroneous packet. The basic technique is to use the energy of the previously transmitted signal along with the new retransmitted signal to decode the block. There are two main schemes for H-ARQ, Chase combining and Incremental redundancy. Chase Combining It involves the retransmission of the same data packet which was received with errors. Once the retransmission is received, the receiver combines the soft values of the original signal and the retransmitted signal weighted by the SNR prior to decode the data packet. Interested readers can refer to [3] and [10] for further information. Advantages: each transmission and retransmission can be decoded individually (self-decodable), time diversity gain, may be path diversity gain. Disadvantage: transmission of the entire packet again which is wastage of bandwidth. Incremental Redundancy (IR) Incremental Redundancy is used to get maximum performance out of the available bandwidth. Here the retransmitted block consists of only the correction data to the original data that carries no actual information (Redundancy). The additional redundant information is sent incrementally when the first, second … retransmissions are received with errors.

Figure 3: IR–Data Block Retransmission (courtesy: Agilent IR Application note)

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Advantages: Reducing the effective data throughput/ bandwidth of a user and using this for another user. Disadvantage: The systematic bits are only sent in the first transmission and not with the retransmission which makes the retransmissions non-self decodable. So, if the first transmission is lost due to large fading effects there is no chance of recovering from this situation. Partial Incremental Redundancy The Partial IR is the combination of chase combining and IR. The disadvantage with IR is removed here by adding the systematic bits along with the incremental redundant bits (different puncturing bits) in the retransmissions as in [5] and [6]. This makes both original and retransmitted signals self-decodable.

III. CONCLUSIONS The HSDPA concept facilitates peak data rates exceeding 2 Mbps and theoretically reaching 10 Mbps. The cell throughput gain over previous releases has been evaluated to be in the order of 50-100% or more, which is highly dependent on factors such as the radio environment and the service provision strategy of the network operator. Practical HSDPA user bit rates even in large macro cells can be similar to broadband home DSL lines. As HSDPA enables more bits to be transferred with the same radio frequency, it also enables lower cost per bit than Release'99 based WCDMA.

The H-ARQ technique which is best suited in HSDPA would be partial incremental redundancy. Performance of partial IR is in between chase combining and IR.

Further evolution of HSDPA peak data rates can be achieved with multiple-input multiple-output (MIMO) antenna techniques of 3GPP Rel.'6. No changes are required to the networks except increased capacity within the infrastructure to support the higher bandwidth.

Figure 4: Simulated slot error rate versus Ior/ Ioc in dB for 4QAM, 3 transport blocks and rate 0.25 white & black markers are for chase &IR [7]

Ior: total received power

Ioc: total interference power

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Figure 5: Simulated slot error rate versus Ior/ Ioc in dB for 64QAM, 27 transport blocks and rate 0.75 white & black markers are for chase & IR [7]



REFERENCES

[1] Douglas McCarthy, “UMTS Evolution to High Speed Downlink Packet Access,” EETS 8316

Term Paper, Fall 2003.

[2] A. Das, F. Khan, A. Sampath, H. Su, “Design and Performance of Downlink Shared Control Channel for HSDPA,“ IEEE PIMRC 2002.

[3] D. Chase, “Code combining: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets,” IEEE Transactions on communications, Vol. com-33, No.5, May 1985.

[4] Ericsson, “WHITE PAPER: WCDMA Evolved,” 2004

[5] Motorola, “Performance comparison of Hybrid-ARQ schemes,” 3GPP input paper TSGR#17(00)1396, 2000.

[6] Motorola, “Performance comparison of Hybrid-ARQ schemes: Additional results,” 3GPP input paper TSGR1#18(00)0044, 2001.

[7] P. Frenger, S. Parkvall, E. Dahlman, “Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA,” Ericsson Research, SE-164 80 Stockholm, Sweden.

[8] D. Messerschmitt and J.--P. Hubaux, “Opportunities for Electronic Commerce in Networking,” IEEE Communications Magazine, vol. 37, no. 9, pp. 95-98, September 1999

[9] Peter Rysavy, “Data Capabilities: GPRS to HSDPA”, White paper developed for 3G America, September 2004.

[10] Motorola, “Performance Comparison of Hybrid-ARQ Schemes”, TSG-RAN WG#17, 20th-24th Oct 2000.

[11] Samir Kallel, Robert link and Sattar Bakhtiyari, “Throughput Performance of Memory ARQ Schemes”, IEEE Transactions on Vehicular Technology, Vol. 48, No#3, May 1999

[12] Douglas N. Rowitch and L.B. Milstein, “On the Performance of Hybrid FEC/ ARQ Systems Using Rate Compatible Punctured Turbo (RCPT) Codes”, IEEE Transactions on communications, Vol. 48, No.6, June 2000


Naveen Shankpal2563451

[email protected]: Dr. Christian Hartmann

Seminar on Topics in Communications Engineering-------------------------------------------------------------------------------------------------------------------

Road Map-----------------------------------------------• Motivation• HSDPA Technology Overview• Different Hybrid ARQ Approaches• Outlook• Conclusion

MOTIVATION-----------------------------------------------• HSDPA is a 3GPP release 5 feature for

UMTS FDD/TDD• Designed for data service applications• Background services: file download, email

delivery• Interactive services: web browsing, server

access, database retrieval• Streaming services: audio/video services

MOTIVATION-----------------------------------------------• Reduced delay• Increased cell and user throughput • Downlink peak data rates up to and more

than 10Mbps• Attracts new subscribers due to new

services• Decreased cost per bit for the operator

HSDPA Technology Overview-----------------------------------------------• Shared channel transmission• Adaptive Modulation and Coding (AMC)• Fast Hybrid Automatic Repeat Request

(H-ARQ)• Fair and fast scheduling at Node B• Fast cell site selection (FCSS)• Short transmission time interval (TTI)

Shared channel transmission-----------------------------------------------Common resource for all users in the cell• Channel codes • Transmission power• Spreading Factor = 16

Additional Physical Channels

High Speed Physical Downlink Shared Channel (HS-PDSCH)

1. HS-Downlink Shared Channel (HS-DSCH)

2. HS-Shared Control Channel (HS-SCCH)

High Speed Dedicated Physical Control Channel (HS-DPCCH)

Resource shared in Code and Time-----------------------------------------------

Courtesy: RYSAVY RESEARCH

Adaptive Modulation and Coding

Adaptive Modulation and Coding(Radio environment dependent)

-----------------------------------------------• Modulation Schemes:

QPSK 16QAM

• Code Rates used:1/4, 1/2, 5/8 and 3/4

Courtesy: Ericsson research

The HS-DSCH transmission schemeConstellation re-arrangement is only used when 16-QAM is active

Courtesy: Nokia website

Modulation-Code Rate-Throughput-----------------------------------------------

Fast and fair scheduling

Fast and fair scheduling-----------------------------------------------

CQI:channel quality indicator

TCP:transmit power controlCourtesy: Nokia website

Fast cell site selection-----------------------------------------------• 20 to 30% UE‘s on soft handover• Tacking of active set of Node B‘s

connected to a UE• Selection of the Node-B with the best

current transmission characteristics• High data rates can be achieved

Short transmission time interval-----------------------------------------------• Shorter TTI (2ms) than the release’99• To reduce the link adaptation delays• Increase the granularity in the scheduling

process• Better tracking of time varying conditions

Fast Hybrid Automatic Repeat Request (H-ARQ)

-----------------------------------------------• Block Error Rate (BLER) after first

transmission: 10-20%• Aim to reduce the delay and increase the

spectral efficiency of retransmission• N-channel stop-and-wait (SAW)

N channel stop-n-wait ARQ-----------------------------------------------

Number of H-ARQ processes = 1 to 8 per UE

Courtesy: Siemens website

H-ARQ Schemes-----------------------------------------------• Chase combining: also called Type I• Incremental Redundancy:

Type IIType III

Chase combining-----------------------------------------------• Coding is applied to transmission packets• Soft combining of original and

retransmitted signals is done at receiver before decoding

• Advantage: self decodable, time diversity, path diversity

• Disadvantage:wastage of bandwidth

Incremental Redundancy (Type II)-----------------------------------------------

Received data block

Courtesy: Agilent IR Application note

Incremental Redundancy-----------------------------------------------

IR-data block retransmissionCourtesy: Agilent IR Application note

Incremental Redundancy-----------------------------------------------• Advantage:

Reducing the effective data throughput/ bandwidth of a user and using this for another user

• Disadvantage:non-self decodable

Chase Combining Vs IR-----------------------------------------------

Chase-IR simulated slot error rate versus Ior/ Ioc in dB for 64QAM 27 transport blocks and rate 0.75 white & black markers are for chase & IR respectively



Courtesy: Ericsson research [7]

Incremental Redundancy: Type III-----------------------------------------------• Incremental Redundancy Type III is also

called Partial Incremental Redundancy• Capability: between chase and IR

Advantage:retransmitted packet information is self decodable

HSDPA Spectral Efficiency----------------------------------------------------


Outlook-----------------------------------------------New 3GPP work Items• New work items related to receiver performance

(release 6)• New work item on HS-DPCCH ACK/NACK

Enhancement (release 6)• New work item Enhanced Uplink / High Speed Uplink

Packet Access (release 6)

Conclusion-----------------------------------------------• Peak data rates exceeding 2Mbps and

theoretically 10Mbps (and more with MIMO)• Lower cost per bit• No changes are required to the network

infrastructure• Practical HSDPA bit rate similar to broadband

home DSL lines• Highly dependent on: Radio environment and

network operator

Thank you for your attention!

Comments and questions

welcome!

Incremental Redundancy-----------------------------------------------

Courtesy: Agilent IR Application note

Throughput requirements required for different applications

-----------------------------------------------• Microbrowsing (e.g., WAP): 8 to 16 kbps• Multimedia messaging: 8 to 32 kbps • Video telephony: 64-384 kbps • General purpose web browsing: 32 kbps

to 384 kbps• Enterprise applications, including e-mail,

database access, virtual private networking: 32 kbps to 384 kbps

• Video and audio streaming: 32-384 kbps

Latency of Different Technologies-----------------------------------------------


high speed downlink packet access (hsdpa): higher...

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