增量冗余Incremental redundancy

White Paper

Incremental Redundancy

Doron Porat

July 2002

Incremental Redundancy - 1 -

Incremental Redundancy

1 Introduction The promise of EGPRS (Enhanced General Packet Radio Services) in wireless networks holds such potential, that we in the industry appear prepared to go to enormous lengths to make it a practical reality. The basic problem in achieving it, however, is that currently in GSM and GPRS, transferring data over the network is not supported at a sufficiently high level of quality. To be accepted in the market, a data service cannot fall short of the quality received when connecting to a wired-line network via modem. Addressing this problem, Incremental Redundancy (IR) is described in the EGPRS standard as a mandatory feature for the EGPRS mobile handset. Together with Link Adaptation, it is used to achieve superior delay/throughput performance over a wide range of operating conditions. Incremental Redundancy (also denoted as Hybrid ARQ II) copes with link quality variations, utilizing Hybrid ARQ II schemes to improve the efficiency of packet data. In the Incremental Redundancy scheme, information is first sent with very little coding, yielding a high bit-rate if decoding is immediately successful. If decoding fails, additional coded bits (redundancy) are sent, until decoding succeeds. The more coding that has to be sent, the lower the resulting bit-rate and the higher the delay. 2 GSM/GPRS data transfer problems In order for packet switched networking and transferring of data to succeed, EGPRS must solve certain problems.

Delay Todays GSM/GPRS systems suffer from delay problems. Capacity In order to supply a high data rate, the operator must dedicate a

large number of time slots to GPRS. This causes a capacity problem for the operator.

High data rate The user expects to receive data rates that are higher than 20Kbps. This is not achievable with GSM/GPRS.

Delay EGPRS with Incremental Redundancy can lessen the severity of delay problems. The unique IR method reduces the delay by reducing the need for packet re-transmision, which is required to a greater extent with GPRS. Capacity EGPRS solves the capacity problem by improving the coding schemes, reaching capacities that are 3 times higher than for GPRS. EGPRS enhances the air interface in order to enable it to support higher data rates, thus enabling the network to handle data traffic rates up to three times faster than GPRS. It accomplishes this by supplementing the GMSK (Gaussian Minimum Shift Keying) modulation used by GPRS, with 8PSK (Eight Phase Shift Keying) modulation. The increased throughput is enabled through the introduction of the second modulation format (8PSK) into the GSM/GPRS system.


High Data Rate EGPRS with Incremental Redundancy can provide data rates of up to ~59Kbp/s per time slot. This improves the data rate of GPRS by 3, as GPRS can only provide a maximum of ~20Kbp/s per time slot. Incremental Redundancy in EGPRS is a feature that improves transmission quality and user experience by delivering high data rates with reduced delay. 3 What is Incremental Redundancy Incremental Redundancy is unique method for reducing delay and improving throughput of data transmissions. A new technique introduced for EGPRS, called Link Quality Control (LQC), combines Link Adaptation (LA) with Incremental Redundancy (IR). Link Adaptation functionality adapts coding and modulation, relative to signal quality. In this case, information is first sent with minimum coding, using high modulation and coding schemes. This yields a high bit-rate if decoding is immediately successful. If decoding fails, additional coded bits (redundancy) are sent, using lower modulation and coding schemes, until decoding is successful. The more coding that has to be sent, the lower the resulting bit-rate. EGPRS also features backward error correction functionality, enabling the request for retransmission of erroneously received blocks. This mechanism is called Automatic Repeat Request (ARQ). Incremental Redundancy is then, an enhanced variant of ARQ called type II Hybrid ARQ. In this method, the information is first coded and punctured according to a specified puncturing scheme. If the decoding fails, the information is retransmitted using a different puncturing scheming. Since the retransmission is combined with the previously transmitted block, the process yields a lower bit-rate, which can facilitate decoding.


The Process of receiving data blocks:

Figure 1: IR Receiving data blocks

When a data block is received, it is decoded. If the data block is decoded successfully and no errors are detected, it is immediately transferred to the upper layers (L2/L3). If the data block is decoded unsuccessfully and errors are detected, it is stored in the IR database and a request is sent for retransmission. (See figure 1). Receiving of Re-Transmitted Data Block: An advantage of hybrid II ARQ is that if only one modulation scheme is used, no link quality measurements are needed. However, since one single modulation scheme may not

Figure 2: IR Data block retransmission

Data Block

IR Database

Accept Data Block

Error Detection

Error

No error

Deliver to Upper Layers

Data Block

Information from IR database

Combine Error Detection

IR Database

Accept Data Block

No error

Error

Deliver to Upper Layers


When an unsuccessful decoded data block is received, it is stored in the IR database. As soon as the retransmitted block is detected, the data from the IR database is combined with the data from the new data block, thereby increasing the likelihood of its successful decoding. The successfully decoded data block is then delivered to the upper layers (L2/L3). If, on the other hand, the combined data block was once again unsuccessfully decoded, then the newly transmitted data block is stored in the IR database, additional data blocks are retransmitted and so on, until the process is successful. (See figure 2). 4 Using different coding schemes In the Incremental Redundancy scheme, an initial MCS is selected based on the link quality. For the retransmissions, the same or another MCS from the same family can be selected (See figure 3). For example, if MCS-7 is selected for the first transmission, any MCS of the family B can be used for the retransmissions. The network controls the selection of MCS.

Scheme Modulation Family MCS-9 A MCS-8 A MCS-7 B MCS-6 A MCS-5

8-PSK

B MCS-4 C MCS-3 A MCS-2 B MCS-1

GMSK C

Figure 3: MCS families

According to the Incremental Redundancy method, the information is first sent with one of the initial code rates using the puncturing scheme (PS) 1 of the selected MCS. If the Data Block is received in error, the same data is re-transmitted with a different puncturing scheme (PS) 2 and decoded together with the already received data, and so on until decoding is successful.


5 Benefits of Incremental Redundancy Incremental Redundancy reduces delay and improves throughput, resulting in better utilization of the network and better user experience.

The graph shows that from SNR 8, the best data rates can be achieved using MCS-9 with IR. Therefore MCS-9 with IR should be considered the best choice for data applications, thus reducing the effort invested in link adaptation management.

Transferring files with and without IR, indicates that the transfer time of a 2K byte file is faster with IR than without.

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6 Comsys Implementation Comsys provides the full Incremental Redundancy functionality required in the physical layer (L1). Comsys EDGEwareTM Rx path Incremental Redundancy support consists of the following: 1. Saved bad block memory management 2. Retransmitted block detection 3. Retransmitted blocks combined with saved blocks 7 EDGEware 7.1 Product Description A Comsys product, EDGEwareTM is a physical layer (L1), base-band, software based modem solution for EDGE. EDGEwareTM implements the EGPRS standard as specified by ETSI (European Telecommunication Standards Institute) and 3GPP. EDGEwareTM is an ideal packet-optimized solution for silicon vendors, handset manufacturers and base station manufacturers designing EGPRS-based mobile/base stations. Providing excellent performance above ETSI GSM standard specifications, it enables the design of solutions that are characterized by low MIPS count and low power consumption, while minimizing RAM and ROM system costs. The products modular interface structure enables resource allocation control and timing handling, while providing a simple and easy-to-manage solution. EDGEwareTM provides all the functionality required for upgrading an existing GPRS mobile station/base station to an EGPRS-enabled mobile station/base station. EDGEware TM implements Incremental Redundancy and dynamic Link Adaptation that result in high throughput, lower delays and lower memory requirements. Built as a DSP firmware, EDGEware TM features include an 8PSK and GMSK demodulator, a state-of-the-art Viterbi equalizer optimized for the EGPRS wireless channel, a channel codec supporting MCS 1-9, Incremental Redundancy (IR) implementation with optimized memory resource usage, and support for multi-slot capabilities and all GSM carrier frequency bands.


7.2 EDGEware Benefits The innovative EDGEware architecture brings users cutting-edge advantages: Decreased time to market Packet-optimized EGPRS solution Excellent performance above and beyond ETSI GSM and 3GPP standard

specifications Easily upgradeable from GSM/GPRS to EGPRS Fast porting to most DSP platforms Minimal development time Advantageous trade-off between DSP cost and performance Compatibility with wide variety of products, from low to high end Supporting the full range of mobile stations Software based solution

Specially designed for highly demanding environments, EDGEwareTM means: Lower DSP costs Minimal RAM and ROM cost Higher resource allocation control A simple and easy to manage solution

Easy to port Easy to integrate

EDGEware from Comsys is the next logical step in the evolution to 3G. Specifications subject to change without notice. 2001 Comsys Ltd. All rights reserved. EDGEware is a trademark of Comsys. All other brands and corporate names are trademarks of their respective owners. The information presented herein is to the best of our knowledge true and accurate. No warranty or guarantee expressed or implied is made herein regarding the capacity, performance or suitability of any product. For further information please contact Comsys Communication & Signal Processing Ltd., Mr. Doron Porat, EDGE/GPRS/GSM Project Manager, Telephone: +972 9 971 7892, [email protected] http://www.comsysmobile.com/

增量冗余Incremental redundancy

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