POWER AND SPECTRUM EFFICIENT ACCESS SERVICES USING DYNAMIC LINKS

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1 POWER AND SPECTRUM EFFICIENT ACCESS SERVICES USING DYNAMIC LINKS Dr. Richard Gedney, Dr. William Thesling, and Mark Vanderaar Efficient Channel Coding (ECC), Inc. 600 Safeguard Plaza, Suite 100 Brooklyn Heights, OH Phone: Fax: Introduction A recent report 1 on commercial satellite Internet systems has indicated that more Internet subscribers per transponder are required to reduce the transmission cost, which is a major component of the overall service cost. The current service cost of around $70 a month for two-way consumer satellite Internet in the US appears to be too high to attract a large number of subscribers. Existing satellite systems used QPSK modulation with concatenated Convolutional and Reed-Solomon error correction codes as per the DVB standard. In addition, the RF links are run using fixed margins with the size of the margin dependent upon the maximum rain attenuation predicted for the service area. One way to increase the system data throughput is to manage each user terminal separately and have each one use as high a level modulation in combination with as high a code rate as the instantaneous link conditions allow. As link conditions fade for each individual terminal, the modulation level and code rate is changed to maintain BER requirements. Since only a low percentage of user terminals in a beam will encounter large rain attenuation at any time, this technique, called Dynamic Link Assignment (DLA), significantly increases average information throughput per unit bandwidth. References 2 and 3 have shown this increase to be on the order of 250 to 400 percent for Ka-band geostationary satellite systems. The exact increase depends upon the rain rate region for the service location and the satellite configuration. In addition to these increases in capacity, the technique may lower satellite and user terminal EIRP requirements. These gains allow the service cost to be reduced to under $30 a month. Additionally, DLA provides a significant advantage for spot beam systems. In the typical spot-beam system, the satellite antenna gain from edge of coverage to beam center varies by about 4 db. As a result, the satellite EIRP varies by the same amount over the beam coverage. Dynamic links as opposed to fixed links allows each user terminal to automatically sense where it is in the spot beam and to operate at as high a modulation level and code rate as possible. This also significantly increases the transponder data throughput. 1

2 The use of adaptive rain compensation has been proven for geostationary satellite systems by the ACTS program 4. Efficient Channel Coding (ECC), Inc. has developed a system using advanced error correction and higher-level modulations that greatly improves on the ACTS technique. The ECC system was developed in conjunction with CodeSpace LLC and Shin Satellite Public Company. It is incorporated into Shin Satellite s ipstar satellite service. In this paper the technique and user modems are described, the performance presented and the availability of the chip sets for the user s set-top-box is discussed. It is widely accepted that Forward Error Correction (FEC) is a valuable technique to increase power and spectrum efficiency and has an important role in any satellite system. ECC has performed significant development on a class of codes that offer performance closer to Shannon s limit than traditional concatenated codes. These codes, which we call Turbo Product Codes (TPCs), involve the iterative soft decision decoding of a product code 5. TPCs have a wide range of flexibility in code rate with significant increase in gain. For example for QPSK with a rate code, TPCs are able to provide an additional gain of approximately 1.6 db over that for the traditional concatenated Reed Solomon outer code with a Viterbi inner code at a BER of This result is based on data from back-to-back modem tests (and thus includes implementation loss effects). ECC s efficient decoding algorithm enabled the world s first practical turbo code error correction chips and software in late TPCs are used to provide all FEC in the ipstar physical layer. The outline for this paper is as follows. Section 1 describes the approach for dynamic links and the channelization architecture. In section 2 the modem performance is presented. Sections 3 and 4 discuss, respectively, the consumer set-top-box and the conclusions. 1. Dynamic Links Using Adaptive Modulation and Coding The basic architecture for the physical and network layers are described in this section. Dynamic Link Assignment (DLA) For dynamic links, variable modulation and TPC codes rates are utilized in an adaptive fashion. The signal to noise ratio (SNR) for the forward link is constantly monitored by each user terminal. The SNR for the return link from each user terminal is monitored by the gateway. As the SNR varies with changes in link attenuation, the modulation and/or code rate is automatically changed on both the forward and return links on an individual user terminal basis to ensure that its BER requirement is maintained. 2

3 Eb/No (db) :43:12 7:12:00 7:40:48 8:09:36 8:38:24 9:07:12 9:36:00 10:04:48 Time (Z) Figure 1: 20 GHz Rain Fade Event Maryland 1/20/95 Figure 2: % Probability of Fade Slope for GHz ACTS 5-year data 3

4 Under clear air conditions, each terminal uses a high level modulation in conjunction with a high code rate. The maximum bits per symbol per Hz that is achievable with the present design is 3.52 using 16-ary modulation with a rate TPC. The minimum bits per symbol per Hz that can be used for the current system is 0.65 using QPSK with a rate TPC. When a threshold delta in Es/No is exceeded, the user terminal sends the information to the gateway. The gateway then decides on the change to be made in code rate and if necessary the modulation level. Notification of the new modulation and code rate is then sent back to the user terminal for the next transmission. The total time for this process to take place is less than 1 second. Figure 1 shows a 20 GHz rain event as recorded by an ACTS user terminal located in Maryland. Although the rain fade slopes appear large in this time-compressed display, the rain fades rates experienced are quite small. As shown by Figure 2, the probability of a 20 GHz rain fade rate exceeding 0.9 db/sec is very small. Therefore, a nominal 1 db of system margin should be adequate to maintain the service BER requirements. Channelization Architecture The channelization architecture design is summarized in table 1. The forward link is composed of multiplexed data channels providing a composite rate of 54 Msymbols per second. For ipstar, the satellite uses transponders that can accommodate up to 9 of these 54 Msps bands in a bent pipe mode of operation. Since the transponders have a wide bandwidth and must accommodate multiple channels, they are operated in a semilinear gain regime. Each user s data is time multiplexed in a conventional fashion onto a forward link channel. The forward link modulation and coding can be changed on a per user terminal basis rapidly without resynchronization. The range of selectable modulations and TPC rates are shown in table 1. Table 1: Forward and Return Link Channelization Architecture Link Channelization User Access Forward 54 Msymbols Time per second Division Return MF-TDMA ALOHA, KHz Slotted MHz ALOHA, MF-TDMA Selectable Modulation Selectable TPC Rate QPSK, ary, 16-ary QPSK, 8-ary Bits Per Symbol The return link is also composed of a large number of channels that are accessed using Multiple Frequency Time Division Multiple Access (MF-TDMA). The channel bandwidth, which is selectable for each user, varies between KHz and MHz in powers of 2. The access can be accomplished via Aloha, Slotted Aloha or MF- 4

5 TDMA schemes. Aloha is primarily for a user terminal to gain entry into the system. Slotted Aloha is used for low return channel data rate applications where real time performance is not required, e.g. web browsing. MF-TDMA is used for higher data rate packet transmissions or for continuous circuits. The modulation and TPC rates are selectable as given in table 1. The gateway controller allocates the modulation and coding for the forward and return links based on an individual terminal s need to accommodate any link attenuation. The controller also assigns the capacity for each terminal and ensures that quality-of-service requirements are met. Once a user terminal has been accepted into the system, it is both symbol and frame synchronized, minimizing overheads for any transmissions on the forward and return links. 2. Modem Performance Care has been taken to optimize the combined modulation and coding performance. TPCs are block codes, and the block sizes of encoded data that have been utilized cover the range from 2K to 16K bits in power of two. The TPC is applied in a stream fashion where by users in a specific code rate/modulation category are combined into a common group so that packing of packets of varying lengths is done efficiently Information Bits Per Symbol ary Modulation Capacity 16-ary Modulation Capacity Unconstrained Capacity QPSK Capacity 16K Block Size 2K Block Size Es/No Figure 3: Back-to-back modem performance in terms of information bits 5

6 The communications performance is enhanced by the application of the TPCs as compared to the DVB-S concatenated code consisting of a Reed Solomon outer code and a convolutional inner code. The back-to-back (including tuner) modem performance is presented in figure 3 as a plot of information bits per symbol versus Es/No. The actual performance for the modems is divided into three bands across the figure. The left, center and right bands are for, respectively, QPSK, 8-ary and 16-ary modulations. The single line above these bands represents the Shannon capacity for each modulation. The horizontal location within each band is dependent upon the TPC block size with the best (smallest) Es/No performance being for the 16K block size and worst performance being for the 2K-block size. Over 40 combinations of modulation and code rate are possible. For the system which has been developed, the average bits-per-symbol step size is approximately 0.08 which corresponds to a 0.36 db step size in Es/No. The gateway automatically selects the operating point, which maximizes the bits per symbol. With this find of granularity in modulation/coding operating points, the capacity (information throughput) is optimized. 3. Set-Top-Box A consumer set-top-box a long with head-end modems have been developed by ECC for the ipstar System to provide a last mile broadband access solution in Asia for Internet, Multicast, Broadcast, VoIP and continuous circuit services. Each user terminal operates in a star configuration with its assigned gateway. Figure 4 is the block diagram for the customer premise indoor equipment. It has the ability to process one band on the forward link with two separate channels per band. Data from each channel can consist of either broadcast (video), multicast or unicast information. The return link is synchronized to the forward band and has the channel characteristics as described in the previous section. With the exception of the external power supply, the network processor and the L-Band Upconverter, the other elements of the block diagram are an ECC chipset to meet the cost requirements for the set-top-box in the consumer market. ECC is currently developing the chipset, and production quantities are scheduled to be available in 2003, which is the time frame for the launch of ipstar satellite by Shin. The ASIC satellite modem design is based in large part on an existing ECC hard copy FPGA implementation. This hard copy implementation has been tested in the field and is being used by Shin to roll out commercial service using their current Ku-band satellites. The user data rates available on the return link can be up to 4.1 Mbps, while those on the forward link can be up to 6 Mbps for a single channel. The actual data rates for each user depend on the combination of services being provided, the number of forward link channels being processed, the link conditions, and antennae patterns. 6

7 With this broad a range of traffic types, provisions have been made to provide a variable Quality of Service. The rain fade mitigation technique (adaptive modulation and coding) described in the previous sections is fully incorporated. Return Link L-Band Upconverter Forward Band L-Band Tuner ASIC ECC Chipset (Converter, Modem, FEC and Transport) External Power Supply To Video Processor SDRAM Network Processor USB Ethernet Figure 4: Block Diagram for Consumer Set-Top-Box 4. Conclusion A two-way satellite Internet modem system has been developed by ECC using advanced error correction and dynamic links, which greatly increases the data throughput per unit bandwidth. Depending upon the rain attenuation region and the RF frequency for operations, the benefit of Dynamic Links with Turbo Product Codes versus Fixed links with concatenated Reed Solomon/Viterbi code varies from one and one half to four times the information throughput. This system is currently being used by Shin Sat to provide commercial services in Asia using their existing Ku Band satellites. In late 2003, Shin Satellite plans to launch the spot-beam ipstar-1 satellite. ipstar-1 with its high degree of frequency reuse coupled with the advanced error correction and DLA will provide the most cost effective broadband services of any satellite system. The price requirement for the set-top-box is being met using chipsets developed by ECC. Production quantities for the chipsets will be available in

8 5. References 1. R. Rusch, Forecast for Broadband Satellite Services Part 2, SkyBroadband ( February 19, R. Gaudenzi and R. Rinaldo, Adaptive Coding and Modulation for Next Generation Broadband Multimedia Systems, Proceedings of the 20 th AIAA International Satellite Communications Systems Conference, Montreal, Canada, May W. Thesling, M. Vanderaar, Mark Thompson, Peter Hamilton, P. Panuvattanawong and R. Gedney, Two-Way Internet Over ipstar Using Advanced Error Correction and Dynamic Links, Proceedings of the 20 th AIAA International Satellite Communications Systems Conference, Montreal, Canada, May R. Gedney and T. Coney, Effective Use of Rain Fade Compensation for Ka Band Satellites, 17 th AIAA International Communications Satellite Systems Conference, Yokohama, Japan, February Technical Description of Turbo Product Codes, Version 4.0, June 1999 at Mbps Turbo Product Code Encoder/Decoder Chip, Product Specification AHA4501 at 8