IEEE C802.16d-04/50r1. IEEE Broadband Wireless Access Working Group <

Transcription

1 IEEE C80.6d-04/50r Project Title Date Submitted Source(s) IEEE 80.6 Broadband Wireless Access Working Group < OFDMA PHY Enhancements for better mobility performance [ ] John Liebetreu, Jeff Foerster, Jose Puthenkulam, Randall Schwartz, David Johnston, Hassan Yaghoobi, Intel Corporation Panyuh Joo, Seungjoo Maeng, Jaeho Jeon, Soonyoung Yoon, Jeong-Heon Kim, Jaehyok Lee, Myungkwang Byun, JeongTae Oh, Wonil Roh, Inseok Hwang, Jaehee Cho, Sanghoon Sung, Hun Huh, Jiho Jang, Ikbeom Lee, HeeSang Seo, Sijun Cho, Chiwoo Lim, Youngbin Chang, Jaeweon Cho, Jaeyoel Kim, Sung- Eun Park, Samsung Electronics Co. Ltd. Naftali Chayat, Tal Kaitz, Mohammed Shakouri, Vladimir Yanover, Marianna Goldhammer, Alvarion Ltd. Shawn Taylor, Gordon Antonello, Ron Murias, Lei Wang, Wi-LAN Inc. J. Pierre Lamoureux, Frank Draper, Jon Labs, Rainer Ullmann, Wavesat Wireless Inc Martin Lysejko, Ofer Kelman, David Castelow, Airspan Raja Banerjea, Don Leimer, Proxim Inc Phil Barber, Broadband Mobile Technologies Dale Branlund, Lalit Kotecha, Mike Webb, BeamReach Networks Re: Working Group Review of P80.6-REVd_D3 Abstract Purpose Notice Release Patent Policy and To propose enhancements to the OFDMA PHY in 80.6REVd_D3 draft for better perfo This document has been prepared to assist IEEE It is offered as a basis for discus individual(s) or organization(s). The material in this document is subject to change in fo contributor(s) reserve(s) the right to add, amend or withdraw material contained herein The contributor grants a free, irrevocable license to the IEEE to incorporate material con thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE s nam may include portions of this contribution; and at the IEEE s sole discretion to permit oth IEEE Standards publication. The contributor also acknowledges and accepts that this co The contributor is familiar with the IEEE 80.6 Patent Policy and Procedures < including the statement "IEEE standards may include the known use of patent(s), includ receives assurance from the patent holder or applicant with respect to patents essential

2 Policy and Procedure s receives assurance from the patent holder or applicant with respect to patents essential portions of the standard." Early disclosure to the Working Group of patent information to reduce the possibility for delays in the development process and increase the likeliho publication. Please notify the Chair <mailto:chair@wirelessman.org> as early as possibl technology (or technology under patent application) might be incorporated into a draft Working Group. The Chair will disclose this notification via the IEEE 80.6 web site <h 3 Page

3 OFDMA PHY Enhancements Introduction In this contribution we propose enhancements to the WirelessMAN OFDMA PHY, so that it can perform more optimally in channel bandwidths ranging from.5mhz to 0 MHz with fixed subcarrier spacing but dynamic selection of the number of subcarriers for mobile applications. The following are some of the parameters that are required to meet the requirements from service providers. Bandwidth For service providers who would like to deploy a high speed public cellular network, the system bandwidths are limited to.5,.5, 5, and 0 MHz for licensed bands and 0 and 0MHz for unlicensed bands. 3 Sampling Frequency According to the allowed bandwidth, the sampling frequency needs to be the same as bandwidth. 4 FFT Size and CP duration In order to support full coverage and full mobility with low overhead for CP duration, the FFT size corresponding to the bandwidth should be scalable, i.e., 8-FFT for.5 MHz BW, 56-FFT for.5 MHz BW, 5-FFT for 5 MHz, 04-FFT for 0 MHz BW, and 048-FFT for 0 MHz BW. The CP duration is kept to be /8 of the OFDMA symbol duration since the OFDMA symbol durations for all bandwidth configurations are equal and the maximum delay of multipath channel up to 0 us should be supported. 5 Frame Length Frame length is from msec to 0msec with identical frame structure for various channel bandwidths in licensed operation. 6 Proposed Text Changes [Change the existing text in 8.4. Introduction as follows] The WirelessMAN-OFDMA PHY ([B39]), based on OFDM modulation, is designed for NLOS operation in the. GHz frequency bands. The allowed channel bandwidths shall be.5,.5, 5, and 0 MHz for licensed bands and 0 and 0MHz for unlicensed bands. [Insert a following section Basic system parameters after Transmitted signal ] Basic system parameters The basic system parameters to characterize an OFDMA signal are described in Error! Reference source not found.. Table Basic system parameters Parameters Values System bandwidth.5 MHz.5 MHz 5 MHz 0 MHz 0 MHz Sampling frequency (F s ).5 MHz.5 MHz 5 MHz 0 MHz 0 MHz Sample time (/F s ) 800 nsec 400 nsec 00 nsec 00 nsec 50 nsec FFT size (N FFT ) Number of used subcarriers Number of data subcarriers Number of pilot subcarriers Page

4 Subcarrier frequency spacing Useful symbol time (T b =/_f) CP time (T g =T b /8) OFDMA symbol time (T s =T b +T g ) khz 0.4 _s.8 _s 5. _s Enhancements to the frame structure Current frame structure in 80.6REVd/D3 048 FFT OFDMA mode has a limitation to support bandwidth scalability and consistent mobility performance simultaneously. For example, fixed FFT size cannot provide a unified frame structure since symbol period increases as the bandwidth decreases. In addition, the 048 FFT size gives a very coarse granularity in symbol axis when system bandwidth is scaled down to.5 MHz. The problem becomes more severe in TDD system when a short frame length is required for fast link control. To handle these limitations and improve overall system performance, a new frame structure is proposed. 8 Design Principles For bandwidth scalability, these two factors are considered for the system parameter selection.. Fixed tone spacing to maintain consistent mobility performance. Symbol time tradeoff between guard time overhead for delay spread and ICI from Doppler spread To enhance the system performance, new features are introduced. The main features are summarized as follows: ) AMC subchannels to maximize multi-user diversity gain by utilizing frequency selectivity of each frequency band (in contrast to diversity subchannels to provide full frequency diversity in entire band for mobile users) ) Dynamic allocation of diversity subchannels and AMC subchannels depending on SS s channel conditions 3) Dedicated signaling channels for fast adaptive modulation coding and H-ARQ 4) Dedicated uplink time slot for initial random access to reduce interference to traffic channels 5) Safety channels for service quality of users in cell boundary 6) Support AAS mode for system throughput and/or coverage extension 7) Support frequency reuse of and 3 9 Enhancements using diversity subchannels In order to mitigate inter-cell interference, diversity subchannels shall be designed such that hits between subchannels in different cell are minimized. Furthermore, it is desirable that the number of hits between subchannels remains unchanged regardless of the subchannel index or cell id. One more thing to consider is the number of differentiable cells. The number of differentiable cells proposed by the subchannelization in the current 80.6-REVd is only 3, which is unacceptable for the typical cellular networks. In this contribution, new subchannelization for downlink is proposed. With this subchannelization, the number of differentiable cells is 04 for 0 MHz bandwidth configuration and the number of hits between subchannels is almost the same regardless of the subchannel index or cell id. As for the uplink, one must consider uplink channel estimation performance as well as hit characteristics and the number of differentiable cells. If tones constituting a subchannel are scattered in the whole band like current subchannelization of REVd, it is difficult to estimate the channel since available pilot tones are too sparse. In this contribution, a new diversity subchannelization for uplink is proposed, which can be more helpful for channel estimation. 0 Enhancements using AMC subchannel The frequency diversity of a channel can be aggressively exploited with band AMC. The comparison between the average channel power of whole band and the fraction of the band is presented. Further, a permutation of the subcarrier mapping order within the band AMC subchannel is proposed. Band AMC is aimed to exploits the frequency diversity of the channel. When BS has knowledge of the channel conditions of each MS, BS can allocate the best AMC band for each MS. Figure shows the band selection gain for the different channel models (ITU PED A/B, VEH A/B). The band selection gain means the ratio of the average channel power of AMC band (subchannel) to the average channel power of the whole band. The AMC band (subchannel) whose average channel power is larger than that of other AMC band is selected. A AMC band consists of 54 subcarriers. For PED B and VEH A, AMC band selection gain more than 3 db occurs around 90%. For PED A and VEH B, the gain is not so large. The selection gain directly increases system throughput with some proper implementation. These observations encourage using band AMC and exploiting the frequency diversity. Further, the AMC band selection gain extends coverage of system. In case that the average SINR of the whole band is less than the SINR threshold of the most robust AMC level but the average SINR of a AMC band is above the threshold, BS allocates Page 3

5 3 AMC band to make the link. It is usual case when MS is located near cell edge. PED -A Band = 4 subcarriers PED -B Band = 4 subcarriers VEH-A Band = 4 subcarriers VEH-B Band = 4 subcarriers Figure. AMC band selection gain for different channel models Reasons for dedicated control subchannel There are three control channels in uplink, ranging channels, CQI channels, and ACK channels. In order for ranging channels to perform better, contiguous tones should be used. On the other hand, frequency diversity must be provided for CQI channels and ACK channels. Control channel subchannelization is to provide unified scheme for control channels, which provides diversity for CQI/ACK channels and contiguous tones allocation for ranging channels. The control channels can be allocated on diversity or AMC subchannels. Introducing Safety channels In order to operate the OFDMA system with frequency reuse factor of, the high inter-cell interference at cell edge should be controlled. If an SS is located at cell edge or at the region of adjacent cell area but the SS is still connected to the serving cell, the SS may cause high uplink interference to adjacent cell and also the link performance of the SS itself is degraded. Since the soft hand-off is not valid in the OFDMA system, several frequency bands (safety channel) are reserved for the SSs which are connected to other cells but are located at his cell area. By introducing the safety channel, link quality for the SSs may be improved and also the uplink interference from the SSs can be mitigated. By the concept of safety channel, we may operate the OFDMA system with effective frequency reuse factor of non-one dynamically. In order to solve the problem defined above, we propose a safety channel. Safety Channel is defined as reserved bins in each cell on which no SS or BS transmits any traffic in its own cell. BS in each cell reserves N bins for safety channels where N is a system parameter, and the locations of bins for safety channel should be different for each cell. In order to indicate the locations of safety channel for each cell, AP punctures N bins in the second downlink preamble at the locations of safety channels. Figure illustrates an example of a usage of the safety channel. Page 4

6 3 4 3 Proposed Text Changes [Replace IEEE P80.6-REVd/D Frame structure with the following text.] Frame Structure Frame Structure for TDD Systems Figure TDD Frame Structure For a TDD system, each frame starts with downlink, a BS to SS transmission. The downlink transmission begins with two preamble symbols followed by a SICH (System Information CHannel) symbol as shown in Figure. In the uplink, transmission begins with control symbols. In order to allow BS to turn around, TTG and RTG shall be inserted between downlink (DL) and uplink (UL) in the middle of a frame and at the end of a frame, respectively. The number of downlink and uplink symbols can be changed with a granularity of six symbols (boundary between DL and UL subframes). Both in downlink and uplink, there are two kinds of subchannels, diversity subchannels and AMC subchannels. Accordingly, transmission period can be divided into diversity subchannel period and AMC subchannel period. Diversity subchannel consists of 54 distributed tones within multiple symbols in downlink. In uplink, a tile, which is composed of the set of 3 contiguous subcarriers through 6 contiguous symbols, is a basic allocation unit for diversity subchannel. A diversity subchannel is made up of 3 tiles, which are spread over whole frequency band in uplink. A tile structure is shown in Fig.. For AMC subchannel, a bin, which is the set of 9 contiguous subcarriers within an OFDMA symbol, is a basic allocation unit both in downlink and uplink. A bin structure is shown in Figure 3. The pilot locations of within bins and tiles can be changed to get better filtering gain for channel estimation. 8 data tones 6 data tones + pilot tones pilot tone 3 Figure Bin Structure Figure 3 Bin Structure A group of 4 rows of bins is called a band. An AMC subchannel consists of 6 bins. These may be structured to support a variety of requirements including AAS, frequency diversity, or band-selectivity. A frame consists of multiple bands. Total number of bins in a whole frequency band depends on bandwidth as shown in Error! Reference source not found.. Page 5

7 3 Table Number of bands and bins Bandwidth.5 MHz.5 MHz 5 MHz 0 MHz 0 MHz N FFT Number of bands Number of bins per bands The downlink symbol right after DL preamble constitutes an SICH symbol on either a diversity or an AMC subchannel. To indicate the serving BS s safety channel quickly, some bins of the second symbol in DL preamble are punctured. In uplink, the first three OFDMA symbols are used for control symbols. Ranging channels, ACK channels, and CQI channels are transmitted through control symbols. For reuse 3 deployment, set of bands with the same index of modulo 3 consists a frequency group. The location of the safety subchannel, which consists of the reserved bins for safety operation, is also to be broadcasted in SICH symbol Downlink frame structure Downlink frame structure is shown in Error! Reference source not found.4. The DL preamble can be used for initial timing synchronization, BS identification, carrier offset estimation and channel estimation. Downlink transmission period can be divided into diversity subchannel period and/or AMC subchannel period. A diversity subchannel consists of distributed tones within multiple symbols and an AMC subchannel consists of 6 contiguous bins as described in The number of diversity symbol (D) and AMC symbol (A) can vary depending on distribution of SS s channel conditions. Band b- Band b Band b+ Symbol : D D 0 0 D D D A A A A A A Safety channel Bin for AMC SC Bin for Safety SC Tone for diversity SC Note: Safety channel is each BS s reserved frequency bin not used for the SS of the serving BS. Safety subchanne(sc) is the frequency bin allocated for the SS which is connected to the serving BS and has requested the bin in safety mode. This bin is other BS s safety channel. Figure 4 Downlink Frame Structure 3 4 Page 6

8 Band 0 Band Band Band 3 tile Bin for AMC subchannel # Bin for AMC subchannel # Bin for AMC subchannel #3 Bin for AMC subchannel #4 Bin for AMC subchannel #5 Bin for AMC subchannel #6 Tile for diversity subchannel # Tile for diversity subchannel # Tile for diversity subchannel #3... Safety Channel Band B- bins Bin Pilot tone OFDM symbol Figure 5. Uplink Frame Structure Uplink frame structure In the uplink, there are two kinds of subchannels, AMC subchannels and diversity subchannels. The first three OFDMA symbols are used for ranging channels, ACK channels, and CQI channels as shown in Figure 5. Basic allocation unit for diversity subchannel is a tile. A diversity subchannel consists of 3 tiles spread over whole frequency band. Basic allocation unit for AMC subchannel is a bin. One AMC subchannel consists of 6 bins. As in downlink, the configuration of AMC and diversity uplink symbols can vary. Since uplink time interval for initial random access are separated from data symbols, interference from SS in initial access to traffic channels can be reduced. In addition, fast downlink link control such as adaptive modulation coding and H-ARQ is enabled through dedicated control channels such as ACK channels, and CQI channels. The H-ARQ is essential for higher system throughput and reliability under imperfect channel quality measurement, interference variation from traffic burst and packet scheduling delay Frame structure for FDD systems 3. Downlink and uplink frame structure for FDD mode are identical to TDD ones in Error! Reference source not found.4, Figure 5, respectively. Downlink transmission period can be divided into diversity subchannel period and AMC subchannel period in the same way as in TDD mode. Uplink frame structure in FDD mode is also basically the same as the one in TDD mode. 4 Proposed Text Changes We propose the following remedies in IEEE P80.6-REVd/D3 [Replace the section OFDMA subcarrier allocations with the following text] OFDMA subcarrier allocations Subtracting the DC subcarrier and the guard subcarrier from N FFT, one obtains the set of used subcarriers N used (assume N used is even for the time being). For both uplink and downlink these used carriers are allocated to pilot carriers and data carriers. To constitute AMC subchannels both in downlink and uplink, nine contiguous subcarriers including one pilot carrier, all within a symbol are grouped into a bin. The structures of diversity subchannels are, however, different for downlink and uplink. In downlink, pilot carriers are allocated first and the remaining carriers are used exclusively for data transmission. For the uplink diversity subchannels, however, the set of adjacent subcarriers in time-frequency plane forms a tile and then a pilot carrier is allocated from within the tile. Page 7

9 In what follows, subcarriers are identified by a subcarrier index k and the corresponding frequency offset index is specified as k foi k N = k N used used /, k / +, k < where k foi is frequency offset index, k is subcarrier index, and N used is the number of used subcarriers. This relation with IFFT index is shown in Error! Reference source not found.. Each BS can reserve safety channels to provide a shelter to MS connected to other BS. For both uplink and downlink, AMC subchannels are defined with bins after excluding reserved ones for safety channels. The subcarriers in downlink diversity subchannels are punctured if their subcarrier indexes collide with the subcarriers allocated for safety channels. k DC N used / N used / + N used / +.. N used - N used - Null Null.. Null Null 0.. N used / - 3 N used / - N used / - k foi N N N used / - N used / Null Null.. Null Null -N used / -N used / used used / / IFFT Figure Subcarrier index mapping (.) Downlink Preamble The first and second symbol of the downlink transmission are the preamble; there are types of preamble carrier-sets, those are defined by allocation of even or odd subcarriers for each one of them; those subcarriers are modulated using a boosted BPSK modulation with a specific code. The preamble carrier-sets are defined using the following formula: ( q [ m] ), N used N used ID k = m, m = 0,, L, cell, S 4 N used N used N used PID [ k] = ( qid [ m ] ), k = m, m = +, +, L cell, s cell, S 4 4 0, otherwise {}{}{}0,,6,0,,7,/,/,,/cellFFTFFTFFTIDskNNN + LLL N, used Page 8

10 3 4 If [],cellidspk is IFFT-processed, it results in a pattern repeating itself once in the time-domain. In the previous equation, is multiplied so that the DL preamble has the same average power level as that of the data OFDMA symbols and q [ m] defined as follows. [],(6mod8), where mod80,,,58 0,,,43(mod), where mod86,78cellidsm ID cell, s is 5 6 Where, mod888()(, (mod8)),6mod80,,,3838srcellrmrrwhidrrm = += += L All the sequences regarding to (m) T should use the codes shown in Table 3 where, H 8( i, j) is the ( j) i, th element of the length 8 Walsh Hadamard matrix, i, j = 0,, L 7, and s denotes the sector ID. The elements of first row of H are all ()8,0,,7mll = L 8m 8m, so we should use the matrix except first row. means th permutation, represents largest integer not larger than the indexes shown in Table. 8m. Those entire permutation indexes required for these preambles should use (0,,,7)l=L Table Permutation 0 ( l), 65, 97, 3,, 5, 7, 6, 63, 94, 47, 86, 43, 84, 4,, 75, 00, 50, 5, 77, 03, 4, 57, 93,, 8, 59, 9, 46, 3, 74, 37, 83, 04, 5, 6, 3, 7, 98, 49, 89, 09, 9,, 6, 95, 0, 55, 90, 45, 87, 06, 53, 9, 08, 54, 7, 76, 38, 9, 7, 36, 8, 9, 69, 99,, 56, 8, 4, 7, 66, 33, 8, 05, 7, 3, 4, 6, 3, 78, 39, 8, 4, 85, 07, 6, 58, 9, 79, 0, 5, 88, 44,,, 68, 34, 7, 73, 0, 5, 0, 60, 30, 5, 70, 35, 80, 40, 0, 0, 5, 67, 96, 48, 4,, 6, 3, 64, 3, 6, 8, 4,, , 77, 03, 4, 57, 93,, 8, 59, 9, 46, 3, 74, 37, 83, 04, 5, 6, 3, 7, 98, 49, 89, 09, 9,, 6, 95, 0, 55, 90, 45, 87, 06, 53, 9, 08, 54, 7, 76, 38, 9, 7, 36, 8, 9, 69, 99,, 56, 8, 4, ( ) 7, 66, 33, 8, 05, 7, 3, 4, 6, 3, 78, 39, 8, 4, 85, 07, 6, 58, 9, 79, 0, 5, 88, 44,,, l 68, 34, 7, 73, 0, 5, 0, 60, 30, 5, 70, 35, 80, 40, 0, 0, 5, 67, 96, 48, 4,, 6, 3, 64, 3, 6, 8, 4,,, 65, 97, 3,, 5, 7, 6, 63, 94, 47, 86, 43, 84, 4,, 75, 00, 50, 0 7, 98, 49, 89, 09, 9,, 6, 95, 0, 55, 90, 45, 87, 06, 53, 9, 08, 54, 7, 76, 38, 9, 7, 36, 8, 9, 69, 99,, 56, 8, 4, 7, 66, 33, 8, 05, 7, 3, 4, 6, 3, 78, 39, 8, 4, 85, 07, 6, 58, 9, ( ) l 79, 0, 5, 88, 44,,, 68, 34, 7, 73, 0, 5, 0, 60, 30, 5, 70, 35, 80, 40, 0, 0, 5, 67, 96, 48, 4,, 6, 3, 64, 3, 6, 8, 4,,, 65, 97, 3,, 5, 7, 6, 63, 94, 47, 86, 43, 84, 4,, 75, 00, 50, 5, 77, 03, 4, 57, 93,, 8, 59, 9, 46, 3, 74, 37, 83, 04, 5, 6, 3, 0 Table 3 (), 0,,,47Tmm=L ID cell s sequence PAPR _ _ _ _ _ _ Page 9

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