Intel Technology Journal

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1 Volume 8 ssue Publshed, August, N 55-86X ntel Technology Journal WMAX calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN A compled verson of all papers from ths ssue of the ntel Technology Journal can be found at:

2 ntel Technology Journal, Volume 8, ssue, calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN Hassan Yaghoob, ntel Communcatons Group, ntel Corporaton ndex words: OFDMA, calable, EEE 8.6, WrelessMAN, subchannel, subcarrer ABTRACT The concept of scalablty was ntroduced to the EEE 8.6 WrelessMAN Orthogonal Frequency Dvson Multplexng Access (OFDMA) mode by the 8.6 Task Group e (TGe). A scalable physcal layer enables standard-based solutons to delver optmum performance n channel bandwdths rangng from.5 MHz to MHz wth fxed subcarrer spacng for both fxed and portable/moble usage models, whle keepng the product cost low. The archtecture s based on a scalable subchannelzaton structure wth varable Fast Fourer Transform (FFT) szes accordng to the channel bandwdth. n addton to varable FFT szes, the specfcaton supports other features such as Advanced Modulaton and Codng (AMC) subchannels, Hybrd Automatc Repeat Request (H-AR), hgh-effcency uplnk subchannel structures, Multple-nput-Multple- Output (MMO) dversty, and coverage enhancng safety channels, as well as other OFDMA default features such as dfferent subcarrer allocatons and dversty schemes. The purpose of ths paper s to provde a bref tutoral on the EEE 8.6 WrelessMAN OFDMA wth an emphass on scalable OFDMA. NTRODUCTON The EEE 8.6 WrelessMAN standard [] provdes specfcatons for an ar nterface for fxed, portable, and moble broadband wreless access systems. The standard ncludes requrements for hgh data rate Lne of ght (LO) operaton n the -66 GHz range for fxed wreless networks as well as requrements for Non Lne of ght (NLO) fxed, portable, and moble systems operatng n sub GHz lcensed and lcensed-exempt bands. Because of ts superor performance n multpath fadng wreless channels, Orthogonal Frequency Dvson Multplexng (OFDM) sgnalng s recommended n OFDM and WrelessMAN OFDMA Physcal (PHY) layer modes of the 8.6 standard for operaton n sub GHz NLO applcatons. OFDM technology has been recommended n other wreless standards such as Dgtal Vdeo Broadcastng (DVB) [] and Wreless Local Area Networkng (WLAN) []-[], and t has been successfully mplemented n the complant solutons. Amendments for PHY and Medum Access Control (MAC) layers for moble operaton are beng developed (workng drafts [5] are beng debated at the tme of publcaton of ths paper) by TGe of the 8.6 Workng Group. The task group s responsblty s to develop enhancement specfcatons to the standard to support ubscrber tatons () movng at vehcular speeds and thereby specfy a system for combned fxed and moble broadband wreless access. Functons to support optonal PHY layer structures, moble-specfc MAC enhancements, hgher-layer handoff between Base tatons (B) or sectors, and securty features are among those specfed. Operaton n moble mode s lmted to lcensed bands sutable for moblty between and 6 GHz. Unlke many other OFDM-based systems such as WLAN, the 8.6 standard supports varable bandwdth szes between.5 and MHz for NLO operatons. Ths feature, along wth the requrement for support of combned fxed and moble usage models, makes the need for a scalable desgn of OFDM sgnalng nevtable. More specfcally, nether one of the two OFDM-based modes of the 8.6 standard, WrelessMAN OFDM and OFDMA (wthout scalablty opton), can delver the knd of performance requred for operaton n vehcular moblty multpath fadng envronments for all bandwdths n the specfed range, wthout scalablty enhancements that guarantee fxed subcarrer spacng for OFDM sgnals. The concept of scalable OFDMA s ntroduced to the EEE 8.6 WrelessMAN OFDMA mode by the 8.6 TGe and has been the subject of many contrbutons to the standards commttee [6]-[9]. Other features such as AMC subchannels, Hybrd Automatc Repeat Request calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

3 ntel Technology Journal, Volume 8, ssue, (H-AR), hgh-effcency Uplnk (UL) subchannel structures, Multple-nput-Multple-Output (MMO) dversty, enhanced Advanced Antenna ystems (AA), and coverage enhancng safety channels were ntroduced []-[] smultaneously to enhance coverage and capacty of moble systems whle provdng the tools to trade off moblty wth capacty. The rest of the paper s organzed as follows. n the next secton we cover multcarrer system requrements, drvers of scalablty, and desgn tradeoffs. We follow that wth a dscusson n the followng sx sectons of the OFDMA frame structure, subcarrer allocaton modes, Downlnk (DL) and UL MAP messagng, dversty optons, rangng n OFDMA, and channel codng optons. Note that although the EEE P8.6-REVd was ratfed shortly before the submsson of ths paper, the EEE P8.6e was stll n draft stage at the tme of submsson, and the contents of ths paper therefore are based on proposed contrbutons to the workng group. MULTCARRER DEGN REUREMENT AND TRADEOFF A typcal early step n the desgn of an Orthogonal Frequency Dvson Multplexng (OFDM)-based system s a study of subcarrer desgn and the sze of the Fast Fourer Transform (FFT) where optmal operatonal pont balancng protecton aganst multpath, Doppler shft, and desgn cost/complexty s determned. For ths, we use Wde-ense tatonary Uncorrelated catterng (WU), a wdely used method to model tme varyng fadng wreless channels both n tme and frequency domans usng stochastc processes. Two man elements of the WU model are brefly dscussed here: Doppler spread and coherence tme of channel; and multpath delay spread and coherence bandwdth. A maxmum speed of 5 km/hr s used here n the analyss for support of moblty. Wth the excepton of hgh-speed trans, ths provdes a good coverage of vehcular speed n the U, Europe, and Asa. The maxmum Doppler shft [5] correspondng to the operaton at.5 GHz (selected as a mddle pont n the - 6 GHz frequency range) s gven by Equaton (). ν 5m / s f m 8Hz λ.86m Equaton () The worst-case Doppler shft value for 5 km/hr (5 m/s) would be 7 Hz for operaton at the 6 GHz upper lmt specfed by the standard. Usng a KHz subcarrer spacng, the nter Channel nterference (C) power correspondng to the Doppler shft calculated n Equaton () can be shown [6] to be lmted to -7 db. The coherence tme of the channel, a measure of tme varaton n the channel, correspondng to the Doppler shft specfed above, s calculated n Equaton () [5]. 9 T. ms C 6 π f m Equaton () Ths means an update rate of KHz s requred for channel estmaton and equalzaton. The maxmum delay spread for fxed broadband wreless s specfed by the tanford Unversty nterm (U) channel model [7]. The worst-case rms delay spread correspondng to U-6 (Terran Type A: hlly terran wth moderate-to-heavy tree denstes) channel s 5. µs. The nternatonal Telecommuncatons Unon (TU-R) Vehcular Channel Model B [8] shows delay spread values of up to µs for moble envronments. The subcarrer spacng desgn requres a flat fadng characterstc for worst-case delay spread values of µs wth a guard tme overhead of no more than % for a target delay spread of µs. The coherence bandwdth of the channel (5% frequency correlaton) correspondng to the µs delay spread, gven by Equaton () [5], s shown to be approxmately KHz. B KHz C 5 5 µ s σ τ Equaton () Ths means that for delay spread values of up to µs, multpath fadng can be consdered as flat fadng over a KHz subcarrer wdth. An OFDM system s also senstve to phase nose and the negatve mpact of mparment ncreases for narrower subcarrer spacng, whch makes the desgn more expensve and complex. The above ratonale, based on the coherence tme, Doppler shft, and coherence bandwdth of the channel, s the bass for the consderaton of a scalable structure where the FFT szes scale wth bandwdth to keep the subcarrer spacng fxed. mulaton results generated n [6] for a.5 MHz channel bandwdth when the FFT sze s kept at 8 shows a consderable amount of degradaton n performance plot (Bt Error Rate vs. gnal to Nose Rato) whch s clearly recognzable for 6-AM and hgh moblty. calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

4 ntel Technology Journal, Volume 8, ssue, Table : OFDMA scalablty parameters Parameters Values ystem bandwdth (MHz) amplng frequency (F s,mhz) ample tme (/F s,nsec) FFT sze (N FFT ) ubcarrer frequency spacng Useful symbol tme (T b / f).679 khz 89.6 µs Guard tme (T g T b /8). µs OFDMA symbol tme (T s T b T g ).8 µs Wthout scalablty, performance s reduced or cost s ncreased for low- and md-sze channel bandwdths. Table summarzes the man scalablty parameters as recommended for adopton n the standard. Note that n Table, the over-samplng factor used s 8/7 (Fs floor(8/7 BW/.8)x.8) as globally specfed n the standard for all OFDMA operatons. The guard tme can attan any of the four possble values /, /8, /6 and /. By settng the value to /8 of an OFDM symbol, a maxmum of. µs delay spread can be tolerated wth an overhead of around %. WrelessMAN OFDMA supports a wde range of frame szes (see Table ) to flexbly address the need for varous applcatons and usage model requrements. Wth a 8 FFT sze, the number of OFDM symbols n the short frame sze, (e.g., ms), wll be very small for narrow bandwdths (less than OFDM symbols for.5 MHz band) whch makes the short frame szes practcally unusable (due to hgh overhead). Another advantage of scalablty s to guarantee a lower bound on the number of OFDM symbols per frame (partcularly a problem for small bandwdth and frame szes). Table : calable OFDMA frame szes Frame zes Frame zes (msec) (OFDM symbols) n the remander of ths paper, the followng tems are emphaszed as the drvers of scalablty and are revsted frequently. a. ubcarrer spacng s ndependent of bandwdth. b. The number of used subcarrers (and FFT sze) should scale wth bandwdth. c. The smallest unt of bandwdth allocaton, specfed based on the concept of subchannels (to be defned later), s fxed and ndependent of bandwdth and other modes of operaton. d. The number of subchannels scales wth FFT sze rather than wth the capacty of subchannels. e. Tools are provded to trade moblty for capacty. Note that fxng the capacty of the subchannel may not be the best choce especally for low-bandwdth systems where typcal applcatons are dfferent n nature. BAC OF OFDMA FRAME TRUCTURE There are three types of OFDMA subcarrers:. Data subcarrers for data transmsson.. Plot subcarrers for varous estmaton and synchronzaton purposes.. Null subcarrers for no transmsson at all, used for guard bands and DC carrers. Actve subcarrers are dvded nto subsets of subcarrers called subchannels. The subcarrers formng one subchannel may be, but need not be, adjacent. Bandwdth and MAP allocatons are done n subchannels. The plot allocaton s performed dfferently n dfferent subcarrer allocaton modes. For DL Fully Used ubchannelzaton (FUC), the plot tones are allocated frst and then the remanng subcarrers are dvded nto data subchannels. For DL Partally Used ubchannelzaton (PUC) and all UL modes, the set of used subcarrers, that s, data and plots, s frst calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

5 ntel Technology Journal, Volume 8, ssue, parttoned nto subchannels, and then the plot subcarrers are allocated from wthn each subchannel. n FUC, there s one set of common plot subcarrers, but n PUC, each subchannel contans ts own set of plot subcarrers. another B rather than a servng B. multaneous ULs can be data allocatons and rangng or bandwdth requests. OFDMA ymbol Number ub-channel Logcal Number k k k k5 Preamble DL-MAP FCH UL-MAP UL-MAP DL Burst Broadcast DL Burst DL Burst Multcast k7 k9 k k k5 k8 k k k DL Burst DL Burst DL Burst (from B) DL Burst UL Burst UL Burst UL Burst UL Burst Preamble DL-MAP UL-MAP Plot ets Varable et # Varable et # Fxed et # Fxed et # RNG/BW-RE L ub-carrer Physcal ndex DL ub-frame TTG UL ub-frame RTG Fgure : Plot dstrbuton for FUC Fgure : OFDMA frame structure (TDD, PUC) n a DL, subchannels may be ntended for dfferent (groups of) recevers whle n UL, ubscrber tatons () may be assgned one or more subchannels and several transmtters may transmt smultaneously. The subcarrers formng one subchannel may, but need not be, adjacent. Fgure shows the OFDM frame structure for Tme Dvson Duplexng (TDD) mode. Each frame s dvded nto DL and UL subframes separated by Transmt/Receve and Receve/Transmt Transton (TTG and RTG, respectvely) gaps. Each DL subframe starts wth a preamble followed by the Frame Control Header (FCH), the DL-MAP, and a UL-MAP, respectvely. The FCH contans the DL Frame Prefx (DLFP) to specfy the burst profle and the length of the DL-MAP mmedately followng the FCH. The DLFP s a data structure transmtted at the begnnng of each frame and contans nformaton regardng the current frame; t s mapped to the FCH. Accordng to the OFDMA specfcatons, a DL-MAP message, f transmtted n the current frame, shall be the frst MAC PDU n the burst followng the FCH. An UL- MAP message shall mmedately follow ether the DL- MAP message (f one s transmtted) or the DLFP. f Uplnk Channel Descrptor (UCD) and Downlnk Channel Descrptor (DCD) messages are transmtted n the frame, they shall mmedately follow the DL-MAP and UL-MAP messages. multaneous DL allocatons can be broadcast, multcast, and uncast and they can also nclude an allocaton for UBCARRER ALLOCATON MODE There are two man types of subcarrer permutatons: dstrbuted and adjacent. n general, dstrbuted subcarrer permutatons perform very well n moble applcatons whle adjacent subcarrer permutatons can be properly used for fxed, portable, or low moblty envronments. These optons enable the system desgners to trade moblty for throughput. n the followng secton, varous subcarrer allocaton modes are dentfed and ther man characterstcs are summarzed. DL Dstrbuted ubcarrer Permutatons: Fully Used ubchannelzaton (FUC) Ths method uses all the subchannels and employs fullchannel dversty by dstrbutng the allocated subcarrers to subchannels usng a permutaton mechansm. Ths mechansm s desgned to mnmze the probablty of hts (probably of usng the same physcal subcarrers n adjacent cells and sectors) between adjacent sectors/cells by reusng subcarrers whle frequency dversty mnmzes the performance degradaton due to fast fadng characterstcs of moble envronments. Table summarzes the subcarrer allocaton structure parameters. n DL FUC, there are varable and fxed sets of plots. The fxed sets are used n all OFDM symbols whle the varable sets are dvded nto subsets that are used n odd and even symbols alternatvely. Ths provdes an approprate tradeoff between allocated power and frequency dversty on plots for channel estmaton. Fgure shows the dstrbuton of varable and fxed sets calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

6 ntel Technology Journal, Volume 8, ssue, of plots n the case of 8 FFT. Plot sets for other FFT szes are subsets of those for the 8 FFT. Table : DL dstrbuted subcarrer permutaton (FUC) Parameters Values ystem bandwdth (MHz) FFT sze (N FFT ) 8 N/A 5 8 Number of guard subcarrers N/A Number of used subcarrers 6 N/A Number of data subcarrers 96 N/A Number of plot subcarrers (uses both varable and constant sets) 9 N/A 8 66 Number of subchannels N/A 8 6 ubcarrer Permutaton Uses Permutaton Type for Tone Dstrbuton (Eq. 7 []) varable set only FFT sze of 56 s not supported Plot sub-carrers Plot sub-carrers Odd ymbols Even ymbols llustrated n Fgure, that spans over three OFDM symbols (n tme) of four subcarrers, each wth total of four plot subcarrers. Note that because of the DL and UL, cluster and tle structures are composed of two and three OFDM symbols, respectvely; the DL and UL subframe sze and the granularty of the DL and UL allocatons are also two or three OFDM symbols, respectvely. Table : DL dstrbuted subcarrer permutaton (PUC) Parameters Values ystem bandwdth (MHz) FFT sze (N FFT ) 8 N/A 5 8 Number of guard subcarrers Number of clusters/subchannels Number of used subcarrers Number of data subcarrers Number of plot subcarrers ubcarrer permutaton Cluster renumberng N/A / N/A /5 6/ /6 85 N/A N/A 6 7 N/A 6 Uses Permutaton Type for Tone Dstrbuton (Eq. 7 []) Actvated Fgure : DL PUC cluster structure DL and UL Dstrbuted ubcarrer Permutaton: Partally Used ubchannelzaton (PUC) Accordng to the OFDMA specfcaton, all OFDMA DL and UL subframes shall start n DL and UL PUC mode, respectvely. n DL PUC, subchannels are dvded and assgned to three segments that can be allocated to sectors of the same cell. The method employs fullchannel dversty by dstrbutng the allocated subcarrers to subchannels. A permutaton mechansm s desgned to mnmze the probablty of hts between adjacent sectors/cells by reusng subcarrers, whle frequency dversty mnmzes the performance degradaton due to fast fadng characterstcs of moble envronments. Table summarzes the parameters of DL PUC subcarrer allocaton. DL PUC uses a cluster structure, as llustrated n Fgure, whch spans over two OFDM symbols (n tme) of fourteen subcarrers, each wth a total of four plot subcarrers per cluster. Table 5 summarzes the parameters of UL PUC subcarrer allocaton. UL PUC uses a tle structure, as Optonal DL Dstrbuted ubcarrer Permutaton: Fully Used ubchannelzaton (OFUC) Ths method employs full-channel dversty by dstrbutng the allocated subcarrers to subchannels usng a permutaton mechansm desgned to mnmze the probablty of hts between adjacent sectors/cells by reusng subcarrers, whle frequency dversty mnmzes the performance degradaton due to fast fadng characterstcs of moble envronments. Table 6 summarzes the parameters of OFUC subcarrer allocaton. n OFUC, plots are mapped as specfed below, whch s dfferent from the assgnment n the FUC mode. Compared to FUC mode, the number of used subcarrers n ths method s consderably larger (68 vs. 79). As a result, complance wth spectral mask requrements, wthout a change n the over-samplng factor, may be a challenge for ths mode. calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN 5

7 ntel Technology Journal, Volume 8, ssue, Table 5: UL dstrbuted subcarrer permutaton (PUC) Table 6: DL dstrbuted subcarrer permutaton (optonal FUC) Parameters Values Parameters Values ystem bandwdth FFT sze (N FFT ) 8 N/A 5 8 ystem bandwdth FFT sze (N FFT ) 8 N/A 5 8 Number of guard subcarrers N/A 8 67 Number of guard subcarrers 9 N/A Number of tles N/A 55 Number of subchannels N/A Number of subcarrers per tle Number of used subcarrers Tle permutaton N/A 97 N/A Uses Permutaton Type for Tle Dstrbuton (Eq. 9 []) Number of used subcarrers Number of data subcarrers Number of plot subcarrers (Nplots) Number of data subcarrers per subchannel 9 N/A N/A N/A N/A ubcarrer permutaton Uses Permutaton Type for ubcarrer Dstrbuton (Eq. []) Optonal UL Dstrbuted ubcarrer Permutaton: Partally Used ubchannelzaton (OPUC) Ths method employs full-channel dversty by dstrbutng the allocated subcarrers to subchannels usng a permutaton mechansm desgned to mnmze the probablty of hts between adjacent sectors/cells by reusng subcarrers, whle frequency dversty mnmzes the performance degradaton due to fast fadng characterstcs of moble envronments. ymbol ymbol ymbol Plot sub-carrers Plot sub-carrers Fgure : UL PUC tle structure Table 7 summarzes the parameters of UL OPUC subcarrer allocaton. UL OPUC uses a tle structure, as llustrated n Fgure 5, that spans over three OFDM symbols (n tme) of three subcarrers each wth one plot subcarrer per tle. Number of subchannels N/A 8 6 ubcarrer permutaton Plot subcarrer ndex Uses Permutaton Type for Tone Dstrbuton (Eq. 8 []) 9km, for k,,, Nplots and m[symbol ndex] mod Optonal DL and UL Adjacent ubcarrer Permutaton: Advanced Modulaton and Codng (AMC) Ths method uses adjacent subcarrers to form subchannels. When used wth fast feedback channels t can rapdly assgn a modulaton and codng combnaton per subchannel. The AMC subchannels enable the use of water-pourng types of algorthms, and t can be used effectvely wth an AA opton. Table 8 summarzes the AMC subcarrer allocaton parameters. n AMC, plots are mapped as specfed below. calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN 6

8 ntel Technology Journal, Volume 8, ssue, Table 7: Optonal UL dstrbuted subcarrer permutaton (OPUC) Table 8: UL/DL adjacent subcarrer permutaton (optonal AMC) Parameters Values Parameters Values ystem bandwdth FFT sze (N FFT ) 8 N/A 5 8 Number of guard subcarrers Number of used subcarrers 9 N/A N/A Number of tles 6 N/A Number of tles per subchannel Number of data subcarrers per subchannel 6 N/A N/A Number of subchannels 6 N/A 8 96 ubcarrer permutaton Uses Permutaton Type for Tone Dstrbuton (Eq. []) ystem bandwdth FFT sze (N FFT ) 8 N/A 5 8 Number of guard subcarrers Number of used subcarrers (Nused) 9 N/A N/A Number of plots (Nplots) N/A Number of data subcarrers 96 N/A Number of bands N/A 8 Number of bns per band N/A Number of subcarrers per bn (8 data plot) 9 N/A Number of subchannels N/A 8 6 ub-carrer permutaton Plot subcarrer ndex None 9km, for k,,, Nplots and m[symbol ndex] mod Fgure 5: UL OPUC tle structure Zone wtchng OFDMA PHY also supports multple subcarrer allocaton zones wthn the same frame to enable the possblty of support for and coexstence of dfferent types of s. Fgure 6 llustrates zone swtchng wthn the DL and UL subframes. The swtchng s performed usng an nformaton element ncluded n DL-MAP and UL-MAP. Fgure 6: Multple zones n Uplnk and Downlnk subframes DL and UL subframes both start n PUC mode where groups of subchannels are assgned to dfferent segments by the use of dedcated FCH messages. The PUC subcarrer allocaton zone can be swtched to a dfferent type of subcarrer allocaton zone through a drectve from the PUC DL-MAP. Fgure 6 shows the zone swtchng from the perspectve of a PUC segment. n the fgure, the PUC FCH/DL-MAP for a segment wth DCell X s followed wth another possbly data PUC calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN 7

9 ntel Technology Journal, Volume 8, ssue, zone for DCell X. A PUC zone for another sector/cell wth DCell Y (Y n general s dfferent from X) s allocated next. An FUC zone for DCell Z s shown next n the fgure. Note that DCell Z may be the same as DCell X whch means that a PUC to FUC swtchng s scheduled wthn the segment for Frequency Reuse One operatons. A swtchng to DCell can be planned for all network broadcast operatons. users. The fgure llustrates a four-antenna confguraton where the AA preamble and AA DL MAPs structure are repeated multples of four tmes to support the correspondng four groups of users. Optonal PUC, FUC, and AMC zones n DL subframes and optonal PUC and AMC zones n UL subframes can be smlarly scheduled. Allocaton of AMC zones enables the smultaneous support of fxed, portable, and nomadc moblty users along wth hgh moblty users (supported n PUC/FUC zones). DVERTY OPTON OFDMA PHY supports AA and also a set of second-, thrd-, and fourth-order transmt dversty optons. Wth the AA opton, the system uses a multple-antenna transmsson to mprove the coverage and capacty of the system whle mnmzng the probablty of outage through transmt dversty, beam formng, and null steerng. Transmt dversty optons consst of a comprehensve set of methods based on second- or fourth-order dversty n DL and second-order dversty n UL that can be flexbly chosen to tradeoff capacty and coverage. The set ncludes both closed- and open-loop optons and also supports patal Multplexng (M) for maxmum spectral effcency. Advanced Antenna ystems Two optonal AA modes are supported n OFDMA PHY: Dversty-Map can and Drect gnalng Method. Dversty-Map can supports both dversty (FUC and PUC) and adjacent (AMC) subcarrer permutaton optons. The Drect gnalng Method supports adjacent subcarrer permutaton wth less overhead n control sgnalng. We now dscuss the Dversty-Map can opton when appled to the AMC subcarrer allocaton method. Fgure 7 shows the AA Dversty Map Zone wthn a frame. The DL subframe ncludes a non-aa secton and an AA secton specfed by nformaton elements provded n the DL MAP. Wthn the AA zone, subchannel numbers and N- (N s the ndex for the last logcal subchannel) are allocated to the AA DL MAP where a ponter to a beamformed broadcast DL MAP s specfed. The broadcaset DL MAP provdes beamformed prvate DL and UL MAPs for AA Fgure 7: AA dversty MAP zone Wthn the AA zone, the AA B specfes allocatons to be used for Rangng. n TDD mode, the B can extract the channel nformaton requred for beam formng from the Rangng Request messages receved from the s. n FDD mode, beam formng s done through the AA Feedback Request and Response messages where channel response nformaton along wth mean Receved gnal trength ndcator (R) and Carrer to nterference plus Nose Rato (CNR) are reported back to the B by the. Transmt Dversty OFDMA mode supports second-, thrd- and fourth-order transmt dversty optons n DL and second-order transmt dversty n UL. All dversty optons are applcable to both dversty and adjacent subcarrer permutatons. pace Tme Codng (TC) based on Alamout algorthm [9] and Frequency Hoppng Dversty Code (FHDC) are two optons for second-order dversty n DL. Although not specfed by the standard, the number of receve antennas can be specfed dependng on the performance requred. econd-order TC econd-order TC n DL supports codng rates of and usng the followng two transmsson format matrces. calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN 8

10 ntel Technology Journal, Volume 8, ssue, calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN 9 A Equaton () B Equaton (5) Here k s are OFDM symbols n the frequency doman rght before FFT operaton. The optonal TC transmt dversty s also supported n UL usng the transmsson format matrx A of Equaton (). Matrx B of Equaton (5) can be used by two s n a collaboratve specal multplexng mode. Fourth-Order TC The fourth-order transmt dversty n DL supports rates,, or usng the followng transmsson format matrces A, B, and C, respectvely. A Equaton (6) B Equaton (7) C Equaton (8) Here, k s are OFDM symbols n the frequency doman rght before the FFT operaton. Thrd-Order TC The thrd-order transmt dversty n DL supports rates,, or usng the followng transmsson format matrces A, B, and C, respectvely. A Equaton (9) B Equaton () C Equaton () n Equatons (9) and (), we have Equaton () where ) / (tan θ, k k k j, θ j k k e X for,8,, L k and s X k ' are OFDM symbols n the frequency doman rght before the FFT operaton. Precodng A general KxL precodng matrx W s specfed to be appled to the output X of any second-, thrd- or fourthorder dversty opton mentoned earler. Ths way an L th order output vector Z of the TC block s transformed nto a fnal K th order vector for transmsson on antennas. X W Z Equaton () Precodng can be performed ether n closed-loop or open-loop form. n the case of open-loop, the B weghts the transmsson accordng to the channel measurement performed on the UL sgnal, where a recprocty assumpton can be made for a TDD mode, for example. n the case of closed-loop, B uses the Channel ualty ndcatons feedback from the. RANGNG N OFDMA The OFDMA PHY specfes a rangng allocaton that can be used for rangng as well as bandwdth request. ntal and perodc rangng processes are supported to synchronze the s wth the B at the ntal network entry and also perodcally durng the normal operaton. Bandwdth request mechansm s supported so that s can request UL allocatons for transmsson of data to the B. A set of 56 specal pseudo-nose bt-long rangng codes are dvded nto three groups for ntal Rangng, Perodc Rangng, and Bandwdth Requests, such that the B can determne the purpose of the receved code by the subset to whch the code belongs. One or more groups of sx adjacent subchannels are allocated to rangng where the rangng codes are BPK modulated to the allocaton. The randomly selects one

11 ntel Technology Journal, Volume 8, ssue, code from the allocated set of codes and transmts back to the B through rangng allocaton. Dfferent s can collde on ther rangng and/or bandwdth requests and the B s stll able to receve smultaneous requests. To process an ntal Rangng request, a rangng code s repeated twce and transmtted n two consecutve OFDM symbols wth no phase dscontnuty between the two OFDM symbols (see Fgure 8). Ths way, the B can properly receve the requests from un-ranged s wth a larger value of synchronzaton msmatch when the frst attempt s made to enter the network. The can optonally use two consecutve rangng codes transmtted durng a four-ofdm symbol perod (see Fgure 9). Ths opton decreases the probablty of falure and ncreases the rangng capacty to support larger numbers of smultaneous rangng s whle at the same tme t further ncreases the capablty of the system to support larger numbers of synchronzaton msmatches. Fgure 8: ntal rangng transmsson Fgure 9: ntal rangng usng two rangng codes bref summary of the supported mandatory and optonal modes are gven here. Fgure : Perodc rangng and bandwdth request transmsson For Perodc Rangng or Bandwdth Requests, the optons are ether to use one or three consecutve rangng codes transmtted durng a one or three OFDM symbol perod (see Fgure and Fgure ). n the case of three rangng codes, the probablty of falure decreases at the same tme as the rangng capacty ncreases, to support larger numbers of smultaneous rangng s. CHANNEL CODNG A detaled dscusson of channel codng optons n OFDMA PHY s beyond the scope of ths paper; only a Based on termnology used n WrelessMAN OFDMA PHY, channel codng conssts of Randomzaton, Forward Error Correcton (FEC), bt nterleavng, and modulaton. Repetton code s used on varous control messages to further enhance the error correcton performance of the system. Repetton codes of,, or 6 are mplemented by utlzng multple subchannels. Randomzaton s performed on both UL and DL data. The data are randomzed usng a PN sequence generator wth a polynomal of degree 5 that s rentalzed at the begnnng of each FEC block wth a seed, whch s a functon of the OFDM symbol offset (from the start of the frame) and the startng subchannel number correspondng to the FEC block. The OFDMA PHY supports mandatory tal-btng Convolutonal Codng and three optonal codng schemes: Zero Talng Convolutonal code, Convolutonal Turbo code along wth H-AR, and Block Turbo code. calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

12 ntel Technology Journal, Volume 8, ssue, The tal btng s mplemented by ntalzng the encoders memory wth the last data bts of the FEC block beng encoded, and the zero talng s mplemented by appendng a zero tal byte to the end of each burst. H-AR mtgates the effect of mparments due to channel and external nterference by effectvely employng tme dversty along wth ncremental transmsson of party codes (subpackets n ths case). n the recever, prevously erroneously decoded subpackets and retransmtted subpackets are combned to correctly decode the message. The transmtter decdes whether to send addtonal subpackets, based on ACK/NAK messages receved from the recever. Bt nterleavng s performed on encoded data at the output of FEC. The sze of the nterleavng block s based on the number of coded bts per encoded block sze. The nterleavng s performed usng a two-step permutaton process. The frst permutaton ensures that adjacent coded bts are mapped onto nonadjacent subcarrers. The second permutaton ensures that adjacent coded bts are mapped alternately onto less or more sgnfcant bts of the constellaton, thus avodng long runs of lowly relable bts. CONCLUON The EEE 8.6 WrelessMAN OFDMA supports a comprehensve set of system parameters and advanced optonal features for moble, portable, and fxed usage models. calablty enables the technology to operate optmally n dfferent usage scenaros. Fgure : Perodc rangng and bandwdth request transmsson usng three codes ACKNOWLEDGMENT The author thanks Dr. C.K. Brght for the support provded durng the wrtng of ths paper and the valuable help on the graphcs. also thank T.J. Cox, D. Andelman, R.C. chwartz, Y. Lomntz, G. Begs and. Talwar for ther valuable revews and comments. REFERENCE []. EEE P8.6-, standard for local and metropoltan area networks Part 6: Ar nterface for Fxed Broadband Wreless Access ystems Name (To be publshed). []. ET 7 rev.., (999-), dgtal broadcastng systems for televson, sound and data servces (DVB-T); framng structure, channel codng and modulaton for dgtal terrestral. []. EEE td 8.a-999, Part, Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) specfcatons; hgh-speed physcal layer n the 5 GHz band. []. EEE 8.g-, EEE tandard for nformaton technology, telecommuncatons and nformaton exchange between systems, local and metropoltan area networks, specfc requrements, Part : Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) specfcatons, Amendment : further hgher-speed physcal layer extenson n the. GHz band. [5]. EEE P8.6e, draft amendment to EEE standard for local and metropoltan area networks, Part 6: ar nterface for fxed and moble broadband wreless access systems, amendment for physcal and medum access control layers for combned fxed and moble operaton n lcensed bands. [6]. EEE C8.6d-_7, applyng scalablty for the OFDMA PHY layer. [7]. EEE C8.6REVd-/5r, OFDMA PHY enhancements for better moblty performance. [8]. EEE C8.6d-/7, addtonal optonal symbol structure. [9]. EEE C8_6e-/88-r, 8 FFT szes for OFDMA PHY. []. C8.6REVd-_5r, OFDMA PHY enhancements for better moblty performance. []. EEE C8.6d-/9, AA enhancements for OFDMA PHY. calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

13 ntel Technology Journal, Volume 8, ssue, []. EEE 8.6d-/65, Enhancng MMO features for OFDMA PHY layer. []. EEE C8.6e-_7r, TC Enhancements for optonal FUC and AMC zones for OFDMA PHY layer. []. EEE C8.6e-/8r, space-tme codes for transmt antennas for the OFDMA PHY. [5]. Rappaport, T.., Wreless Communcatons Prncples and Practce, econd Edton, Prentce Hall PTR, Upper addle Rver, NJ. [6]. L, Y., Cmn, L.J., Bounds on the nterchannel nterference of OFDM n Tme- Varyng mparments, EEE Transactons ON Communcatons, Vol. 9, No., March, pp. -. [7]. EEE 8.6.c-/9r, channel models for fxed wreless applcatons. [8]. Recommendaton TU-R M.5, Gudelnes for evaluaton of Rado transmsson technologes for MT-, 997. [9]. Alamout,. A., mple Transmt Dversty Technque for Wreless Communcatons, EEE Journal on elect Areas n Communcatons, Vol. 6, No. 8, October 998. []. EEE P8.6REVd/D5-, standard for local and metropoltan area networks Part 6: Ar nterface for Fxed Broadband Wreless Access ystems Name. solutons. He s a member of the EEE 8.6 and 8. workng groups. He also serves as secretary of the sub GHz Techncal Workng Group for the WMAX forum, an ndustry group focused on nteroperablty of systems that conform to the EEE 8.6 standard. Pror to ntel, he worked on desgn and modelng of wreless terrestral and satellte recevers for tanford Telecom and on RF network desgn of moble wreless systems for LCC nternatonal. Hs e- mal s hassan.yaghoob at ntel.com. Copyrght ntel Corporaton. Ths publcaton was downloaded from Legal notces at AUTHOR BOGRAPHY Hassan Yaghoob receved a B.. degree from harf Unversty of Technology, Tehran, ran, n 989 and M.. and Ph.D. degrees from the Unversty of Maryland, n 99 and, respectvely, all n Electrcal Engneerng. Hs academc research nterests nclude nonlnear control theory, communcatons theory, and dgtal sgnal processng. Hassan s ndustral experence ncludes communcatons systems engneerng, slcon desgn/functonal defnton, and standards development n the area of broadband communcatons. nce, he has been workng at ntel Corporaton. As an engneer for ntel s Broadband Product Group, he worked on slcon functonal defnton, algorthm desgn, system desgn verfcaton, and valdaton of varous cable modem products. He represented ntel at the DOC. Rado Frequency nterface pecfcaton (RF) and Acceptance Test Plan (ATP) standard commttees at Cablelabs. Hassan s currently workng as a trategc Technologst for ntel s Broadband Wreless Dvson workng on product defntons of ntel s 8.6d/e slcon calable OFDMA Physcal Layer n EEE 8.6 WrelessMAN

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