Patent Application: US-13274308-A

Abstract:
an orthogonal frequency division multiplexing communication system with a transmitter and a receiver . the transmitter is arranged to transmit channel estimation sequences on each of a plurality of band groups , or bands , and to transmit data on each of the band groups or bands . the receiver is arranged to receive the channel estimation sequences for each band group or band to calculate channel state information from each of the channel estimation sequences transmitted on that band group or band and to form an average channel state information . the receiver receives the transmitted data , transforms the received data into the frequency domain , equalizes the received data using the channel state information , demaps the equalized data to re - construct the received data as soft bits and modifies the soft bits using the averaged channel state information .

Description:
referring to fig2 a communication system comprises a transmitter 10 and a receiver 12 . the transmitter 10 comprises a convolutional coder / puncturer 14 which is arranged to receive data in the form of scrambled psdus , and to perform coding and puncturing on it . a bit interleaver 16 is arranged to receive the data from the coder 14 and to perform an interleaving operation on it . a mapper 18 , which in this embodiment is a dcm mapper , is arranged to receive the interleaved data and to map it as will be described in more detail below . finally an inverse fast fourier transform ( ifft ) unit 19 is arranged to transform the data into an analogue signal y t ( n ) in the form of an ofdm symbol for transmission . the receiver 12 comprises an fft unit 20 which is arranged to transform the received signal yr ( n ) back to the digital frequency domain . a channel equalizer 22 is arranged to perform channel equalization based on channel estimation information , and a demapper 24 is arranged to perform demapping on the equalized signal to generate soft bits which have a sign + or − to indicate a value of 0 or 1 and a magnitude , which indicates a reliability associated with the value of the bit . the data is then arranged to be de - interleaved by the de - interleaver 26 and then decoded by a viterbi decoder 28 to obtain the scrambled psdu . this is then unscrambled to obtain the original psdu . referring to fig3 , which shows the functionality of the receiver in more detail , a channel estimation block 30 is arranged to receive channel estimation data sequences sent by the transmitter 10 and received by the receiver 12 and to compare them with stored copies of the same data sequences . from this comparison the channel estimation block 30 is arranged to derive channel estimation data , which is the ratio of the received signal to the transmitted signal for each frequency sub - channel . this ratio includes phase and amplitude information and provides an indication of the different effects of the transmission channel on different frequency components of the transmitted signal , which is used in the equalization process . in this embodiment the channel estimation information is also used to generate channel state information ( csi ) which is used to modify the output of the demapper , as will be described in more detail below . in this embodiment the mapping and demapping is performed using dcm constellation mapping . in dcm constellation mapping , the binary serial input data , coded and interleaved from scrambled psdu , are divided into groups of 200 bits , and then these 200 bits are further grouped into 50 groups of 4 bits . each group of 4 bits is represented as ( b [ g ( k )], b [ g ( k )+ 1 ], b [ g ( k )+ 50 ], b [ g ( k )+ 51 ]), where kε [ 0 . . . 49 ] and in qpsk constellation mapping , the coded and interleaved binary bit values b [ i ] ε { 0 , 1 } are mapped into bipolar symbols x ( i ) ε {− 1 , 1 }. the dcm mapping can be derived from applying two different matrixes to multiply the bipolar symbols x ( i ) of qpsk mapping to obtain a complex number pair [ y n , y n + 50 ], shown in ( 2 ). equation ( 2 ) can be simplified as ( 3 ). the normalization factor , kmod = 1 /√{ square root over ( 10 )}, is used to normalize the average symbol power to 1 in the dcm . therefore the complex number pair [ y n , y n + 50 ] is formed into two four - dimensional constellations as shown in fig5 a and 5 b . moreover the two complex numbers are allocated into two individual ofdm sub - carriers separated by at least 200 mhz , which can have frequency diversity gain for robustness against multi - path and interference . in the ofdm modulation , the ofdm sub - carriers suffer from different noise power , for example , echoes , deep fades , etc . particularly the noise effect of the frequency domain equalization process can degrade the soft - decision demapping . each ofdm sub - carrier position has a dynamic estimation for the data reliability . this dynamic estimation in frequency - domain is defined as channel state information ( csi ). each data carrier has a potentially different csi based on the power of the channel estimate at that frequency . therefore the more csi that can be taken the more reliable the csi estimation is in the presence of multipath interference to offer a better demapping result . least square ( ls ) equalization is one of the popular equalization methods for the ofdm based systems and has low complexity to implement . ecma - 368 defines 6 stored channel estimation ( ce ) sequences in blocks of 122 contained in the 6 ofdm symbols of the plcp preamble . the basic ls csi for each equalized data sequence can be calculated from the received and stored ce sequences transmitted on the same band group ( band ?) and given by where cer is the received ce sequence and ces is the stored a priori ce sequence . it should be noted that cer / ces includes both phase and amplitude information , i . e . information about the q and i components of each frequency component of the sequences , whereas csi as the modulus of cer / ces is a scalar term indicative of the power of each frequency component of the sequences . a time - frequency code ( tfc ), shown in table 1 , is given to the phy from the mac to define the hopping sequence across the selected band group ( bg ) while hopping is only performed in tfc1 - 4 with tfc 5 - 7 each representing use of only one band gap and no hopping [ 5 ]. taking the 6 ce sequences and the selected tfc code ( tfc = 1 ) for the band hopping can create the 6 different blocks of csi . moreover , averaging the different blocks of csi derived for the same band can produce a more reliable csi in the time invariant or slowly changing channel with respect to the frame time . the first block of csi is averaged with the fourth block of csi as both are derived from data transmitted on the first band , while the second one is averaged with the fifth one as both of them are derived from data transmitted on the second band , and the third one is averaged with the sixth one as both of them are derived from data transmitted on the third band . then the three new averaged csi blocks and the copy of these blocks replace the previous csi blocks in order , shown in fig4 . when the dcm is involved , one constellation point is related to two different ofdm sub - carriers allocated separately by 200 mhz , so the frequency diversity can lead to two different csi ( csi n , csi n + 50 ) for any particular bit of data , associated with the two data sub - carrier frequencies related to the constellation point onto which it was coded . then the last stage of the proposed csi is , for any soft bit , to use the smaller of the two averaged csi ( csi s ) as the more reliable csi from the two different csi , to modify the soft bit . the dcm soft demapping of this embodiment exploits csi as will now be explained . the receiver converts each time - domain ofdm symbol into the frequency - domain via the fast fourier transform ( fft ). then channel estimation and symbol equalization follow . the dcm demapper 24 performs demapping the equalized complex numbers , related to two different sub - carriers , back to a group of 4 soft bits , and then outputs groups of 200 soft - bits . the dcm soft - demapper 24 employs a related matrix factor to combine the two equalized complex numbers previously transmitted on different sub - carriers into maximum likelihood ( ml ) soft bits . by eliminating the factor √{ square root over ( 10 )}/ 5 the dcm soft - demapper can still remain the same demapping performance . therefore the group of 4 soft bits can be obtained as shown in ( 6 ), b 1 = 2 i r n + i r n + 50 b 2 = i r n − 2i r n + 50 b 3 = 2 q r n + q r n + 50 b 4 = q r n − 2q r n + 50 ( 6 ) where i and q can each have values between − 3 and 3 depending on the magnitudes of the received q and i components . multiplying the aforementioned minimum of the two csis with the soft bits ( b 1 , b 2 , b 3 , b 4 ) creates the ml soft bits . specifically , as the csi is a group of relative powers for the groups of frequencies , the powers associated with the n and n + 50 frequencies are multiplied by the i and q values . as a result , the group of 4 soft bits is created from : b 1 =( 2 i r n + i r n + 50 )* min ( csi n , csi n + 50 ) b 2 =( i r n − 2 i r n + 50 )* min ( csi n , csi n + 50 ) b 3 =( 2 q r n + q r n + 50 )* min ( csi n , csi n + 50 ) b 4 =( q r n − 2 q r n + 50 )* min ( csi n , csi n + 50 ) ( 7 ) the soft bits from the dcm demapper are then inputted to the bit deinterleaver , the soft - bit viterbi decoder and then descrambled to recover the psdu . two propagation models are presented here , an awgn channel and foerster &# 39 ; s cm 1 variant 1 [ 11 ]. channel estimation using in cm 1 is also specified in [ 11 ] for the channel environment . as in the mboa tests [ 12 ], we adopted 500 packets with each packet of 1024 octets per psdu in each simulation . we also maintain strict adherence to timing and use a hopping characteristic of tfc = 1 and 100 channel realizations in multi - path environment . the packet error rate ( per ) performance is calculated using the mean for the best 90 % channel realizations ( top 90 % ile [ 12 ]). noise figure ( nf )= 6 . 6 db is added in the receiver rf front end . in multi - path environment the proposed dcm has gained improvement by exploiting csi coupling with hopping information . fig6 a depicts particularly the performance difference between using csi and no csi in 480 mbit / s mode and estimated data of 480 mbit / s performance from batra &# 39 ; s result [ 7 ] in cm 1 . the proposed dcm increases the propagation distance for 480 mbit / s mode to 4 . 3 m for reliable reception in cm 1 ( per & lt ; 8 %), which is 13 % improvement in comparison of batra &# 39 ; s result − 3 . 8 m [ 7 ]. fig6 b depicts the performances for the proposed dcm exploiting a csi aided scheme coupled with the band hopping information at the required high data rate transmission . the performance in awgn channel is also simulated , shown in fig6 c . because csi exits in multi - path environment only , the performance with csi is as same as no csi in awgn channel . it can therefore be seen that the dcm provides fast and reliable data modulation for mapping and demapping high data rate at 320 mbit / s , 400 mbit / s and 480 mbit / s achieved by various levels of coding and diversity within the mb - ofdm system . the dcm demapper of some embodiments of this invention exploits a csi aided scheme coupled with the band hopping information resulting in improved performance in high data rate transmission , particularly in the 480 mbit / s mode , thereby increasing the propagation distance to be 4 . 3 m for reliable reception in cm 1 . federal communications commission , “ new public safety applications and broadband internet access among uses envisiged by fcc authorization of ultra - wideband technology ”, press release ] 4 feb . 2002 , http :// www . fcc . gov / bureaus / engineering_technology / news_releases / 2002 / nr et0203 . html a . batra , et al , “ multi - 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02 / 490 - sg3a . 7 feb . 2003 , http :// grouper . ieee . org / groups / 802 / 15 / pub / 2003 / mar03 / 02490r1p802 - 15_sg3a - channel - modeling - subcommittee - report - final . zip mboa standard “ multiband ofdm physical layer proposal for ieee 802 . 15 . 3a ”, september 2004 , http :// www . wimedia . org / imwp / idms / popups / pop_download . asp ? contentid = 65 16