Patent GB-1505139-A (GB)
Title: Ultrafast adaptive digital modem
IPC: H04L27/06
Filed: 1975-10-21
Abstract: 1505139 Equalizers; data transmission; APC systems HYCOM Inc 21 Oct 1975 [11 Nov 1974] 43210/75 Headings H4R H3A and H4P In an adaptive digital modem particularly of the type in which a quadrature modulated signal is demodulated by a carrier signal from a generator 73, Fig. 3, which is not phase locked to the received signal and in which phase correction is applied subsequently, the received analogue signal is sampled, at 67, and timing error of the sampling is corrected, at 94, in response to a timing error signal derived as a function of the magnitude of an angle which has a trigonometric function which varies with the quotient of the magnitude of the phase corrected signal at a first sampling instant divided by the magnitude of the phase corrected signal at the immediately adjacent sampling instant. As described the receiver Fig. 3 is of the type described in Specification 1,459,501 in which an incoming quadrature modulated signal at a symbol rate of 1600 Hz is sampled, e.g. at 6400 Hz and non- coherently demodulated into I and Q channel signals using 1600 Hz quadrature carriers from the read only memory 73. The resulting signals are passed through transversal equalization network 79, which is of the type described in Specification 1,459,501 and a phase correction network 81 where phase errors, due to the carriers from ROM 73 not being coherent with the input signal carrier, are corrected prior to detection of the transmitted signal in a detector 83 which can be of the type described in Specification 1,504,609. Normal adaptation of the timing, equalization and phase correction is carried out during transmission according to the systems described respectively in Specification 1,459,501, U.S.A. Specification 3,906,347, and U.S.A. Specification 3,971,996. On start-up however a special fast-beam preamble is transmitted to the receiver and used by fast learn network 97 to bring the receiver rapidly to adapt to the incoming signal. The fast learn preamble comprises, Fig. 5, a first period, of ten band intervals, of unmodulated carrier of 1600 Hz, which is used to set the level of the incoming signal in the a.g.c. circuit 65. During a second 10 band period, T 1 to T4, the 1600 Hz carrier is modulated by a 400 Hz tone, which is transmitted in only the Q channel to allow correction for the phase of the demodulating carrier from 73. Impulses 105 and 107 are transmitted at times T5 and T6 respectively 10 band and 20 band intervals after the end of the tone modulated carrier 103 to allow fine phase and timing adjustment and also equalizer adaptation. Phase adjustment on the 400 Hz modulated carrier is effected in accordance with simultaneous samples of the I and Q channel outputs. If the signal is only transmitted in the Q channel the adjustment required is ##=-Arctan(YI 0 )/YQ 0 ) where YI 0  and YQ 0  are the symbols at the output of the phase correction network 81 at the sampling instant. The arcsine function can be used but is less accurate. In order to restrict the correction to less than 45 degrees, where the arctan is between 0 and 1, a fixed phase jump of 60 degrees is provided if the magnitude of YI 0  is greater than or equal the receiver reference level l 0 . Fig. 4 shows the fast learn network 97 of Fig. 3 and includes the numerical value taker 135 and comparator 137 which provides an output to provide the fixed phase jump of 60 degrees when |YI 0 |#l 0  the direction and amplitude of the jump being determined by the multipliers 127 and 131 giving a signal -60‹ sqn. (YI, YQ) on line 133. The inverse ROM 155 and multiplier 153 provide a YI/YQ signal to select the angle ## from the tan<SP>-1</SP> ROM 157 to feed to the phase control network via line 161 when the angle ## is less than 45 degrees, i.e. |YI|<l 0 . With the phase of the demodulating signal correctly adjusted the timing of the sampling in the A to D converter has to be adjusted. Correct sampling timing is controlled by comparing the amplitude of successive samples and effecting a time shift #T where #T=-0À5+1(1/90) Arctan (YQ -1 /YQ 0 ) where the value of YQ 0 #YQ -1 ; correct timing giving sample amplitudes of the 400 Hz modulating signal which are equal in amplitude. The factor YQ -1 /YQ 0  is determined by multiplier 153, with switch 141 operated to connect to contact 145 to insert the one band delay 177 in the Q symbol feed to the multiplier 153. Operating on the value YQ +1 /YQ 0  provides the adjusting signal on line 183. Modification of timing will however affect the phase of the signal which will require a proportional correction derived as -360‹ #T on line 199. A more accurate fine timing and phase correction is achieved using the pulse 105, Fig. 5, of the fast learn preamble. The timing adjustment #T can be determined as -0À67 (YQ 1 /YQ 0 ) where YQ 0  and YQ 1  are the main and first trailing symbol samples of the response to the impulse. This value is provided on line 203 of Fig. 4. Accumulated phase error can be corrected as described above by the signal on the line 161 using the signals YI 0  and YQ 0  derived from the main pulse response, and a correction corresponding to the timing correction is derived by multiplying the signal on line 203 by - 360 degrees in multiplier 205. Preferably the timing phase correction and the phase correction derived by comparison of the signals in the I and Q channels are combined in adder 207 to provide a signal on line 209. Equalization takes place on the final impulse 107 of the fast learn preamble. Since phase error may still exist until the pulse arrives and appropriate adjustment be effected, the response to the pulse is stored in shift registers 217 and 219, Fig. 13, and phase correction on the lead in symbols effected when a phase error ## is detected to give corrected signals Yš j  = YI j cos## + YQ j sin## and Y #j  = YQ j cos## - YI j sin## where j is the sampling instant and varies from - 3 to 0 in the described embodiment. The corrected symbols are normalized in multipliers 237 and 239, by multiplication with 1/YQ 0 , and loaded into registers 241 and 243. The equalizer coefficients are calculated, as shown in Fig. 13 in accordance with the equations CI k  = -hI k {k>O } CI 0  = 1À0 CI -1  = -hI -1  CI -2  = (CI -1 )<SP>2</SP>-(CQ -1 )<SP>2</SP> - hI -2  CI -3  = CI -1 . CI -2 -CQ -1 CQ -2 -hI -2  CI k  = -hI k {k< -3} where hI k =normalized Y #k  {k#0} and CQ k =hQ k {k>0} CQ 0 =0 CQ -1 =hQ -1  CQ -2 =2CI -1 CQ -1  + hQ z-1  CQ -3 =CI -1 CQ -2  + CQ -1 CI -2 +hQ -3  CQ k  = hQ k {k<-3}  where hQ k  = normalized Yš k {k#0}.