Showing papers by "Wai-Choong Wong published in 1984"

Simultaneous transmission of speech and data over an analog channel

Abstract: A technique for transmitting an entire analog speech signal (S(t)) and a modulated data signal (D(t)) over a transmission channel (20) such as a common analog telephone speech channel. The present technique multiplexes the entire modulated data signal within the normal analog speech signal frequency band where the speech is present and its signal power density characteristic is at a low level. Separation of the speech and data signals at the receiver (30) is effected by recovering the modulation carrier frequency (fc) and demodulating (33) the receiver signal (X(t)) to recover the data signal. The data signal is then remodulated (34) with the recovered carrier and is convolved with an arbitrary channel impulse response in an adaptive filter (35) whose output signal is subtracted (37) from the received composite data and speech signal (X(t)) to generate the recovered speech signal (S(t)). To improve the recovered speech signal, a least mean square algorithm (36) is used to update the arbitrary channel impulse response output signal of the adaptive filter (35).

Transmission errors in companded PCM over Gaussian and Rayleigh fading channels

Abstract: Transmission errors in μ-law pulse-code-modulated systems are capable of inflicting considerable distortion in the recovered analog signals. We analyze this distortion when the transmission is over radio channels, and when either noncoherent frequency shift keying or coherent phase shift keying (CPSK) modulation is employed. We consider both Gaussian and Rayleigh fading channels, and for the latter we investigate the effect of bit scrambling prior to transmission and two types of diversity. We determined the gain in overall signal-to-noise ratio due to using the minimum distance code compared to the conventional folded binary code for low values of input signal power. In addition to the theoretical results, we present computer simulations of μ-law pulse-code-modulated encoded speech transmitted over the same channels, using CPSK modulation, bit scrambling, and diversity where appropriate, i.e., for the same conditions as employed in the derivation of our theory.

Transmission errors in companded PCM over Gaussian and Rayleigh fading channels

Abstract: Transmission errors in μ-law pulse-code-modulated systems are capable of inflicting considerable distortion in the recovered analog signals. We analyze this distortion when the transmission is over radio channels, and when either noncoherent frequency shift keying or coherent phase shift keying (CPSK) modulation is employed. We consider both Gaussian and Rayleigh fading channels, and for the latter we investigate the effect of bit scrambling prior to transmission and two types of diversity. We determined the gain in overall signal-to-noise ratio due to using the minimum distance code compared to the conventional folded binary code for low values of input signal power. In addition to the theoretical results, we present computer simulations of μ-law pulse-code-modulated encoded speech transmitted over the same channels, using CPSK modulation, bit scrambling, and diversity where appropriate, i.e., for the same conditions as employed in the derivation of our theory.

Weighting strategies for companded PCM transmitted over Rayleigh fading and Gaussian channels

Abstract: We present three methods of weighting N-bit μ-law Pulse Code Modulation (PCM) with binary modulation, and evaluate their performances theoretically for transmission over Rayleigh fading and Gaussian channels. The digital modulation methods considered are noncoherent frequency shift keying and coherent phase shift keying. Ideal selection combining and ideal maximal ratio combining diversity techniques are employed when the transmission is over fading channels. Weighting System 1 is the conventional weighted pulse code modulated system, where the bits in every PCM word have the same weighting profile. Weighting System 2 has 2N unique weighting profiles, while System 3 has an addendum to System 2, where every bit in a particular word is further weighted by a unique multiplicative factor. For Rayleigh fading channels and the encoder operating at an input level that provided maximum signal-to-quantization noise-ratio, we obtained gains in overall signal-to-noise ratio (s/n) over unweighted μ-law PCM of 3, 4.5, and 6 dB for Systems 1, 2, and 3, respectively. When the systems were used in conjunction with Gaussian channels, the corresponding gains were 10, 12, and 17 dB, for a channel s/n of 10 dB. In addition to the theoretical results, we conducted computer simulations using four concatenated speech sentences transmitted via our weighted μ-law PCM systems over mobile radio channels. The simulation performances were in good agreement with our theoretical results, which were based on input signals having an exponential distribution.

Soft decision demodulation to reduce the effect of transmission errors in logarithmic PCM transmitted over Rayleigh fading channels

Abstract: In this paper we investigate soft decision demodulation applied to μ-law Pulse Code Modulated (PCM)-encoded signals transmitted over Rayleigh fading channels by means of coherent phase-shift keying modulation. Each bit in the μ-law PCM word is assigned its own soft decision demodulation erasure threshold. These thresholds are theoretically determined as a function of the input power level, channel s/n, and the relative mean square error power δ that occurs because of the replacement by interpolation or prediction of those samples discarded in the soft decision demodulation process. We find that there is no advantage in applying soft demodulation to more than the first 4 bits of the 8-bit μ-law PCM words, μ = 255. When the input signal level was −17 dB (corresponding to peak overall speech s/n in the absence of transmission errors), the gains in overall speech s/n compared to basic μ-law PCM and fixed weighted μ-law PCM were 7 and 3.5 dB, respectively, when the channel s/n was 30 dB. More significantly, when the input signal level was reduced to −40 dB, the corresponding gains in overall speech s/n were 18 and 12 dB. Simulations were performed using four concatenated speech sentences and a fading channel that was obtained from a mobile radio Rayleigh fading hardware simulator. The simulations were in reasonable agreement with our theoretical results.

Transmission simultanee de la parole et d'informations par un canal analogique

Abstract: La presente invention concerne une technique de transmission d'un signal de la parole analogique complet (S(t)) et un signal module d'informations (D(t)) par un canal de transmission (20) tel qu'un canal de transmission telephonique analogique commun de la parole. La presente technique effectue le multiplexage du signal module d'information complet dans la gamme de frequence du signal analogique normal de la parole ou la parole est presente et sa caracteristique de densite de la puissance du signal est a un faible niveau. La separation des signaux de la parole et d'information au niveau du recepteur (30) est effectue en recuperant la frequence porteuse de modulation (fc) et en demodulant (33) le signal recepteur (X(t)) pour recuperer le signal d'information. Le signal d'information est ensuite remodule (34) avec la frequence porteuse recuperee puis est convolutionne avec une reponse d'impulsion de canal arbitraire dans un filtre adaptatif (35) dont le signal de sortie est soustrait (37) du signal composite recu d'information et de parole (X(t)) pour produire le signal recupere de parole (S(t)). Pour ameliorer le signal recupere de parole, un algorithme quadratique moyen minimal (36) est utilise pour mettre a jour le signal de sortie de reponse d'impulsion du canal arbitraire du filtre adaptatif (35).