WO2016174631A1

WO2016174631A1 - Carrier time division multiplexing (ctdm)

Info

Publication number: WO2016174631A1
Application number: PCT/IB2016/052449
Authority: WO
Inventors: Mostafa EL-SAYIED
Original assignee: El-Sayied Mostafa
Priority date: 2015-04-28
Filing date: 2016-04-29
Publication date: 2016-11-03
Also published as: WO2016174631A8

Abstract

Multi-band OFDM transmission with overlapping bands. Each band has a different carrier frequency but the bandwidths overlap each other. In time domain, the modulated carriers of each band are set to zero during so-called free time periods within the carrier period except one carrier which is sampled and decoded during this free time period. Successive Interference Cancellation (subtraction of a remodulated replica) is performed, and then the next band can be decoded and so on. In order that the samples at the receiver all fall within the free time periods, the sampling frequency must be aligned with the carrier period, it is therefore proposed there for a sampling frequency not coinciding with a multiple of the carrier frequency to truncate said carrier. FFT is then performed on each separated band.

Description

Title of Invention : Carrier Time Division Multiplexing

(CTDM)

Technical Field

Electrical Communication Engineering

Background Art

Multiplexing or multiple access by using time division methods (TDMA or TDM): where time is divided between a number of users or data streams.

Multiplexing or multiple access by using frequency division methods (FDMA or FDM): where frequency is divided between a number of users or data streams.

Multiplexing or multiple access by using code division methods (CDMA):

where orthogonal codes are divided between a number of data streams.

Orthogonal Frequency Division Multiplexing (OFDM): it is a subsidiary of frequency division method, where a number of carriers there frequencies are orthogonal to each other are used.

Summary of Invention

Carrier Time Division Multiplexing is a method that makes a specific

bandwidth for a digital modulated signal to virtually appear as a single

frequency, this can be approached by time divide the carriers so that each carrier can be independently extracted and demodulated, the application of this concept in a communication system can increase the transmission medium utilization.

Technical Problem

Bandwidth limitation Solution to Problem

The main principal of Carrier Time Division Multiplexing CTDM is to represent any signal with a limited frequency Fm in a discrete form only a limited number of samples are needed, not the whole signal , the number of needed samples must equal to or greater than the maximum signal frequency.

Carrier Time Division is a Multiplexing method for digital modulation

techniques where a number of carriers are time divided so that for each carrier at least two equispaced samples can be extracted with in its period to comply with Nyquist rate and Discrete Fourier Transform (DFT) process, the sampling rate Fs must be Fs>2^*Fmax where Fs is the Sampling rate (Samples/sec) and Fmax is the largest frequency component in the signal that is going to be sampled.

Carrier time division process is to free time periods (TF) for each symbol in each carrier so that no any other signals values exist in these periods except for the previous signals as shown in Fig.1.

Fig.1 is an example of a carrier time division multiplexed signals (3 QAM modulated signals), the first carrier frequency is 100 KHz, the second 125 KHz and the third 150 KHz, the time period from TO to the end of the symbol time contains symbol 1 for signal 1 , from T1 to T4 contains symbol 1 for signal 2 and from T2 to T3 contains symbol 1 for signal 3, the total symbol time (Tt) in this example is [3 / (4*100 KHz)] = 7.5 usee where only ¾ or its multiples of the full sinusoidal cycle is used because of the fact that DFT requires

equispaced samples and carrier time division requires free time period to be placed at the beginning and end of each symbol, this leads to the difficulty of using samples spaced by π and its multiples which if used will result in some constrains in the DFT process, as an example, a sine wave with zero phase and equispaced samples spaced by π or its multiples will have zero value for all its samples which leads to false results for the DFT process at the receiver, so it is not possible to use a full cycle sinusoidal symbol.

The processes of demodulating and demultiplexing of the first symbol for the received signal occurs by sampling the received signal at the free time periods of the first symbol for the first signal then processing a Fast Fourier Transform (FFT) or a direct low pass filtering of the samples to demodulate, for

demultiplexing an Inverse Fast Fourier Transform (IFFT) and a subtraction process from the total signal or a direct subtraction process when direct low pass filtering is used to demultiplex, the same processes are repeated for the demodulation and demultiplexing of each symbol. The minimum required bandwidth of a digital QAM equals to 1/TS where Ts is the Symbol time, Fig.2 (1 ) shows the bandwidth allocation for a normal QAM signal and Fig.2 (2) shows the bandwidth of a carrier time division multiplexed QAM, Fig.2 (3) shows the single frequencies (Virtual Bandwidth) of the carrier time division multiplexing QAM in the demodulation and demultiplexing processes, as shown in Fig.2 huge bandwidth saving can be attained by well allocating the carriers frequencies and the free time periods.

A block diagram for a simple carrier time division multiplexing transmitter is shown in Fig.3 (1 ), the serial data stream is split into N-1 sub-streams and each stream is modulated using a digital modulation scheme, Inverse Fast Fourier Transform (IFFT) is used to transform the results to time domain, symbols in time domain are shifted and centred to produce a carrier time division multiplexed signal, then the total sampled signal is converted to a continuous one by using a digital to analog converter (DAC), an optional RF modulator can be used if the operating frequency of the system is so high.

A block diagram for a simple carrier time division multiplexing receiver is shown in Fig.3 (2), the samples are extracted from the received signal at the free time periods of each symbol, a Fast Fourier Transform (FFT) is used to transform the time domain signal to its frequency domain so it can be demodulated, at the same time an Inverse Fourier Transform (IFFT) is used to convert the same signal to its time domain so it can be subtracted from the total received signal, the same processes processed for extracting the other symbols.

A multi fast Fourier transform (FFT) can be performed for each symbol to increase the immunity against noise in the receiver.

Advantageous Effects of Invention

Can reach more data rate with less bandwidth consumption.

Brief Description of Drawings

Fig.1

[Fig.1 ] illustrates the architecture of the invented carrier time division multiplexing technique. Fig.2

[Fig.2] illustrates Bandwidth Comparison between Normal QAM Modulation and Carrier Time Division Multiplexing QAM Modulation

Fig.2 (1 ) Normal QAM Modulation Spectral Allocation and Bandwidth

Fig.2 (2) Carrier Time Division Multiplexed Signals Bandwidth

Fig.2 (3) Carrier Time Division Demultiplexed Signals Virtual Bandwidth (Single Frequencies)

Fig.3

[Fig.3] illustrates the block diagram of the invented (CTDM) transmitter and receiver.

Claims

[Claim 1] The method of Carrier Time Division Multiplexing, including the process of time division between the carriers.

[Claim 2] A method as claimed in Claim 1 , in which each symbol must contain free time periods with respect to the symbols of the signals that follow it, not the past ones and with the subtraction process the values of the past values can be eliminated and only the real value of the processed symbol will be attained.

[Claim 3] A method as claimed in Claim 2, in which one or more fast Fourier

transform (FFT) are processed on samples of the free time periods region so the whole symbol can be attained.