On-off keying-7-phase shift keying modulation system and method for fiber communication

A modulation system includes a modulator configured to employ a modulation mechanism on data. The mechanism includes a signal constellation configured to map sub-carriers which include a signal to be modulated. The signal constellation has a plurality of points asymmetrically disposed on a circle about an origin and a point at the origin wherein a number of sub-carriers becomes variable over different symbol intervals. Corresponding demodulators and corresponding methods are also disclosed.

BACKGROUND

1. Technical Field

The present invention relates to signal modulation and more particularly to a modulation system and method that improve power efficiency and reduces inter-carrier interference (ICI).

2. Description of the Related Art

Orthogonal Frequency Division Multiplexing (OFDM) has achieved great success in wireless and cable communications due to its robustness against multipath fading and its potential for high spectral efficiency. However, OFDM is sensitive to frequency distortion. For optical fiber communication, chromatic dispersion (CD) and polarization mode dispersion (PMD) have similar frequency distortion effects. To compensate for these dispersions, and to take advantage of the high spectral efficiency, the OFDM applications in optical systems have begun to be investigated.

Directly applying OFDM into optical systems has at least the following problems: (a) sensitivity to carrier frequency offset (CFO) caused by the misalignment in carrier frequencies between transmitter and receiver; (b) sensitivity to the receiver In-phase and Quadrature (IQ) imbalance due to manufacturing inaccuracies. Both CFO and IQ imbalance will cause Inter-Carrier Interference (ICI) among the sub-carriers and decrease the Carrier-to-Interference Ratio (CIR), thus degrading system performance.

Referring toFIG. 1, a signal constellation for 8-PSK (8-phase shift keying) is illustratively shown. OFDM systems using conventional 8-PSK, whose signal constellation is shown inFIG. 1, use all sub-carriers simultaneously to transmit modulated symbols. The number of sub-carriers and the separation between the sub-carriers are fixed all the time. When the CFO and the IQ imbalances exist, all sub-carriers will contribute to the ICI, which will degrade system performance. Due to the fact that all sub-carriers are used all the time, there is no power efficiency improvement.

An OFDM system using 8-PSK modulation uses modulated signal waveforms for 8-PSK which can be expressed as

sm⁡(t)=g⁡(t)⁢cos⁡[2⁢π⁢⁢fc⁢t+2⁢π8⁢(m-1)]⁢⁢1≤m≤8,0≤t≤T⁢⁢where(1)
g(t) is the pulse shape, t is time, T is the period or symbol duration, fcis the carrier frequency.

SUMMARY

A modulation system includes a modulator configured to employ a modulation mechanism on data. The mechanism includes a signal constellation configured to map sub-carriers which include a signal to be modulated. The signal constellation has a plurality of points asymmetrically disposed on a circle about an origin and a point at the origin wherein a number of sub-carriers become variable over different symbol intervals. Corresponding demodulators and corresponding methods are also disclosed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present embodiments present an orthogonal frequency division multiplexing (OFDM) optical system using On-Off-Keying 7-Phase-Shift-Keying (OOK-7PSK) modulation that provides improved power efficiency and reduced inter-carrier interference (ICI). The OFDM system using On-Off-Keying 7-Phase-Shift-Keying (OOK-7PSK) modulation, which includes seven evenly spaced points on a circle plus a point at the origin, reduces the ICI in high-data-rate fiber communication systems. The numerical and simulation results show that the OFDM system using OOK-7PSK is more robust against both CFO and IQ imbalance compared to the OFDM system using conventional 8-PSK modulation. The OOK-7PSK modulation reduces Inter-Carrier interference and improves the power efficiency. These benefits are achieved without additional system complexity.

Embodiments described herein may be entirely hardware, entirely software or including both hardware and software elements. In a preferred embodiment, the present invention is implemented in hardware with possible software elements. Software includes but is not limited to firmware, resident software, microcode, etc.

The present embodiments may be provided and used in any transmitter/receiver application. The present system and method may be employed in electrical signaling systems, optical signaling systems or any other signaling system or network.

Referring now to the drawings in which like numerals represent the same or similar elements and initially toFIG. 2, a block/flow diagram of an OFDM system100is depicted in accordance with the present principles. System100shows both a transmitter150and a receiver160. These components may be employed together or separately as needed. An OOK-7PSK modulator102is employed in this system, whose constellation is shown inFIG. 3. The effects of CFO and IQ imbalance have been considered and will be discussed hereinafter.

An input data stream101from a data source is first modulated by OOK-7PSK modulator102, resulting in a serial complex symbol stream. This symbol stream101is passed through a serial-to-parallel converter (S/P)104, whose output is a set of N parallel symbols. Then, these N parallel symbols in frequency domain are converted into samples in time domain by taking an inverse Fast Fourier Transform (IFFT)106. The IFFT106yields the OFDM symbols including the sequence of length N, which corresponds to samples of the multi-carrier signal that includes linearly modulated sub-carriers. Then, in block108, a cyclic prefix (CP) is added to the OFDM symbol to avoid Inter-Symbol Interference (ISI), and the resulting time symbols are converted from parallel to serial (P/S) (block108). The time symbols are passed through a digital to analog (D/A) converter and a low-pass filter (LPF)110.

A final resulting OFDM signal is then upconverted using mixer112to a carrier frequency fc114and provided on a channel116.

A receiver portion160performs the opposite functions of the transmitter150. An OFDM signal is downconverted using mixer118to the carrier frequency fc120from channel116. The transmitted signal from channel116may be filtered by a channel impulse response and corrupted by Additive White Gaussian Noise (AWGN) in simulations. The transmitted OFDM signal is impaired by the CFO (Δf ′) and IQ imbalance. The time symbols are passed through a low-pass filter (LPF) and an analog to digital (A/D) converter122. In block124, the cyclic prefix (CP) is deleted from the OFDM symbol, and the resulting time symbols are converted from serial to parallel (S/P) (block124). Then, these N parallel symbols in time domain are converted into samples in frequency domain by taking a Fast Fourier Transform (FFT)126. The symbol stream is passed through a parallel-to-serial converter (P/S)128, which takes a set of N parallel symbols and serially inputs them to a demodulator130. Demodulators130demodulates by OOK-7PSK, resulting in a serial complex symbol stream output to a data sink131.

Referring toFIG. 3, OOK-7PSK in accordance with the present invention includes a 7-PSK constellation202plus a point204at the origin. When data information is mapped to the point204at the origin, the corresponding sub-carrier is not used for transmission, i.e., there is no power to be transmitted on that sub-carrier, which will not contribute to ICI. Hence, the resulting number of sub-carriers for the present system using OOK-7PSK becomes variable, which means different numbers of sub-carriers are transmitted over different symbol intervals, resulting in a variable sub-carrier OFDM system. Since not all sub-carriers are transmitted simultaneously all the time, power efficiency can be increased and the average ICI in the OFDM system can be decreased, which results in performance improvements.

The OFDM system using OOK-7PSK modulation, whose signal constellation includes seven evenly spaced points208on a circle206plus the point204at the origin, provides signal waveforms that can be expressed as:

sm⁡(t)={g⁡(t)⁢cos⁡[2⁢π⁢⁢fc⁢t+2⁢π7⁢(m-1)](1≤m≤7)0(m=8)⁢⁢0≤t≤T(2)
where g(t) is the pulse shape of the transmitted signal and T is the duration of the symbol.

When data is mapped to the point at the origin, the corresponding sub-carrier is not used for transmission, i.e., there is no power to be transmitted on that sub-carrier, which will not contribute to the ICI when the CFO and the IQ imbalance exist. Hence, the resulting numbers of sub-carriers using OOK-7PSK modulation becomes variable, so that the average separation between sub-carriers is increased, resulting in the probability of symbol error being improved. The points208are asymmetrically disposed about the axes passing through the origin.

Not all sub-carriers being transmitted at the same time has ⅛ probability of not being transmitted. Hence, the average CIR for the present OFDM system using OOK-7PSK modulation can be expressed as

IQ Imbalance: The effect caused by IQ imbalance is now considered. Referring toFIG. 4, IQ imbalance at the receiver is illustratively shown. In a conventional OFDM system, IQ imbalance can be characterized by two parameters: the amplitude imbalance between I and Q channel, and the phase imbalance. The complex baseband signal x(t) is up-converted to the desired carrier frequency (fc), and then amplified before transmission. At the receiver, a down-converted and low-pass filtered (LPF) RF signal is sampled to yield the sequence for FFT demodulation. The IQ imbalance at the receiver distorts this received signal. The received signal on the kthsub-carrier after the FFT can be expressed as:
Y(k)=αX(k)+βX*(N−1−k)+N(k)  (4)
where
k=0,1, . . . N−1
α=0.5{1+(1+δ)(cos φ−jsin φ)}
where δ is the amplitude imbalance
β=0.5{1−(1+δ)(cos φ+jsin φ)}
and φ is the phase imbalance at the receiver.

Equation (4) shows that the gain and phase mismatches in the receiver cause the symbol at the sub-carrier k to be multiplied by the complex factor α. In addition, a spurious component will be present which is equal to the conjugate of the symbol at the (N−1−k)thsub-carrier multiplied by another complex factor β. The symbol at the kthsub-carrier will include an interference related to the symbol at the (N−1−k)thsub-carrier, and vice versa.

As mentioned, in OFDM systems using conventional 8-PSK modulation, all sub-carriers are used to transmit information, hence, the IQ imbalance will result in each sub-carrier being interfered with by its frequency mirror-image sub-carrier. All of them will contribute to ICI. However, in the OFDM system using OOK-7PSK modulation, the sub-carriers may not be transmitted at the same time, thus the average ICI caused by using OOK-7PSK modulation is just ⅞ of the ICI caused by using 8-PSK modulation when IQ imbalance exists.

Joint Impairment: We have considered the effects of CFO and IQ-imbalance separately. Normally, the received signal may be affected by their joint impairments. We need to consider the joint effects of CFO and IQ imbalance. The received signal on the kthsub-carrier after the FFT processing can be expressed as

while this joint IQ imbalance and CFO causes more ICI, the average CIR for an OFDM system can be expressed as The average CIR for the proposed OFDM system using OOK-7PSK can be expressed as

Referring toFIG. 5, average CIRs between an OFDM system using 8-PSK and the OFDM system using OOK-7PSK for a normalized frequency offset ε varying from 0.01 to 0.25 with different IQ imbalances is shown. The number of sub-carriers is 256.FIG. 5shows CIRs versus normalized frequency offset with different IQ imbalances. Note that, as the CFO increases, the CIR decreases. The CIRs of OOK-7PSK perform at least 0.58 dB better than the ones of 8-PSK. Also, when the IQ imbalance increases, the CIRs for both OFDM systems decrease over all CFOs.

Referring toFIGS. 6-7and tables 1-2, symbol error rate (SER) performances for an OFDM system using conventional 8-PSK and the OFDM system using OOK-7PSK with different IQ imbalances at different CFOs are shown. As CFO and IQ imbalance changes, the signal to noise (SNR) gains between the present OFDM system and a conventional OFDM system are also changed. InFIG. 6, the SNR gain is 1.8 dB in favor of the OOK-7PSK at a SER of 10−3under the conditions of ε=0.01, δ=0.1, and φ=10°.FIG. 7shows SER versus SNR for the following conditions: ε=0.05, δ=0.2, and φ=10°.

Table 1 shows the gains between the OOK-7PSK and conventional 8-PSK with the fixed IQ imbalance of δ=0.05, φ=5° at different CFOs. When the CFO is increased from 0.01 to 0.10, the gain between the OOK-7PSK and conventional 8-PSK is increased from 1.85 dB to 4.62 dB.

Table 2 shows the gains between the OOK-7PSK and conventional 8-PSK with the fixed CFO of ε=0.10 at different IQ imbalances. We note that, when the IQ imbalance is changed from δ=0.05, φ=5° to δ=0.20, φ=10°, the gain between the OOK-7PSK and conventional 8-PSK is changed from 1.85 dB to 6.70 dB at a SER of 10−3.

A comparison betweenFIGS. 6-7and Tables 1-2 shows that the OFDM system using OOK-7PSK modulation is rather robust to CFO and IQ imbalances compared with an OFDM system using conventional 8-PSK modulation. Therefore, the present OFDM system can be used in the case where CFO and IQ imbalance exist.

An OFDM system using OOK-7PSK modulation has been disclosed and the SER performances under the effects of both CFO and IQ imbalance have been analyzed. By using OOK-7PSK modulation, the present OFDM system obtains SNR gains and improves power efficiency by at least 12.5%. All simulation results have shown that the present OFDM system using OOK-7PSK modulation performs better than an OFDM system using conventional 8-PSK modulation especially under the effects of both CFO and IQ imbalance.

Having described preferred embodiments for an on-off keying-7-phase shift keying modulation system and method for fiber communication (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.