Bit and power loading method for OFDM-PON based on hierarchical modulation

Abstract

The application discloses a kind of OFDM-PON bits and power loading method based on layered modulation, belong to passive optical network field.The application includes: based on the signal-to-noise ratio between each branch optical network unit and optical line terminal in OFDM-PON, each optical network unit is grouped;Low-order quadrature phase modulation modulation format is used as base layer in the optical network unit group with low signal-to-noise ratio, high-order quadrature amplitude phase modulation-16 and quadrature amplitude phase modulation-64 modulation format are used as enhancement layer in the optical network unit group with high signal-to-noise ratio;The trough point of the frequency selective fading of each group optical network unit in OFDM-PON is calculated;According to bit allocation method, power allocation method is carried out for bit allocation and power allocation of base layer and enhancement layer.The application realizes the maximum utilization of flexible OFDM-PON in channel condition by simple and easy-to-implement algorithm, to realize the increase of overall OFDM-PON system capacity, satisfy future optical access network to the full use of channel power abundance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical communication technology and passive optical network, and particularly relates to a bit and power loading method based on hierarchical modulation for OFDM-PON (orthogonal frequency division multiplexing passive optical network). BACKGROUND

[0002] With the bandwidth demand of new services such as Internet of Things (IoT), cloud services, 4K / 8K video streaming and online games, passive optical network (PON) has become the main solution for next-generation fiber access because it can provide users with huge transmission capacity. Next-generation PON (NG-PON) is mainly divided into time division multiplexing PON (TDM-PON), wavelength division multiplexing PON (WDM-PON), time-wavelength division multiplexing PON (TWDM-PON) and orthogonal frequency division multiplexing PON (OFDM-PON) according to multiplexing mode. Among them, OFDM-PON is the prospective technology of NG-PON because of its strong anti-fiber dispersion (CD) capability and relatively high spectral efficiency compared with other types of PONs. Based on intensity modulation direct detection (IM / DD), PONs will experience frequency-selective power fading at different frequencies with different fiber transmission distances. Therefore, through appropriate bit and power loading, the channel conditions of the optical fiber link can be fully utilized, and the signal-to-noise ratio (SNR) of the entire optical transmission system can be significantly improved.

[0003] Although bit and power loading technology has been widely studied for a single link. However, due to the different distances between different optical network units (ONUs) and the same optical line terminal (OLT), when the transmission power of the OLT is fixed, different ONUs have different receiving end SNRs, and the total power budget of the system is determined according to the worst ONU SNR of the NG-PON system. Therefore, the problem of waste of system SNR is caused. In view of this problem, the prior art proposes related technologies based on flexible forward error correction coding (FEC), adaptive hierarchical modulation and probability shaping to maximize the achievable capacity of the system. However, so far, there is no related technology for bit and power loading in flexible hierarchical modulation-based OFDM-PON in the prior art. SUMMARY

[0004] The method of the present application performs bit and power loading according to the SNR of the channel for the base layer and the enhancement layer of the modulation symbol on the OFDM subcarrier, thereby solving the important problem of maximizing the improvement of channel conditions in flexible OFDM-PON link and realizing the full utilization of channel conditions.

[0005] The technical scheme adopted by the present application is as follows:

[0006] A bit and power loading method based on hierarchical modulation for OFDM-PON, according to the signal-to-noise ratio of the channel, bit and power are allocated to the base layer and the enhanced layer of the modulation symbol on the OFDM subcarrier, comprising the following steps:

[0007] Step 1, based on the signal-to-noise ratio between the optical network unit and the optical line terminal in each branch of the OFDM-PON, the optical network units are grouped;

[0008] Step 2, in the optical network unit group with low signal-to-noise ratio, the modulation format of low-order quadrature phase modulation is used as the base layer, and in the optical network unit group with high signal-to-noise ratio, the modulation format of high-order quadrature amplitude phase modulation-16 and quadrature amplitude phase modulation-64 is used as the enhanced layer;

[0009] Step 3, calculate the valley point of the frequency selective fading of each group of optical network units in the OFDM-PON;

[0010] Step 4, set a constant total transmission power and a target data rate, and allocate bits to the base layer of the OFDM-PON according to the bit allocation method;

[0011] Step 5, set a constant total transmission power and a target data rate, and allocate power to the base layer of the OFDM-PON according to the power allocation method;

[0012] Step 6, set a constant total transmission power and a target data rate, and allocate bits to the enhanced layer of the OFDM-PON according to the bit allocation method;

[0013] Step 7, set a constant total transmission power and a target data rate, and allocate power to the enhanced layer of the OFDM-PON according to the power allocation method.

[0014] The beneficial effects of the present application are:

[0015] 1, PON topology means that the transmission quality of the channel between different ONUs and OLTs is different. Multi-user bit and power allocation allows balancing according to the quality of service (QoS) of the target ONU. The present application adopts OFDM based on hierarchical modulation, which can provide adaptive modulation on the corresponding subcarrier for each ONU, and loads the bit and power method of OFDM on the base layer (BL) and the enhanced layer (EL) of hierarchical modulation. In this way, the available transmission optical power limited by the characteristics of the transmitting light source can be shared among ONUs.

[0016] 2, the present application adopts hierarchical quadrature amplitude modulation (QAM) at the transmitter end, and sends high-priority and low-priority bits in a single QAM symbol, and after fiber channel estimation, hierarchical bits and power are loaded for optimization transmission.

[0017] 3、The application respectively allocates bits and power according to SNR of channels for base layer and enhanced layer of modulation symbols on OFDM subcarriers, realizes maximum utilization of channel conditions in flexible OFDM-PON through simple and easy realized algorithm, thereby realizes increase of overall OFDM-PON system capacity, and satisfies future optical access network to channel power abundance quantity sufficient utilization. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 It is a schematic diagram of principle of layered modulation in NG-PON.

[0019] Figure 2 It is principle block diagram of OFDM-PON based on layered modulation of the application. DETAILED DESCRIPTION

[0020] In order to better understand the application for the person in the art, the application is further described in detail below in combination with the drawings and the following examples.

[0021] A bit and power loading method based on layered modulation of OFDM-PON, which realizes multi-layer OFDM architecture in downlink PON transmission.

[0022] The method is through adaptive bit and power loading method of controlling influence data rate, wherein the transmitted OFDM signal respectively contains base layer and enhanced layer on each subcarrier.

[0023] Layered modulation in OFDM-PON refers to improving data rate and spectral efficiency through multiplexing different data streams in power domain on each subcarrier. Figure 1 It is 64QAM format composed of 3 layers of quadrature phase shift keying (QPSK). Wherein d1, d2, d3 are respectively Euclidean distance of 3 layers of QPSK signal in a single quadrant from the center axis. h1, h2, h3 are respectively Euclidean distance of 3 layers of QPSK signal between adjacent quadrants. d1, d2, d3 and h1, h2, h3 constitute key parameters-power allocation in power domain in OFDM-PON-layered power ratio d1 / h1, d2 / h2, d3 / h3.

[0024] The appropriate allocation of the layered power ratio in the aforementioned layered modulation-based OFDM-PON depends on the OFDM-PON channel conditions. The channel frequency-selective fading of OFDM-PON is determined by cos... 2 (Dλ 2 Lf 2 The value of / c) is determined. Here, D is the chromatic dispersion coefficient, λ is the wavelength of light, c is the speed of light, L is the length of the optical fiber, and f is the frequency position of the optical carrier. It can be seen that as the optical fiber transmission distance gradually increases, the frequency position corresponding to the trough of frequency fading on the OFDM-PON subcarrier becomes lower.

[0025] Figure 2 The diagram shows the traditional bit and power loading of an OFDM-PON channel and the bit and power loading based on hierarchical modulation.

[0026] Figure 2 The basic layer bit loading method shown is as follows:

[0027] Given a constant total transmit power and a target data rate, the number of bits and power allocated to the k-th subcarrier of OFDM-PON are respectively b. k and power p k The number of bits allocated on the k-th subcarrier is the loaded modulation format, b k With p k The relationship between them is:

[0028] b k =log2(1+p k ·snr) (1)

[0029] Wherein, the power p allocated to each subcarrier k and the increase in power Δp k They are respectively:

[0030]

[0031]

[0032] Set a constant total transmit power and a target data rate, and set b as the number of bits allocated to the base layer and enhancement layer on the k-th subcarrier of OFDM-PON, respectively. k,i and b k,j The calculation steps are as follows:

[0033] 1) Let i and j represent the base layer and enhancement layer levels respectively, j = 0, and b k,j =0

[0034] 2) Set the target data rate:

[0035]

[0036] Among them, B l For each level l, the data rate is independent.

[0037] 3) To Calculate the accumulated level l bit load b k and the accumulated power p k .

[0038] If the total transmission power p tot satisfy:

[0039]

[0040] and Then decrease (increase) b k , get b k,j And based on the increase Δp k , get p k,i .

[0041] 4) If j≥1, then from the currently accumulated b k Extract the bit loading of level j-1. Therefore, the bit loading for each level j is:

[0042]

[0043] Otherwise, if j = 0, b k,0 =b k Then, under the constraint of the maximum number of enhancement layers, we increase j and obtain p. k,j .

[0044] The present invention uses a layered modulation method in the transmitter to allocate bits and power on the BL and EL layers as follows: first, bits and power are allocated on the high-priority base layer; second, bits and power are loaded on the enhancement layer.

[0045] In summary, the method of this invention achieves optimal subcarrier power and corresponding QAM constellation through a simple and easy-to-implement approach, thereby maximizing the utilization of flexible OFDM-PON channel conditions and increasing the overall OFDM-PON system capacity.

[0046] The above description is merely a specific implementation of the present invention in the embodiments, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention.

Claims

1. A method for bit and power loading in OFDM-PON based on hierarchical modulation, characterized in that, Bit and power allocation is performed separately for the base layer and enhancement layer of the modulation symbols on OFDM subcarriers based on the channel's signal-to-noise ratio, including the following steps: Step 1: Based on the signal-to-noise ratio between the optical network units and optical line terminals in each branch of OFDM-PON, group each optical network unit; Step 2: In the optical network unit grouping with low signal-to-noise ratio, a low-order orthogonal phase modulation modulation format is used as the base layer; in the optical network unit grouping with high signal-to-noise ratio, a high-order orthogonal amplitude-phase modulation-16 and orthogonal amplitude-phase modulation-64 modulation format is used as the enhancement layer. Step 3: Calculate the valleys of frequency-selective fading in each group of optical network units in OFDM-PON; Step 4: Set a constant total transmit power and a target data rate. Allocate bits for the base layer according to the bit allocation method, allocate power for the base layer according to the power allocation method, allocate bits for the enhancement layer according to the bit allocation method, and allocate power for the enhancement layer according to the power allocation method. The specific method is as follows: Set a constant total transmit power and a target data rate, and set the bits allocated to the base layer and enhancement layer on the k-th subcarrier of OFDM-PON as follows: and The calculation steps are as follows: 1) Let i and j represent the base layer and enhancement layer levels, respectively. and ; 2) Set the target data rate : in, For each level l, the data rate is independent; 3) To Calculate the accumulated level l bit load b k and the accumulated power p k If the total transmission power satisfy: （5） and Less than or greater than Then decrease or increase ,have to And according to the increase ,get ; 4) If Then from the currently accumulated Extract the bit loading at level j-1; the bit loading for each level j is: Otherwise, if Then, under the constraint of the maximum number of enhancement layers, we increase j and obtain .

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