CN107018097B - Sequence detection method based on generalized likelihood ratio detection principle for wireless optical communication - Google Patents
Sequence detection method based on generalized likelihood ratio detection principle for wireless optical communication Download PDFInfo
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Abstract
The invention discloses a sequence detection method based on a generalized likelihood ratio detection principle in wireless optical communication, which is used for carrying out sequence detection on signals through a full-connection grid diagram searching method and a selective storage method and based on generalized likelihood ratio path measurement and judgment conditions of an observation window aiming at a multi-order modulation system, and carrying out channel estimation and ambient light estimation based on sequence detection values. The grid graph designed by the invention is a full-connection graph and does not require to know the memory length of the channel. The complexity of the grid pattern search algorithm is close to the square of the modulation order and does not increase along with the sequence length, the observation window length and the channel memory length. The selective storage method further reduces the complexity and storage capacity of the system and avoids the problems of decision failure and error floor.
Description
Technical Field
The invention is suitable for the field of wireless optical communication, and particularly relates to a sequence detection method based on a generalized likelihood ratio detection principle. Under the condition of no channel estimation and no ambient light estimation, the sequence detection method can automatically eliminate the influence of the ambient light and can carry out channel estimation and ambient light estimation based on the sequence detection result. The method is also suitable for the communication field of other channels which contain additive constant or slow change interference besides multiplicative channel coefficients.
Background
Compared with traditional radio frequency and infrared communication, the wireless optical communication has the advantages of high transmitting power, no electromagnetic interference, environmental friendliness, energy conservation and the like. Wireless optical communication systems generally use a wireless channel as a transmission medium, and particle scattering and object surface reflection in the wireless channel cause multipath fading. At the same time, sunlight and artificial lighting devices cause ambient light interference. Therefore, the signal detection needs to eliminate the influence of the channel state and the ambient light. Common ambient light elimination methods include optical filtering techniques such as optical filters, but the optical filters can only eliminate ambient light with a different spectrum from the light source of the system. Another method is to perform channel estimation and ambient light estimation based on the electrical signal after photoelectric conversion, and then detect the signal without the influence of the channel and the ambient light. Since channel estimation and ambient light estimation require a large number of pilot symbols, such methods not only increase the overhead of energy and bandwidth, but also increase the system delay. The performance of signal block-by-block sequence detection is superior to symbol-by-symbol detection, the performance increases with the sequence length, but the system complexity also increases exponentially with the sequence length.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a sequence detection method based on the generalized likelihood ratio detection principle for wireless optical communication. The method not only can directly detect the sequence under the condition of no channel estimation and no ambient light estimation, but also can carry out channel estimation and ambient light estimation based on the sequence detection result. The method greatly reduces the system complexity of sequence detection and avoids the problem of error level.
The principle of the invention is as follows: based on the generalized likelihood ratio condition of the observation window, the signal is subjected to sequence detection, and channel estimation and ambient light estimation can be carried out based on the sequence detection value. Because the complexity of the sequence detection algorithm increases exponentially with the increase of the sequence length, the complexity of the system is reduced by adopting the full-connection grid graph search algorithm, and the reduced complexity is close to the square of the modulation order. In addition, aiming at the problem that the data stream cannot be judged when continuous multi-bit same symbols appear, a selective storage method is adopted, and meanwhile algorithm complexity and required memory capacity are reduced.
The technical scheme provided by the invention is as follows:
the wireless optical communication system adopts a modulation mode with a modulation order of M, and the method carries out sequence detection on a signal by receiving a sampled discrete electrical signal sequence through a full-connection grid diagram searching method and a selective storage method based on generalized likelihood ratio path measurement and judgment conditions of an observation window. The method comprises the following steps:
(1) determining the length L of an observation window according to the channel correlation time and the storage capacity, and assuming that the states of a channel and ambient light in the observation window are unchanged;
(2) and detecting and storing the received sequence by a full-connection grid graph search algorithm and adopting a path metric and a judgment condition based on a generalized likelihood ratio detection principle and a selective storage method. (ii) a
In addition, the method further comprises the following step (3) of performing channel estimation and ambient light estimation.
(3) And (3) based on the sequence detection value obtained in the step (2), performing channel estimation and ambient light estimation based on the generalized likelihood ratio detection principle.
The invention relates to a full-connection grid graph searching method similar to a Viterbi algorithm, which comprises the following steps:
(1) and constructing a fully-connected grid graph with M nodes at each moment, wherein each node at the moment k is provided with M branches, the ith branch of each node is connected with the ith node at the moment k +1, and i belongs to {0, 1., M-1 }. The ith node at each moment and the ith branch of each node represent the M-order modulation signal M assumed at the current moment ki. Different from the grid graph used for the traditional Viterbi algorithm, the grid graph designed by the invention is a full-connection graph and is not a grid graph of a system model, the system model does not need to be known, and nodes of the grid graph do not contain a hypothetical modulation signal at the previous moment;
(2) and for each node at the time k, based on the signal in the observation window and the survivor path, selecting one of the M hypothetical paths merged to each node as the survivor path through the generalized likelihood ratio path metric and the decision condition. When the two sequences are in an integer multiple relationship, the corresponding generalized likelihood ratio path metric values are the same, so that the judgment is wrong, and the performance has an error floor. Because the last values of the M hypothetical paths merged into each node are the same, the hypothetical paths cannot be in an integral multiple relationship with each other, and the error floor problem is avoided. Because the search algorithm only needs to calculate and compare M at each moment2The path measurement of the hypothesized paths, the number is independent of the sequence length, the observation window length or the channel memory length, thereby greatly reducing the system complexity;
(3) when M survival paths are converged at the time t (t < k), sequences at the time t and the previous time can be detected and determined;
(4) the survivor paths and the received sequence at the undetected instant (i.e., after the t instant) are stored. Selecting and storing L in the test sequence in chronological orderiA modulated signal M of M orderiAnd a received signal corresponding to the time instant. As shown in equation (1), when more than L consecutive identical symbols occur in the hypothetical path, the path metric will have a zero denominator, resulting in a failure in the decision. Provided that two or more Li>0, assuming the path is not parallel to u, the decision failure can be avoided. Meanwhile, the method also reduces the complexity of the algorithm and the required memory capacity;
(5) at the end of the received sequence, the sequence is detected from the M survivor paths in the method of step (2).
Generalized likelihood ratio path metrics, channel estimates, and ambient light estimates require computation of y (k, L) · u and y (k, L) · xm(k, L) where u is all 1 unit vectors of length L, the amount of computation can be reduced by addition of the newest term and subtraction of the oldest term.
The invention is not only suitable for multi-order modulation of an IM/DD system, but also suitable for a phase modulation system.
Compared with the prior art, the invention has the beneficial effects that:
(1) under the condition of not estimating the channel state and the ambient light, directly realizing the sequence detection of the signal;
(2) the channel state and ambient light can still be estimated based on the sequence detection results after signal detection;
(3) the designed grid graph is a full-connection graph and does not require to know the memory length of a channel;
(4) the complexity of the grid graph search algorithm is close to the square of the modulation order and does not increase along with the sequence length, the observation window length and the channel memory length;
(5) the selective storage method further reduces the complexity and the storage capacity of the system and avoids the problems of decision failure and error floor.
Drawings
FIG. 1 is a fully connected grid diagram.
Detailed Description
The following describes an example of the present invention in detail with reference to the accompanying drawings. The present example is carried out on the premise of the technical solution of the present invention, and detailed embodiments are given, but the scope of protection of the present invention is not limited to the following examples.
The invention provides a method for detecting a sequence receiver by a generalized likelihood ratio of wireless optical communication, which is used for judging a receiving sequence by adopting a generalized likelihood ratio detection principle aiming at M-order modulation of an IM/DD system, reducing the complexity of sequence detection by combining a full-connection grid graph search algorithm and a selective storage method, avoiding the problems of judgment failure and error floor of generalized likelihood ratio detection, and estimating a channel state and ambient light based on a sequence detection result after detection. Comprises the following steps:
(1) establishing a channel model: and determining the length L of the observation window according to the channel correlation time and the system storage capacity, and assuming that the channel and the ambient light state in the observation window are unchanged. The signals within the observation window are represented as follows: the transmission sequence is x (k, L) ═ x (k-L +1),.., x (k)]The receiving sequence is y (k, L) ═ y (k-L +1),.., y (k)]The noise sequence is n (k, L) ═ n (k-L +1),.., n (k)]。xm(k, L) represents the mth of the M hypothetical sequences that converge to the same node, M ∈ {0, 1.To detect the signal. The received signal may be expressed as:
y(k,L)＝Rc(k)x(k,L)+RPambu+n(k,L)
where R is the responsivity of the photodetector, c (k) is the channel gain, PambU is the full 1 unit vector L long, which is the optical power of the ambient light incident on the photodetector. c (k) slowly, assuming negligible change in value over the length of an observation window. The receiver does not estimate the channel state information before detection and possible intersymbol interference components are combined in n (k, L). For simplicity, h (k) ═ rc (k) represents channel state information, and b (k) ═ RP represents channel state informationambRepresenting ambient light.
(2) Searching a grid diagram 1, and detecting and storing a receiving sequence by adopting a path metric and a judgment condition based on a generalized likelihood ratio detection principle and a selective storage method;
(3) based on the sequence detection value obtained in the step (2)
And performing channel estimation and ambient light estimation based on the generalized likelihood ratio detection principle.
The decision complexity due to the generalized likelihood ratio detection principle is ML-1The invention designs a fully-connected grid graph searching method, thereby reducing the complexity of the system. The implementation mode is as follows:
(1) constructing a fully-connected grid graph with M nodes at each moment, as shown in FIG. 1;
(2) for each node at time k, a path metric is measured by a generalized likelihood ratio based on the signal and survivor paths within the observation window
Or
And decision conditions
Selecting one of the M hypothetical paths merged to each node as a survivor path;
(3) when M survival paths are converged at the time t (t < k), sequences at the time t and the previous time can be detected and determined;
(4) the survivor paths and the received sequence at the undetected instant (i.e., after the t instant) are stored. Selecting and storing L in the test sequence in chronological orderiA modulated signal M of M orderiAnd a received signal corresponding to the time instant. Since the present example performs channel estimation and ambient light estimation after detection, a detection value of 0 is still effective for estimation, and thus a detection value of 0 does not need to be discarded. For example, take LiAssuming that the path cannot be parallel to u, i.e. a situation where the decision fails is avoided.
(5) At the end of the received sequence, the sequence is detected from the M survivor paths in the method of step (2). After detection, generalized likelihood ratio detection channel estimation utilizes a detection sequence
Obtained by formula (4) or formula (5):
or
The ambient light estimation is obtained by equation (6) or equation (7):
or
Claims (4)
1. A sequence detection method of wireless optical communication based on generalized likelihood ratio detection principle is characterized in that a modulation method with modulation order M is adopted in a wireless optical communication system, a received sampled discrete electrical signal sequence is detected and stored through a full-connection grid graph search algorithm and by adopting path measurement and judgment conditions based on the generalized likelihood ratio detection principle and a selective storage method, and channel estimation and ambient light estimation can be carried out based on a sequence detection value; the method comprises the following steps:
(1) determining the length L of an observation window according to the channel correlation time and the system storage capacity, and assuming that the states of a channel and ambient light in the observation window are unchanged;
(2) detecting and storing the received sequence by a full-connection grid graph search algorithm by adopting a path metric and a judgment condition based on a generalized likelihood ratio detection principle and a selective storage method; the method specifically comprises the following steps:
①, constructing a fully-connected grid graph with M nodes at each moment, wherein each node at the moment k is provided with M branches, the ith branch of each node is connected with the ith node at the moment k +1, wherein i belongs to {0,1,. and M-1}, the ith node at each moment and the ith branch of each node represent possible M-order modulation signals M at the current moment ki；
②, for each node at time k, based on the signal and survival path in the observation window, through the generalized likelihood ratio path metric and the decision condition, selecting one of the M hypothetical paths merged to each node as the survival path;
③ when M survival paths converge at time t (t < k), the sequence of time t and the previous time can be detected and determined;
④ store the survivor paths and receive sequences at undetected times (i.e., after time t). The L in the detect sequence is selected and stored in time orderiA modulated signal M of M orderiAnd a received signal corresponding to the time instant;
⑤ at the end of the received sequence, the sequence is detected from the M survivor paths in the method of step (2).
2. The method for detecting a sequence based on the generalized likelihood ratio detection principle in wireless optical communication according to claim 1, wherein the method further comprises the step (3) of:
(3) and (3) based on the sequence detection value obtained in the step (2), carrying out channel estimation and ambient light estimation based on the generalized likelihood ratio detection principle.
3. The method for sequence detection based on the generalized likelihood ratio detection principle in wireless optical communication according to claim 1, wherein the generalized likelihood ratio path metric in step ② is obtained by equation (1) or equation (2) or other mathematical equivalents:
where u is the full 1 unit vector of length L, xm(k, L) is a sequence of L nonzero M-order modulation signals traced back from the time k to the mth hypothetical path, and y (k, L) is a receiving sequence at the corresponding time;
or
Wherein xm(k-i) is the ith non-zero M-order modulation signal traced back from the time k in the mth hypothetical path, and y (k-i) is the received signal at the corresponding time;
the decision conditions for selecting a survivor path from the M hypothetical paths and for selecting a detection sequence from the M survivor paths are given by equation (3) or its mathematical equivalent:
4. according to claim 2The sequence detection method based on the generalized likelihood ratio detection principle in the wireless optical communication is characterized in that the generalized likelihood ratio detection channel estimation in the step (3) utilizes a detection sequence
This is obtained by equation (4) or equation (5) or other mathematical equivalents:
or
The ambient light estimate is derived from equation (6) or equation (7) or other mathematical equivalents:
or
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