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In the field of wireless communication, multipath radio signals through a plurality of paths refers arrival propagation phenomena receiving antenna from the transmitting antenna. Atmospheric scattering electric wave, ionospheric reflection and refraction of the waves, as well as mountains, buildings and other objects of surface waves due to multipath propagation are reflected.
In the wireless transmission system, the multipath means simultaneously receives two copies, copies after two different transmission paths, with different transmission delays.
For example: from the signal received by the receiver together with the (non-reflected signal) to the building or other direct transmission signals reflected by an object. This will cause the television receiver "ghost" - One can see that there is a decay of the echo in the horizontal direction is superimposed on the main image.
Another common example is a radio (AM radio in particular), the reflected signal through the ionosphere with a certain delay, and this signal is directly transmitted radio signal received together.
Typically, the multipath had a negative impact on the system, but differ in the MIMO system, a MIMO system specifically different antennas send a copy of the signal, complex receiver system different chip combination is treated to improve system performance.
The impact caused by multipath
Multipath fading cause signal and phase shift. Rayleigh fading is a kind of amplitude of the impulse response of the channel subject to multipath Rayleigh distribution statistical models. For the multipath channel, the direct signal is present, which can be described by statistical models Rician fading.
You can visually see the effects of multipath in the communication quality of the television signal transmission. Longer path signal components arrive slightly later than the receiving antenna with a short signal path to the antenna. Because television scanning electron gun is superimposed on a slightly right of the TV screen from left to right, the late signal will be formed early in the signal of the virtual image.
, A single target may generate one or more virtual images reflected on the terrain since the radar receiver similar reasons. The same as the actual object motion and the virtual image thereof is reflected, and therefore affect the identification of the radar target. To overcome this problem, radar receiver needs to signal to nearby topographic map compared pair appear below ground or removed by the reflected signal generated at a certain height or more.
In the digital radio communication system, the intersymbol interference generated multipath effects (inter-symbol-interference, ISI) will affect the quality of signal transmission. The time domain equalizer, orthogonal frequency-division multiplexing (OFDM) and Rake receiver can be used to combat multipath interference generated.
The basic idea is to use a time domain equalization in the transversal filter utilizing the delay time determines a coding sequence encoding a currently received sequence, other than removing the error code determination rule, thereby eliminating the error code present in reduced intersymbol interference . For example a known coding sequence of 11001 should be 10, if there 01 is removed, and then determines the next sequence, until the restore the correct coding sequence.
Orthogonal frequency-division multiplexing (OFDM) technique is one of LTE (UMTS Long Term Evolution standard) of key technologies, the basic idea is to decompose the data stream into several low-speed bit streams separate, from said frequency domain It is divided into a plurality of sub-carriers, and transmitted in parallel. When such a high-speed transmission can be effectively reduced, due to multipath intersymbol interference caused by transmissions. In order to maximize the elimination of multipath effects and intersymbol interference caused by other factors, the OFDM technology is still in each signal set an idle period of the transmission, called a guard interval, which time period is greater than the maximum channel delay, so as not to generating intersymbol interference caused by the delay of the next signal. As shown, the dashed line is a non-idle signal segment, although at this time due to the overlapping transmission occurs before and after the multipath signals, but because no signal idle period, and therefore does not interfere with the overlapping portion. In practice, since the idle period of no transmission waveform, at this time, if the overlapped portion of a plurality of carriers, the orthogonality is destroyed, due to multi-path transmission due to inter-channel interference (ICI, Inter Channel Interference), idle for this period also fill signal, called a cyclic prefix, then when receiving a signal to discard this paragraph. The dotted line portion of the signal waveform is added after the cyclic prefix.
Multipath effects not only frequent causes of fading, and is one of the basic factors limiting the transmission bandwidth or transmission rate. In HF communication, in order to ensure the maximum multipath delay in the transmission circuits and the minimum delay difference is not larger than a predetermined value, the operating frequency requires a certain percentage of not less than the maximum usable frequency of the circuit. This percentage is called multipath reduction factor is an important basis to determine the lowest usable frequency circuit. FIG multipath reduction factor and path length. Measure the anti-multipath channel tropospheric propagation, usually suppressing the reflection surface, and a narrow beam antenna diversity reception and the like.
Remedy multipath interference
Anti-multipath interference measures have the following main aspects:
(1) improve the distance measurement accuracy of the receiver, such as the narrow correlator code tracking loop, phase ranging, smoothed pseudorange and the like;
(2) anti-multipath antenna;
Smart antennas using a combination of a plurality of antenna elements for signal processing, automatic adjustment of transmit and receive patterns to different signal environment for optimal performance. Smart antenna is a space division multiple access (SDMA) techniques, consists of two aspects: spatial filtering and direction of arrival (DOA) estimation. Spatial filtering (also called beam forming), the main idea is to use the distribution of signal, interference and noise in the space, the use of linear filtering technique to suppress interference and noise as possible, in order to obtain the best possible signal estimate.
By controlling the weighting smart antenna adaptive algorithm automatically adjusts the antenna pattern, it is formed in the direction of interference nulling, canceling the interference signal to form the main beam direction of the useful signal, the purpose of interference suppression. Automatic adjustment is the formation of the beam weighting coefficients. Smart antenna beam forming the multi-user interference is greatly reduced, while reducing inter-cell interference.
(3) multipath signal processing and adaptive noise cancellation technology.
Multiple access interference due to the consequences of using a conventional single user reception scheme in a multi-user system caused. Single-user receiver matched filter as a tool-related decisions, does not consider the presence of MAI, each user is detected not consider the impact of other users, for a single-user detection strategy. In general, the MAI does not exist a single user transmission, but in a multi-user environment, when increasing the number of users or interfere with their transmission power increases, MAI will not be ignored. Thus Multiuser Detection promised born, which has optimum detection algorithm and algorithm suboptimal detection algorithms.
In CDMA systems, multi-user detection problem is actually observed value is the random variables from the plurality of linear combinations of the extracted noise random variable target process. In general, multi-user receiver needs to know not only all the user needs to spread information but also as the system becomes constantly updated. Further, the user needs to estimate the magnitude, phase, and timing information for detecting the receiving end, this will inevitably cause an increase in computational complexity. Because of this limitation, most multi-user detection is applied to one side of the base station, the mobile station will be applied to one side, one implementation is to send known training sequence the receiver parameters adaptively adjusted to a desired operating state . This approach has obvious drawbacks: when the channel changes in response to either mutation or the number of users, it is necessary to re-send a training sequence, the training sequence transmitted frequently cause a great waste of spectrum resources. For these reasons, developers do not need to spread information to all users, no need to send blind multiuser detection algorithm training sequence to become the new hot spot industry research. Linear detection, for example, linear multi-user detection is blind without knowing interfere with the user spreading information, the process by which the weight vector in the case of the training sequence is not required. Since all users operate independently in the same modulation scheme, the user may be assumed between the information symbols and codes different element of the same user are independent and identically distributed, and the magnitude of the difference can be reflected in the mixed channel response coefficient matrix.