[Organize] a most complete text to read "diversity" and "reuse"

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Diversity and Multiplexing

  Multiplexing and diversity are two typical wireless transmission technology . For the concept and purpose of diversity and multiplexing, the following expressions are given many different ways to promote a comprehensive understanding.

1. The concept of diversity multiplexing and

1.1 Diversity

  Embodiment 1: Diversity is identical in a plurality of independent paths to transmit data, the receiving end by a set of technical points and against channel fading, improving transmission reliability, reduce the error rate ;

  Option 2: The signal on principle, if the original transmitted signal replica other degree of attenuation provided to a receiver, it helps to correct the decision of the received signal. This to enhance the rate of the received signal is the correct decision by providing multiple copies of the transmission signal is called a diversity method .

  Embodiment 3: Diversity receiving end it fading characteristics received independently of one another specific processing approaches to reduce the signal level fluctuates.Diversity refers to a dispersion of transmission and reception concentrated. The so-called dispersion is transmitting the reception side to obtain a plurality of statistically independent,Carry the same informationThe fading signal. A receiver receiving a plurality of focus statistically independent fading signals received are combined (the combined selection) to reduce the effects of fading .

  Embodiment 4: In the field of communications, diversity (Diversity) technology is the same signal transmitted on multiple transmission paths independent from each other. That is, the same signal, independent channels , the same signal is transmitted through separate channels to each other out, obviously does not improve the efficiency of the channel, on the contrary, reducing the efficiency of channel utilization. So, the purpose of doing so is only one: to improve the reliability of information transmission, accuracy .

1.2 Multiplexing

  Embodiment 1: multiplexing different data is transmitted on a plurality of independent paths, full use of system resources and improve system capacity, i.e., the total data rate .

  Embodiment 2: refers to an integrated multiplexing multiple channels on the transmission path, and then restore the original release mechanism or each channel multiplexing process in the terminal. Multiplexing at the transmitting side is the combined multiplex signal (e.g., radio and television front-end mixer), to realize transmission of a dedicated physical channel, the receiving end then the composite signal is separated .

  Embodiment 3: multiplexed (Multiplexing) technique in the communications field, refers to multiple independent signals transmitted on the same transmission path. In other words, different signals, a common channel .At the transmitting end, a plurality of independent signals into one multiplexed signal, called a multiplexed; at the receiving end, the signal is demultiplexed into signals of each individual process, called demultiplexing。

2. The purpose of diversity and multiplexing

Objective diversity 2.1

  Embodiment 1: Diversity is a signal through a plurality of paths reach the receiving end, the advantage is to reduce the signal is subjected to the possibility of channel depth of fading can not be identified; diversity technique is used to compensate for fading channel loss, which typically utilize radio propagation environment of the same irrelevant characteristics between the individual samples of the signal, using a predetermined signal to improve received signal combining techniques, to resist the adverse effects of fading . Spatial diversity means spatially selective fading can be overcome, but the distance between the basic conditions for a diversity receiver is greater than 3 times the wavelength satisfied. As with the equalizer, it without increasing the transmission power and bandwidth, and improve transmission quality of wireless communication channels. In mobile communication, a base station receiver and the mobile station are diversity techniques may be employed.

  Embodiment 2: in a wireless propagation environment, radio signals encounter a variety of fading, influence the receiver to correctly receive signals.By diversity, the same signal on different paths from each other separate transfer experience different fading; merge at the receiving end the signals of different paths, a diversity gain can be obtained, to improve the correctness of the received signal。

2.2 diversity order and diversity gain

  Diversity order(Diversity order) refers to the number of independent fading branches: If the fading between each pair of transmit and receive antennas are independent, then the diversity order of transmit antennas fraction = $N_t$× number of receiving antennas $N_r$; More diversity order, the greater the available maximum diversity gain, the more improved system performance; diversity order will increase in multipath channel, the diversity order of the number of transmit antennas = $N_t$ × number of receiving antennas $N_r$ × Nobumichi multi diameter of several $L$ . The actual space-time code design will reduce the diversity gain.

  Diversity gain(Diversity gain) Definition: $d = -adhesive (lg (P e) / Ig (SNR))$ , the limit value tends to infinity as the signal to noise ratio SNR. Bit Error Rate $P_E = α * (SNR) ^ {- d}$ , coding gain factor α and the original coefficient is obtained as compared to the foregoing, at high SNR, the diversity gain curve to determine the slope error, and error coding gain curve to determine the translation distance.

2.3 multiplexed object

  Embodiment 1: multiplexing a plurality of signals simultaneously over multiple paths served the receiving end, the benefits of sending and receiving end isAn increase in throughput。

  Mode 2: the communication field, the common feature is multiplexed in the multiplex transceiver to ensure the transmission quality information, while improving the utilization efficiency of the transmission path of a resource (personal understanding: for example, frequency division multiplexing, frequency path to ensure the transmission quality at the same time improve the utilization of time and space resources). In data communication,Multiplexing channel transmission efficiency improves(This means that, in the case where the same channel capacity, throughput may be raised), with a wide range of applications.

  Annex: Analysis channel capacity and throughput: "channel capacity" and the "throughput" Analysis

3. Classification diversity and multiplexing

  Dividing a small portion of a resource (sub-channels), respectively, using these small sub-channel parallel data transmission. This technique is often called a multiplexing technique, such as: TDM, FDM, OFDM.
  This technique is used to distinguish the uplink (transmitting side) is called userMultiple access technologySuch as TDMA, FDMA, OFDMA. (PS: downlink multiple access without distinction.) Variety of multiple access techniques may be reference to the following: large-scale spatial division multiple access MIMO transmission
  this technology for distinguishing uplink and downlink data, called duplexing techniques, such as the FDD and TDD.
  This technique is used to distinguish the data transmitted in the radio channel is still called the multiplexing .
  This technique is used to transmit the same data to reduce the error probability of the system, to improve application performance of the system, it is called a diversity technology .

3.1 Diversity techniques

  Common diversity There are two main ways: macro-diversity and differential set.

  Macrodiversity also known as "Multi-station diversity", primarily is a diversity technique for cellular systems . In macro diversity, the plurality of base stations disposed in different geographical locations and in different directions, and while a mobile station communicating in the cell. As long as the signal propagates in all directions at the same time and not serious slow fading or shadowing affected by terrain, this approach can maintain normal communication. It is a slow fading reduction technique.

  Micro-diversity is a method of reducing the influence of fast fading diversity techniques are often used in various wireless communication systems. The main micro-diversity techniques are employed: space diversity, polarization diversity, frequency diversity, field components diversity, angular diversity, time diversity aliquots diversity technique.

• Spatial diversityThe basic principle is to receive the same signal at two different arbitrary positions, as long as the distance between the two positions is large to some extent, the two received signal fading is uncorrelated, i.e. independent space having fast fading. Also referred to as space diversity antenna diversity is used most wireless communication diversity technique .
• Frequency diversityThe basic principle is the fading of the two signals is greater than the coherence bandwidth of the frequency intervals is irrelevant, thus, can transmit the same information with a plurality of frequencies to achieve frequency diversity.
• Polarization diversityThe basic principle is the fading of an electromagnetic wave having two different polarization independent, the transmitting and receiving ends can be very close but different transmit and receive signals polarized antenna with two positions, respectively, to obtain diversity effect. Polarization diversity can be seen as a special case of space diversity, it is also with two antennas (diversity case double), but only the use of electromagnetic waves with different polarities of the uncorrelated fading characteristics, thereby shortening between the antennas distance.
• Angle diversityApproach is that the radio waves through several different paths, at different angles and reach the receiver, and the receiver using a plurality of sharp directional receiving antennas can be isolated in different directions of signal components, since these components have mutually independent signals fading characteristics, it is possible to achieve angular diversity and anti-fading effect is obtained.
• Time diversity: In addition to fast fading with space and frequency independence, it also has a time independent, i.e., the same signal at different times, the retransmission interval a plurality of times, as long as the transmission time of each time interval is sufficiently large, then each time the transmission signal fading occurring will be independent of each other, the receiver receives the same signal are combined repeated, can reduce the influence of fading. Time diversity fading channels primarily for transmission of digital signals .

3.2 Reuse

  Multiplexing two major categories: frequency division multiplexing (i.e., frequency-division multiplexing) and time division multiplexing (i.e. time division multiplexing) , wavelength division multiplexing and statistical multiplexing essentially also belong to this two kinds of multiplexing. There are also other multiplexing techniques, such as code division multiplexing, space multiplexing and polarization division multiplexing .

• Space division multiplexing(SDM) is the same means so that one frequency band is re-used in different spaces, in mobile communication, the basic technology is achieved using an adaptive array antenna spatial division, forming different beams in different directions the user.
• Frequency Division Multiplexing(FDM): carrier bandwidth is divided into a plurality of subchannels of the different frequency bands, each sub-channel signals may be transmitted in parallel way. Process for FDM analog transmission.
• Time Division Multiplexing(TDM): transmitting multiple signals in the same channel intrinsic interaction time interval. TDM is widely used for digital transmission.
• Code Division Multiplexing(CDM): coding each channel as channel bits to achieve transmission (particular pulse sequence). This encoding transmission time by transmitting a unique series of short pulses is completed, but a longer time is used in the bit time segments alternate. Each channel has its own code, and can be transmitted and releasing the asynchronous multiplexed on the same fiber.
• Wavelength Division Multiplexing (WDM): Use of different wavelengths in a single optical fiber simultaneously transmitting multiplexed lightwave signal. For WDM Fiber Channel. And it is applied to FDM WDM lightwave fiber transmission channel based on the same principle.

4. Comparison of Diversity and Multiplexing

  Common: to be using multiple antennas;

  Different points: different content transmitted on multiple antennas, the receiving end further processing of different, such asDiversity will be combined at the receiving end, the receiving end should multiplexed with multi-user detection and isolation. Of course, the strong diversity transmission reliability, stability, high throughput multiplex transmission , reliability may not be as SISO system. The two often contradictory, not seeking to maximize the same time .
  This contradiction may be from the effectiveness and reliability of the communication system to be appreciated that the diversity is to improve the reliability of the communication system, is to enhance the effectiveness of the multiplexed communication system, the reliability and effectiveness is often a contradiction .

The further understanding of the diversity and multiplexing

  Transmitting multiple signals in a path multiplexing, multiple signals how to distinguish each other ? Transmitting the same signal in the plurality of passages diversity technique, how the plurality of passages independent of each other ? In other words, in the field of communication, whether multiplexing or diversity technique involves "Orthogonal"the concept of.

  For instance, frequency division multiplexing is a frequency difference, other multiplexing resources like time slots , i.e.The same time slot, the same space (antenna unit), the same orthogonal codeIn the case where the carrier bandwidth is divided into aDistinguished from each other, a plurality of different frequenciesSubchannels, each transmit different signals. The same principle will be appreciated that time division multiplexing, code division multiplexing, space division multiplexing, in the following table.

   The same signal must be independent of each other different pathways, diversity can play a role . There are many diversity techniques, such as frequency diversity, time diversity, space diversity, angular diversity, polarization diversity, as shown below. Regardless of the type of the diversity technique, there must be independent of path information to ensure that the same signal is subjected to different fading , so that when combined with a reference to each other, complementary, the effect of mutual authentication.

   LTE uses a Multiple Input Multiple Output MIMO technology can play a role in spatial multiplexing and spatial diversity.

Reference material

[1] diversity technique _ Baidu Encyclopedia
[2] multiplexing _ Baidu Encyclopedia
[3] "multiplexing" Analysis of the concept of "diversity" of
[4] Diversity and Multiplexing

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