Diversity Technique for 4-G Wireless Communication

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Wireless transmission involves communication between devices without the help of any wires, cables and like any other physical connections over the distance. The main factors to consider in wireless transmission are cost effectiveness, flexibility, convenience, speed, accessibility and constant connectivity. Wireless communication is made up of a lot of components namely mobile stations, mobile equipment, base stations, subscriber identity module, mobile switching center, base transceiver station and so on. The main moto of wireless communication is allowing many users to share data without any collisions. This can be achieved by using various multiple access techniques known as frequency division multiple access(FDMA), time division multiple access(TDMA), code division multiple access(CDMA), space division multiple access(SDMA). Depending upon the allocated bandwidth the techniques also split into narrowband and wideband systems. There are various factors that affect the transmission of data which are path loss, fading, interference, doppler shift and this parameter is affecting the data rate, reliability and range of transmission. We are mainly concentrating on fading problems and we will see how to solve this by various diversity techniques.

Wireless communication

Cost effectiveness:

In wired communication there are a lot of wires used to provide connection, which will result in more cost and less space. But wireless communication does not require any wire connection to provide better communication which will result in low cost and more effective. Hence any company that uses wireless communication will provide a cheap transmission for the users.

Flexibility:

In wired communication, the people should be in the place for communication. But in the case of wireless communication there is no need for people in place. It is flexible because it will happen irrespective of their location. Due to this people can communicate with each other wherever they are irrespective of their distance and location.

Convenience:

In wireless communication, we use mobile phones for communicating, which are so simple and anyone can use this for communicating and there is no need for physical communication. Without any cable connection we can now connect to anyone, anywhere, anytime.

Speed:

The improvement of speed is also an important term. The improvement in speed and accuracy is a needed parameter. Wireless remote connections are more speedy than wired ones. If anything goes wrong wireless will easily manage and work more faster than wired connection.

Accessibility

The wireless helps easy accessibility as the remote areas where ground lines can’t be properly laid, are being easily connected to the network. Like in rural regions, online education will be possible by help of easy accessibility everyone can get.

Constant connectivity:

Constant connectivity also ensures the stream of data transferring is very smooth and makes it possible. Like GPS connection between devices will change whenever they are moved.

Propagation Mechanism

The transmission data is split into three types depending upon the frequency ranges and distance from the ground. Which will be discussed below:

Ground wave propagation:

It will occur at the frequency range up to 2 Mhz and distance up to 16 kms. It also called as surface wave propagation

Sky wave propagation:

It is also called ionosphere wave propagation, because the transmission occurs depending upon the reflection of the ionosphere. The frequency range is from 2 Mhz to 30Mhz. The propagation distance is 50 Km to 400 Km. It will split into different layers depending upon the critical frequency and distance.

Space wave propagation:

It is also called line of sight propagation. It will split the transmitted signal into two signals, which are direct and indirect waves. Where this propagation depends upon the distance between the transmitter and receiver it is also called line of sight propagation.

Source: https://www.researchgate.net/figure/Multipath-Propagation-Concept_fig1_354748628

Channel Limitations

There are a lot of variables that can affect the channel which are path loss, interference and blockage. These will affect the range of transmission, data rate and reliability of the communication. We will discuss various channel characteristics as follows.

Fading:

Fading refers to fluctuation in signal strength when received at the receiver. It is also classified into two types: slow fading and fast fading. The time gap between the first and last received signal is known as delay spread. Fading will occur because of three mechanisms which are reflection, diffraction, and scattering.

Slow fading:

As per name the fading occurs slowly. It will occur when the receiver is covered by buildings from the transmitter. Which is also called as shadow fading due to big objects like buildings blocking the direct transmission between the transmitter and receiver.

Fast fading:

It refers to the rapid fluctuation in amplitude or phase of the received signal due to interference between multiple versions of received signal arriving at different times.

Source: https://www.peoi.org/Courses/Coursesen/uwb/uwb1.html

Path loss:

Path loss is the ratio of transmitted signal to the received signal power in the given path. It also functions as the propagation path. It is a very important parameter to consider when designing wireless communication networks. It also depends on the number of parameters like radio frequency used and nature of the terrain. Path loss will be low in free space transmission due to direct path between the transmitter and receiver.

Interference:

In wireless transmission the data should encounter interference due to a variety of sources. Where we will see some interference below.

Adjacent channel interference:

In adjacent channel interference the signal will have the components which are outside of the allocated bands for the signal. While transmitting the components will interfere with that outside frequencies that will create interference. It will be avoided by using a guard band between the allocated frequency ranges.

Co-channel interference:

It is also called narrowband interference. It will occur due to nearby systems using the same transmission frequency range for transmission.

Inter-symbol interference:

In this type of interference, the distortion in the received signal is caused by temporal spreading and consequent overlapping of individual pulses in the signal.

Source: https://www.mathworks.com/videos/winner-ii-channel-model-1484949729030.html

Attenuation:

The strength of the received signal decreases as the distance from the transmitter increases. It is also the function of distance, transmission medium, as well as frequency of transmission.

Noise:There are various forms of noise which will degrade the transmission. The most common one is thermal noise. It is due to the thermal agitation of electrons and it is uniformly distributed across the frequency spectrum.

Techniques:

There are various techniques used to overcome the above listed limitations. Some are listed below.

A. Equalizer:

Equalizer is a filter used at the receiver side which will equalize the amplitude and delay in the received signal, whose impulse response is the inverse of the channel impulse response. Which have been used in frequency selective fading channels. Adaptive equalizer has two phases of operation that are training and tracking mode.

B. Channel coding:

It will improve mobile transmission by adding redundant bits to the transmission message. At the baseband portion of the transmitter, a channel coder maps a digital message sequence into another specific code sequence containing a greater number of bits than originally contained in the message. Channel Coding is used to correct deep fading or spectral null.

C. Diversity:

Diversity is another technique used to compensate for fast fading and is usually implemented using two or more receiving antennas. It is usually employed to reduce the depths and duration of the fades experienced by a receiver in a flat fading channel.

Diversity :

Diversity is the technique used in wireless communications systems to improve the performance over a fading radio channel. Here the receiver is provided with multiple copies of the same information signal which are transmitted over two or more real or virtual communication channels. Thus the basic idea of diversity is repetition or redundancy of information. In virtually all the applications, the diversity decisions are made by the receiver and are unknown to the transmitter.

The reception of a signal in a channel transmitted through any type of fading degrades in quality if the signal level attenuation is below the expected operation region of the receiver. In this situation, the received signal power is not expectedly enough compared with signal noise and interference power for reliable reception. The solution to overcome the channel attenuation because of fading problems in the channel is to increase the transmitted power adjusted to the attenuation which is called power control (PC). On the other hand, there are two primary problems with this power control (PC) system.

One of these problems is that the dynamic range of the transmitter and the required transmitting power is extremely high if it’s intended to fully compensate for the fading. This is impossible because of the radiation power limitations, the cost and the size of the amplifiers, and the limited battery power in the portable unit. Moreover, excess transmitted power increases the interference level at the other channels and users in the system unit.

Using a PC the fading can’t be overcome completely but the attenuation may compensate considerably. It can mention that large-scale fading can be compensated as well in the uplink of a system, for example CDMA. But stringent power control is required in order to prevent near far problems in the system. The rate of large-scale fading is simply slow, as a result it can be tracked well and the delay in the feedback of the power control commands can be neglected 39 compared with the rapid fading. On the other hand, small-scale fading can result in such rapid variations in the signal power that even the power control can’t follow them.

Another approach to minimize fading effect in a system is to supply multiple replicas of the transmitted signal to the receiver which already have passed through different fading channels. The result of this approach is that the probability that all replicas of the signal will fade simultaneously is reduced. This is called diversity and it is effectively and commonly used to overcome degradation in performance due to interference and fading. If there is a D number of fading channels and the probability that any one channel may fade under some threshold is P, then the probability of all D signals which fade below the threshold is . The number of diversity channels D is called diversity order in the system.

Diversity Techniques :

Diversity technique is used to decrease the fading effect and improve system performance in fading channels. In this method, we obtain L copies of the desired signal through M different channels instead of transmitting and receiving the desired signal through one channel. The main idea here is that some signals may undergo a fading channel but some other signals may not. While some signals might undergo deep fade, we may still be able to obtain enough energy to make the right decision on the transmitted symbol from other signals. There are a number of different diversities which are commonly employed in wireless communication systems. Some of them are as follows:

Frequency Diversity:
Time Diversity:
Space Diversity:
Polarization Diversity:
Angle Diversity:
Antenna Diversity:
Receive Diversity:
Transmit Diversity:

Frequency Diversity:

In a channel, Transmitted signals with different frequencies are affected in different ways in the frequency domain. The fact is an advantage in frequency diversity technique. Multiple replicas of information signal are sent over several affected frequency bands in this diversity.

Moreover, frequency hopping might be used to achieve such a kind of diversity instead of sending multiple frequency replicas over different affecting frequencies. Frequency bands can be changed many times per symbol in fast frequency hopping and this results in frequency diversity on each transmitted symbol. This process is very beneficial in an environment where there is a partial band jamming in the channel.

However, a different frequency band is used for a burst of symbol in slow frequency hopping. If SFH is combined with time domain coding and interleaving, it is seen as an additional block interleaving in the frequency domain. This kind of spreading of information in a frequency domain introduces frequency diversity benefits. In general, the information signals are modulated through different carriers M in frequency diversity scheme. It is important that different signals undergo independent fading. The carriers should be separated by at least coherent bandwidth from each other. L copies of signals are optimally combined at the receiver to make a statistical decision. The maximum ratio combiner is the optimal combiner.

Source: https://www.sciencedirect.com/topics/engineering/frequency-diversity

Time Diversity:

Interleaving and coding, over symbols across different coherent time periods, is used to obtain time or temporal diversity. This technique utilizes coding of channel and interleaving to mitigate channel fading at a cost of added delay and loss of bandwidth efficiency. It is used on slow fading channels and on channels which are delay sensitive.

Intentional redundancy is introduced into the transmitted signal to achieve time diversity in the temporal domain. Redundancy can be done by repetition of channel coding. To make repetition coding, information bearing signals are transmitted in several time slots. But the separation between time slots should be more or equal than the coherent time of the channel to obtain independent faded signals which helps to gain full diversity advantages. Moreover, it is possible to obtain repetition coding by spreading in direct-sequence code division multiple access.

Channel coding, when there is an error control capability, may be performed separately or with modulation. If we want to perform coding and modulation separately, then we can use conventional error control coding to achieve redundancy in the form of extra symbols. The advantage to this case is that the transmission of extra symbols leads to lower efficiency in system bandwidth. On the other hand, if we want to perform coding and modulation together then the redundancy comes from the channel-set expansion. But the most beneficial process is ceded modulation because it is bandwidth efficient since it allows error control coding without increasing the bandwidth.

Source: https://slideplayer.com/slide/9205161/

Space Diversity:

Multiple antennas are used to transmit signals with carrying information at the transmitter and/or receiver to provide multiple independent fading paths in space diversity. This technique is used to provide significant performance gain without sacrificing any valuable bandwidth on the transmitted power resources. Spatial diversity is widely used because it is easy to implement and it’s cost effective and very simple. This technique has a single transmitting but multiple receiving antennas. The receiving antennas should be at enough distance for that the multiple fading in the diversity will be uncorrelated. There should be a balanced average power between channels and the correlation coefficient should be very low to achieve a good diversity gain. In Space diversity, there are multiple receiving antennas placed at different spatial locations, resulting in different received signals.

Source: https://www.analogictips.com/signal-channel-diversity-fading-part-1-space-diversity/

Polarization Diversity:

Here, the electric and magnetic fields of the signal carrying the information are modified and many such signals are used to send the same information. Thus orthogonal type of polarization is obtained. It enables detection of smaller radar cross-section (RCS) targets, and avoids the physical, mathematical, and engineering challenges of time-of-arrival coherent combining. The advantage of polarization diversity over spatial diversity is that diversity gains are possible with collocated antennas.

Angle Diversity:

The angular diversity schemes may be applied at the base station or at the Mobile unit.

At the base station:

In time Angular Diversity Combine the two received signals at the same time in order to achieve Diversity Gain. It is a microscopic diversity.
Out of time Angular Diversity The signal strength of the mobile unit is constantly monitored at the base station by each beam of a multi-beam antenna system. The strongest beam was used for the traffic link at the time.

Antenna Diversity:

Antenna diversity is a popular and extensively used technique to improve performance in wireless communication systems. The technique reduces fast fading and inter-channel interference effects in the wireless network system. In an antenna diversity system, two or more antennas are used and fixed in positions which will provide uncorrelated signals with the same power level. Then the signals are combined and created an improved signal. The basic method of antenna diversity is that the antennas experience different kinds of signals because of individual channel conditions and the signals are correlated partially. Then we can expect that if one signal from one antenna is highly faded, other signals from other antennas are not faded in such a way and these signals are our expected quality signals. In a multipath propagation environment, each receiving signal experiences an individual fading characteristic.

Receive Diversity:

Multiple antennas are used at the receiver to obtain diversity and employ switching and combining or selection intending to improve the quality of received signal. Since it is easier and cost effective to use multiple antennas at the base station than the terminal which is a positive manner of receiving diversity. This technique may utilize channel state information (CSI) at receiver and it’s fully fit for uplink which is remote to base. But the main problems of receiving diversity are cost, size and necessary power at the remote units. This technique is larger in size and expensive in cost because of multiple antennas, radio frequency chains or selections and its switching circuits.

Transmit Diversity:

Unlike receive diversity, transmit diversity needs multiple transmitting antennas. Moreover, unlike receive diversity, transmit diversity does not utilize CSI in its single information signal. Effective signal processing technique should be used to extract the noisy and distorted received signal in transmit diversity.

Why do we use Transmit Diversity?

Sometimes, a base station has to serve hundreds of thousands remote units. Therefore, it is cost saving to add the necessary equipment to the base stations instead of the remote units. This is the main reason that transmit diversity is very attractive to the wireless service operators. For example, for covering the service area of a base station, one antenna and one transmit chain can be added to that base station to improve the reception quality of all the remote units under the base station. Transmit diversity is more effective than receive diversity for increasing the forwarding link that is the bottleneck in broadband asymmetric applications such as browsing the internet and downloading files.

Conclusion:

The diversity is used to provide the receiver with several replicas of the same signal. Diversity techniques are used to improve the performance of the radio channel without any increase in the transmitted power. As higher as the received signal replicas are de-correlated, as much as the diversity gain can be achieved. Different types of Diversity schemes have their own merits and demerits So in different environments different diversity schemes are selected.

References :

Authors:

Sneha Bagul
Prasad Chatur

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Diversity Technique for 4-G Wireless Communication

Diversity Technique for 4-G Wireless Communication

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