Diversity Techniques

Diversity is a good technique applied in mobile communication receiver circuits where there are multipath environments. The diversity techniques use the nature of the propagation path characteristics for improving the sensitivity of receivers. It will improve the wireless links, at less cost. It does not require prior training because a training sequence is not needed by a transmitter like an equalizer. The diversity technique finds a way of analyzing signal paths for the multipath cellular environment.

The diversity decisions made at the receiver end and they are not known to the transmitter.

Diversity Techniques Concept

The simple concept of diversity is that even if a radio signal path experiences a deep face, there will be another independent signal path available for analysis.

Let us consider the received signal that is observed with many signal level variations. We assume two antennas with a specific separation between them are located on a terminal. They experience different fading levels as the mobile terminal moves. An example of received signal variations is represented in the figure below. The received signal levels differ with their corresponding fading levels. Therefore, we select an antenna that possess higher signal level so that the probability of deep fading can be avoided.

In the figure shown below, the signal level of antenna #1 suffers fading whereas the signal level of antenna #2 fades in seldom cases. Note that branch #1 is deeply faded whereas branch #2 is not highly faded.

Types of Fading

There are two types of fading as under:

• small-scale
• large-scale fading.

Small-scale fading

The small-scale fading are mainly characterized by rapid amplitude fluctuations and deep fades of less wavelength (λ). The small-scale fading is due to signal fades caused by multiple path and reflections with respect to mobile movement.

Large-scale fading

The large-scale fading is generated by shadowing effects which is due to changes in both the nature of surroundings and the terrain profile involved. The large scale fading is log-normally spreaded with particular standard deviation value of approximately 10 decibels in the urban area.

Types of Diversity

The two basic types of diversity are as under:

• Microscopic diversity
• Macroscopic diversity

Microscopic diversity techniques

To counteract small-signal fading that is to avoid deep fading in the signal received under small distances, fading can be minimized by this technique. It can prevent small signal fades in case of less antenna separations, if two antennas are used.

By choosing the signal of higher strength most of the time, the receiver can reduce the fading effects in the graph shown in the figure below. The small-signal fades rapidly whereas the large-signal fades gradually with respect to an indoor environment.

Macroscopic diversity techniques

In large signal fading, the signal strength reduces that is because of shadowing problems. By choosing a base station that is not shadowed when compared to other base stations, the mobile unit can acquire a better signal-to-noise ratio (SNR) in its forward path. Such a type of counteracting to large-scale fading effects or the method of diversity used to reduce large-scale signal fades is termed as macroscopic diversity technique. This is highly useful at the base station receiver end.

Important Diversity Techniques

The important diversity techniques are discussed in the following ways:

Types of Diversity Techniques

1. Space diversity techniques
   • Selection diversity
   • Feedback diversity
   • Maximal-ratio combining method
   • Equal gain combining.
2. Polarization diversity technique
3. Time diversity technique
4. Frequency diversity technique
5. Directional diversity technique
6. Path diversity technique
7. Macroscopic diversity technique
8. Transmitter diversity technique

Each diversity technique is unique in its functionality but aims towards a common goal of reducing the fading effects in multipath receiver circuit.

Space Diversity Technique

The space diversity scheme is also called as ‘antenna diversity scheme‘. In conventional methods of wireless communication, the availability of direct path between transmitter and receiver is not assured. Therefore, the occurrence of Rayleigh fading will be present.

But, the antenna space diversity can achieve independent fading changes by applying spatially separated antennas.

In the space diversity scheme, the receiver configuration is quite simple. Several number of diversity branches are selectable. For producing diversity reception at each and every cell site, multiple base station receiving antennas are used effectively. It is important to note that main scattering takes place in ground which is in the vicinity of the mobile unit, and hence to attain decor relation, the antennas at base station have to be placed with necessary separation distances. This separation distance can be in order or tens of wavelength (λ) value with respect to base station. Generally, the space diversity technique can be used at base station or mobile or at both ends.

Also, in case, the antenna spacing is greater than λ/2, then it is sufficient to obtain low fading correlation between the diversity branches, and antenna spacing of 50λ to 100λ is a must at the base station end.

A general schematic of space diversity is shown in the above figure. There are ‘n’ branches with separate gain values namely G1, G2 … Gn, and a set of demodulators to generate the required output.

Space Diversity Combining Schemes

Selective Diversity

In selective diversity combining, the branches having the strongest received signal will be selected. In selective diversity method, ‘n’ number of demodulators are used and their gains can be adjusted to give mean signal to noise ratio (SNR) for every diversity branch. Then, the antenna signals will be sampled. Finally, the best signal that possess good signal strength will be sent to a demodulator. It is also seen that practical diversity system has to be carefully such that reciprocal of the mobile signal fading rate is a longer than the internal constant values of selection diversity circuitry.

Feedback Diversity

The feedback diversity technique is also known as scanning diversity. In this method, the ‘n’ signals are scanned in a proper sequence and monitored to pick a signal in the sequence which is above the preset threshold value say ‘α’.

Then, a scanning process will be initiated for the received signals. But, the demerit of this method is that the fading level reduction is less than the other diversity techniques. In this method, for the received signals (m), the best signal of better strength is measured by comparing every signal with a preset threshold value ‘α’ as shown in the figure below.

One of the merit of feedback diversity is its easier implementation than other methods.

Maximal Ratio Combining Technique

The concept of this method is that all the branch signals [N] are combined coherently with necessary weighting coefficients for every diversity branch signal so that the reduction of fading will be better leading to overall improvement of system performance.

A block diagram for this method is shown in the figure below. Unlike selection diversity, the signals are co-phased before the addition process and for this, individual receiver and phasing circuits are a must for all the antenna elements.

In the output, signal of maximal ratio combiner will be such that the sum of individual signal to noise ratio (SNR) values will be equal to the SNR of output signal measured.

Advantages of Maximal ratio combiner technique

• Maximal ratio combiner generates an acceptable SNR value.
• Accuracy is high.
• Produces the best reduction of fading

Equal Gain Combiner Technique

In the equal gain combining, all the diversity branches are coherently added with a same weighting factor. On the other hand, this scheme also co-phases all the diversity branches and finally adds them up. As the signals are co-phased from all branches, they provide an equal gain factor. When compared to maximal ratio combining, the configuration of this method is simple. By applying equal gain combining, it is convenient for the receiver to get back the signals.

One of the demerits of this method is that it degrades the SNR value by 0.5 dB at the output of combiner if two branches are involved. If ten branches are involved in the reception, then, the SNR degradation would be roughly upto 1 dB value.

Polarization Diversity Techniques

In polarization diversity, both horizontal and vertical polarization are involved. In case, if a signal is transmitted by a pair of polarized antennas, and they are received by another pair of antennas, then, two uncorrelated fading signals will be received because different fading variations are experienced by horizontal and vertical polarizations and due to different reflection coefficient values of the tall building walls.

The measured vertical and horizontal polarization signal paths between the base station and mobile are found to be uncorrelated. Also, the decorrelation in vertical and horizontal polarization for signals is due to multiple reflections in the radio channel between base stations and mobile antennas. There will be an amount of dependence of received polarization on transmitted polarization.

Time Diversity Techniques

In the time diversity method, the information is transmitted repeatedly at specific time spacings that would exceed the coherence time of the mobile channel, and this will lead to repetition of signals for several times; irrespective of fading conditions.

Thus, when an identical information is sent for different time slots, it is possible to obtain diversity branch signals.

The time diversity technique is well suited for spread spectrum CDMA system, in which, RAKE receiver is used for reception.

Frequency Diversity

In this method of frequency diversity, the information is transmitted on many carrier frequencies. The idea behind this is that if the frequencies are separated by more than that of the coherence bandwidth of the mobile channel, these would be uncorrelated with each other and hence these would not experience same fading status. Also under channels, uncorrelated situations, the occupancy of fading will be multiple of the individual fading probability (occurrence).

The frequency diversity scheme is applied in microwave fields whenever line of sight (LOS) links is used. That is in LOS links, they may carry many channels in the frequency division multiplex mode (FDM). There are chances of deep fades in frequency diversity due to tropospheric propagation and the resulting refractions of the signal.

The fading variation independence factor between the separated frequency components is a main effect with respect to land mobile communication and it is known as frequency diversity effect. Thus, the frequency diversity is a popular diversity reception technique.

Directional Diversity

The received signals would arrive from different incident angles due to any one of the propagation mechanisms namely reflection, diffraction or scattered signals around the mobile terminal. By using selective directive antennas, the independent faded signals (since all the paths arising from various angles are mutually independent of each other) can be received. This type of diversity is suitable to apply in mobile terminal end, where limited directions of signals at base station is linked.

Path Diversity

In path diversity method, the signals are coherently combined. That is both the direct and delayed signal components are combined together. Thus, the diversity branches are generated only after signal reception, and this method is also called as Implicit diversity. As an example, an adaptive equalizer and RAKE diversity are also categorized as path diversity schemes.

Advantages of Path Diversity schemes are as under:

1. In this scheme, no extra power is required.
2. No extra antennas are required
3. No extra frequency spectrum is required.

Disadvantages of path diversity schemes

This diversity method is very sensitive to Rayleigh fading conditions, and hence, the propagation path conditions have to be given more attention.

Advantages and Disadvantages of Different Diversity Schemes

Table: Some of the advantages and disadvantages of different diversity schemes

S. No	Diversity Scheme	Advantages	Disadvantages
1.	Polarization	(i) No space and extra bandwidth are required.	(i) 3 dB extra power is a must (ii) Two branch diversity schemes is only possible.
2.	Space Diversity	(i) Several diversity branches are allowed. (ii) It is also applicable to macroscopic diversity. (iii) No extra bandwidth or power power is required.	(i) Large hardware size is required. (ii) Larger antenna spacing is a must for the microscopic diversity at the base station.
3.	Frequency Diversity	(i) Several diversity branches are allowed.	(i) Relevant power level of frequency spectrum are important.
4.	Time Diversity	(i) Hardware is simple. (ii) Several diversity branches are allowed.	(i) Larger buffer memory is a must when diversity frequency is small. (ii) More frequency spectrum is necessary according to the number of diversity branches.
5.	Angle Diversity	Doppler spread can be reduced.	Diversity gain will depend on the number of obstacles available around the terminal.
6.	Path Diversity	• No space is required. • No extra bandwidth and power are required.	The diversity gain will depend on the delay status.