Limitations And Advantages Of A Directional Antenna

Abstract – A discussion of directional antennas and performance, the limitations or drawbacks and advantages of using directional antennas compare to omnidirectional antennas.

INTRODUCTION

The definition for directional antennas is antennas which radiates in one or more directions that allowing to increases the performance on transmit, receive and reduce the interference from other sources, or in another way to say is the antennas that receives or sends signals most effectively in a particular. The Radio Frequency (RF) energy can be diverted in a particular direction to father distance. Therefore, it has long range coverage but the effective beam width decreases.

Due to the size of directional antennas, the frequencies used are above 200 to 300 MHz. The antenna’s wideband property depends on the type of the antennas and the directional properties of the antennas are a function of their electrical size.

Figure 1 : Directional Antenna with 6 zones. [6]

Each zone is a wedge with radius r spanning π/3 radians. Zone 1 always faces east. The dashed circle shows the omnidirectional communication radius.

A few types of directional antennas are available. [5]

Fixed Beam

Fixed beam antenna has fixed gain profile with a primary lobe pointing in a single direction. We only can steering the beam by changing the orientation of the antenna physically , which permit relatively slow changes at best. This kind of antenna does provide high gain for their cost and are widely deployed in practice.

Figure 2 : Fixed Beam antennas gain pattern

Sector

In sector antennas, there is multiple fixed beams antennas where each of these beam antennas aims in different direction. Each has covering different area and total 360° coverage. Packets may be sent on any sector. Switching between antennas is done electronically and allows the choice of sector to occur on a per packet basis.

Figure 3 : Sector Antenna Gain Pattern

Analog Phase Array

Analog Phase Array antennas work by calculates phase shifts into the signal at antenna’s elements. There will be individual signals after from phase shifts interfere constructively and destructively with each other in order to form a particular gain pattern.

Figure 4 : Analog Phase Array Antenna Gain Pattern

Digital Phase Array

Another description for Digital Phase Array is smart antenna uses digital signal to accomplish phase shifting. Even the additional power required to do this had increase the cost and complexity greater than the Analog Phase Array, but there is some significant function had been added. Lobes and nulls may be steered very precisely to amplify desired signals and eliminate extraneous ones and angle of arrival information for packets may be obtained as well. Multiple patterns may be realized simultaneously using the same set of elements.

Figure 5 : Digital Phase Antenna Gain Pattern

THE GAIN OF DIRECTIONAL ANTENNAS [1]

The definition for directivity according to [7], “The directivity of a wireless antenna is given by the ratio of maximum radiation intensity (power per unit solid angle) to the average radiation intensity (averaged over a sphere). The directivity of any source, other than isotropic, is always greater than unity”. Both omnidirectional antennas and directional antennas have directivity but the difference between these two antennas is the coverage pattern. For omnidirectional antennas, the coverage pattern is torus-shaped. The directivity in directional antennas case in higher than the omnidirectional because of its ability to focus the beam.

Because of the greater gain in the directional antennas, compare to onmidirectional, the signal transmitted with some power will be able to reach wider distance than the signal transmitted in the omnidirectional antennas.

LIMITATION S OF DIRECTIONAL ANTENNAS

Deafness [1]

Deafness is one of the problems happen when using the directional antennas and it had limited the network performance. Deafness is the problem of failed to hearing from the others. In omnidirectional antennas, all neighboring node are capable of listening to all ongoing transmission but not in directional antennas. The node may be turned into particular sector while receiving and the node said to be ‘locked’. In this situation, all the signals that arrive in other sectors cannot be received by the nodes. The nodes said to be Deaf in all other sectors.

Figure 6 : Deafness

In the figure 2 above, it shows that Node A is communicating with Node B. During the communication, Node A is facing to Node B and turned away from Node C. When Node C is sending a transmission to node A, Node A failed to hear for the transmission unless it is available. Node A can be said “Deaf” towards Node C .

The implement of directional antennas said to reduce the interference but it has increase the ratio of packet loss. Multiple retransmissions may also cause the node to misunderstanding that the connection is lost due to mobility and triggering for route discovering search. In another way to say, it causes the destructive interactions with the upper layers.

Drawbacks Specific to Directional MAC (DMAC)

The above layer in OSI model does not seem to harness the features of the model even there is some or specific changes in the physical layer. Reusing the same approaches as that omnidirectional MAC, DMAC has created or bring new problems which were not exists in omnidirectional MAC. There is a few drawbacks :

Heightened hidden terminal

When a node transmits a signal that may affect an ongoing transmission, the hidden terminal problem will be arise. The Ready to send and clear to send are not reaching all the neighbor nodes and this will cause those nodes unaware of ongoing transmission.

Head of line blocking-

The queuing mechanism used is the First-In-First-Out (FIFO). So the node with antennas will pick the first packet in the queues to transmit. The node will send the packets if the channel in the direction that the node wish to communicate with. If the channel is not idle, the node has to wait until the channel is idle and then transmit the packet. There is still has others packet in the queue waiting to transmit, and there is possible that the channel is not idle. Because of the first packet is still waiting for transmission, it blocks all the packets that can be transmitted.

Imperfect virtual carrier sensing

Nodes often do not listen to all the signals around them due to the deafness problem. This causes an incomplete Directional Network Allocation Vector (DNAV) table which doest not consistently store the state of the channel in different directions. This leads to imperfect virtual carrier sensing.

Effect of mobility [1]

Figure 7 : Effect of mobility

The reach-ability due to higher range (Position 1)

In the communication between two nodes, Node X to Node Y, the coverage using omnidirectional antennas is in circle pattern while the coverage pattern using directional antennas is in lobes pattern. If the Node Y moves out of the circle area, it will unable to receive any packet transmitted by Node X. Since the gain of directional gain is more higher, it is possible that Node Y is still in the directional range X and hence, Node Y still able to receive the packet transmitted. If not, Node Y still will be unable to receive the packet transmitted.

Reach-ability in different sector (Position 2)

In any area inside the circle area when using omnidirectional antennas, the Node X will be able to reach the Node Y. While using directional antennas with DMAC, Node X failed to reach the Node Y using the same sector. It is because Node Y has gone out of the range of the signal that is transmitted in that sector.

Un-reach-ability due to omni-discovery (Position 3)

Node X tries to send the packet in its old direction. After failing to reach it after ‘Directional retransmit limit’, Node Y will tries to send an omnidirectional signal. Since the Node Y is unreachable by directional signal, Node X cannot discover the Node Y and thus, Node X will assume that the Node Y is unreachable. Node X will never tries to reach the Node Y using different sector even though it is be done by transmitting directional signal in different sector.

Node X will reports the error to the above routing layer and drops the packet due to no route. In fact this problem can be solved by transmitting the packet directionally in another sector.

Totally unreachable (Position 4)

This is another case where the Node Y moves to out of range for both and directional. Node X will be unable to reach the Node Y both in omnidirectional and directional. This case of mobility will lead to disconnection and cannot be recovered.

ADVANTAGES OF USING DIRECTIONAL ANTENNAS

Security Issue

Use directional antennas to prevent Wormhole attack [6]

Figure 8 : Wormhole attack where the adversary controls nodes X and Y and connects them through a low-latency link.

Wormhole attack means that a forwarded packet from attackers through a high quality out-of-band link and replays those packet at another locations. The attackers will replay the packet received by node A at node B and vice versa. A more intelligent attacker may able to replace the wormhole endpoints at particular locations and this may disrupt nearly all communications to or from a certain node and to all the nodes in the communications.

In directional antennas, based on the signal received, a node can get the approximate direction information. Thus, an attacker cannot execute the wormhole attack if the wormhole transmitter is recognized as a false neighbor, that is not the real neighbor and so ignore the messages. There are three increasingly affective protocols to help to prevent the wormhole attacks. As bidirectional information is added, it is more difficult to allot the attacker to launch the wormhole attack successfully. The three protocols are

Directional neighbor discovery protocol, that is does not rely on any cooperation between nodes and cannot prevent many wormhole attacks.

Verified neighbor discovery protocol, that is preventing wormhole attacks where the attacker control any two endpoints and the victim nodes are at least two hops distant.

Strict neighbor discovery protocol where to prevent wormhole attacks even when the victim nodes are nearby.

To Support Symmetric traffic services In Time Division Duplex (TDD) Code Division Multiple Access (CDMA) [2]

Code Division Multiple Access comprises of two operations , that are Time Division Duplex and Frequency Division Duplex, which to provide two-way simultaneously. A pair of frequency bands is used for uplink and downlink transmissions in FDD. In TDD, the uplink sand downlink transmissions are multiplexed into time slots on the same frequency band, the system is its capability of flexibly to adjusting the uplink and downlink bandwidth by allocating different numbers of time slots. It is more suitable for applications with asymmetric traffic suck as Internet Browsing and file transfer compare to FDD. However, Cross-slot Interference which may seriously degrade the system capacity may happen in TDD-CDMA system during the transmission of asymmetric traffic from adjacent cells. Cross-slot interference is the interference due to opposite direction transmissions between two adjacent cell.

In the [2], this paper has shown that the by applying developed interference analysis framework how the interference between virtual cells can be suppressed due to the directivity of directional antennas and thus proposed a virtual based interference – resolving algorithm to support asymmetric traffic services in TDD-CDMA Systems.

In this paper, it stated that how the directional antennas take effect in supporting the asymmetric system in TDD-CDMA.

By using directional antennas in a trisector cellular system can restrict the strong base-base interference into a hexagon and consequently, it is possible by just coordinating the switching points of downlink and uplink bandwidth ratio in only three sectors for TDD-CDMA.

The cross-slot-interference level in the omnidirectional case is larger if compare with the directional antennas case. This is due to the transmissions power from a mobile station in omnidirectional cellular system is greater than the trisector cellular system. The reason is because the smaller antennas gain.

In ad hoc network

Spatial reuse factor

Figure 9 : Spatial reuse in directional antenna

In figure 9, Node A want s to have communications with Node B , Node C and Node D. In omnidirectional case, the communication between Node C and Node D is not allowed if there is packet sending from Node A and Node B. This is to avoid that the packet from Node C to interfere with Node A to Node B communications. If we using directional antennas, then the sender may focus the beam towards to the receiver. It allows that the coomunications between Node A to Node B and Node C to Node D go on currently. As conclusion, if the nodes use directional antennasthen neighboring nodes that are not in the direction of the signal can go ahead with their transmission. Multiple transmissions can be initiated by different nodes instead of a single transmission if they are not interfere with one another. This increasing the spatial reuse factor.

Extended Range and Energy Savings

Figure 10 : Extended range in directional antenna

In the figure 10, the Node A want to communicate with Node C. in omnidirectional case, the communications cannot reach in single hop. Node A has to transmit the packet to Node B and Node B will transmit the packet to Node C. When using directional antennas, there is larger directional gain. Hence, Node A is able to reach the Node C in single hop. With higher directional gain, focused beam can travel larger distance than those unfocused beam in omnidirectional beam. The sender can reach to receiver with farther away and this has increase the transmission range. Also with higher directional gain, the power required to reach a maximum distance is less than the power used in omnidirectional antennas. This reduce the energy spent by nodes for transmission and reception.

Conclusion

As conclusion, directional antennas have those benefits that is not exist or stronger or solve the problem exist in using omnidirectional antennas but there is also a few problem that occur only in using directional antennas. There is a few solution proposed to reduce the problem but there is still have a lot of space for improvement.