Performance Analysis For The Communication With LEO Satellites Between Dynamic TDMA, CDMA, And OFDMA

Introduction

Wireless communication is a technology that involves data transmission between two or more devices using a wireless signal. There are several layers based on different models. Datalink layer constitutes of media access schemes which assist in relaying of data. They include contention-based schemes, conflict-free schemes and hybrid schemes. CDMA (Code Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access) are part of conflict-free schemes. D-TDMA (Dynamic Time Division Multiple Access) is an example of hybrid schemes. LEO (Low Earth Orbit) satellites are approximately 2000km in altitude. They are used for voice and data communication. They are located nearer to the ground hence require less power and have a small propagation round-trip delay.

Discussion

This paper seeks to find out which one performs better for the communication with LEO satellites between dynamic TDMA, CDMA, and OFDMA.

CDMA

The channel uses code for transmission. Consequently, the scheme uses Spread Spectrum as the concept behind its implementation. This involves use of a wider bandwidth over which signal information is spread. Pseudo-noise spreading codes are used in modulating the signal. It is implemented either as frequency hopping or direct sequence spread spectrum.

Some of the issues worth noting about CDMA include chip rate, bandwidth (5 MHz) and multi-rate. It has its strengths on:

1. The diversity of the frequency – this is due to the spread concept on a bigger bandwidth hence not adversely affected by noise or fading among other impairments related to frequency. It increases reliability and security.
2. It has no fixed number of users who can access the system simultaneously. The level of noise is directly proportional to the number of users. More users, therefore, lead to more error rates. However, the system will gradually degrade to unacceptable levels of error rate.
3. Soft handoff – this may be viewed as an advantage and a limitation. It’s a limitation because it is more complex than hard handoff. However, it is an advantage since the frequency channel in use does not require to be changed during handoff.
4. Power control – it makes the capacity to increase by reducing the interference. This is the concept of soft capacity. Compared to OFDMA and TDMA, there is no wastage of capacity between frequency channels. Moreover, there is more frequency reuse in CDMA by neighbor cells.

OFDMA

It involves using multiple carrier signals that transmit at different frequencies. The orthogonality function ensures optimal use of the spectrum as well as reduce interference between neighbor subcarriers. It has its strength in that the whole signal does not suffer from frequency selective fading but only some of the subcarriers. However, it has its difficulty in inter-carrier interference.

Dynamic TDMA

It is an improvement of the Time Division Multiple Access which uses both time and frequency slices for the channel transmission. In D-TDMA, there are reservation, voice and data slots in the frame. It helps with allocation of successful voice and data. Unsuccessful voice is given a try in the next frame. For unsuccessful data, use probability for retransmission. The main challenge with this scheme is the difficulty in dynamical change. This is because of the fixed reservation slots.

Findings

LEO satellites are found at a lower distance to the ground. It requires less power for transmission. Practical examples of LEO include the GLOBALSTAR LEO satellite system and the IRIDIUM LEO satellite system. The former uses CDMA which proved to be better than the later which implemented FDMA and TDMA. Some of the issues to look out for in LEO satellites include:

1. The Doppler Shift - the frequency of the signal changes as the shift increases
2. Atmospheric drag – it gradually leads to orbital deterioration.
3. Distance – this has to do with free space loss. There is greater loss with increase in frequency or reduced wavelength.

They have their strengths on lower propagation delays and stronger signal for similar amount of transmission power. The best media access scheme must, therefore, be able to withstand these limitations as well as propagate the strengths. From the discussion, Dynamic TDMA would require precise control on its functionality. It depends on available slots for reservation, voice and data. This requires use of guards and accurate synchronization and also timing to function effectively. The atmospheric drag would then make it more difficult or impossible to make accurate estimations. OFDMA will require synchronization of frequency in the subcarriers available. Change of frequency by the Doppler shift would, therefore, greatly affect its function. The distance could cause inter-carrier interference as well as other impairments on frequency such as intermodulation noise. CDMA relies on code for channel transmission. The main control would be the power of the various carriers. This is to reduce interference because different codes may have non-perfect orthogonality. The idea of soft handoff would perfectly deal with any change in the frequency of the signal. The concept of graceful degradation would help in addressing the atmospheric drag issue. Diversity of frequency easily overcomes any impairments on frequency such as noise. The power control ultimately makes it the best performing scheme for communications with LEO satellites.