Introduction to Microservice Architecture Using SpringBoot - Part 1

1. What are the multiple access techniques?

In wireless communication systems, it is often desirable to allow the subscriber to send information simultaneously from the mobile station to the base station while receiving information from the base station to the mobile station.

A cellular system divides any given area into cells where a mobile unit in each cell communicates with a base station. The main aim in the cellular system design is to be able to increase the capacity of the channel, i.e., to handle as many calls as possible in a given bandwidth with a sufficient level of quality of service.

There are several different ways to allow access to the channel. These includes mainly the following-

1. Frequency division multiple-access (FDMA)

2. Time division multiple-access (TDMA) 3. Code division multiple-access (CDMA)

4. Space division multiple access (SDMA)

2. Why CDMA is better than TDMA and FDMA?

The CDMA doesn't split the original RF channel into two or more time slots or sub-channels but assigns a unique code to each of them. Unlike the FDMA method, CDMA has the very same frequency of transmission in every slot. Also, unlike the TDMA method, CDMA transmits individual slots simultaneously. In CDMA every user uses the full available spectrum instead of getting allotted by separate frequency.CDMA is much recommended for voice and data communications.

While multiple codes occupy the same channel in CDMA, the users having same code can communicate with each other. CDMA offers more air-space capacity than TDMA. The hands-off between base stations is very well handled by CDMA.

3. What is BPSK modulation scheme?

Binary Phase-shift keying (BPSK) is a digital modulation scheme that conveys data by changing, or modulating, two different phases of a reference signal (the carrier wave). The constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption.

They are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the complex numbers they represent will be the same and thus so will the amplitudes needed for the cosine and sine waves.

4. What is difference between BPSK and QPSK?

Quadrature phase shift keying (QPSK) and binary phase shift keying (BPSK) modulators are used to change the amplitude, frequency, and/or phase of a carrier signal in order to transmit information. QPSK devices modulate input signals by 0°, 90°, 180°, and 270° phase shifts. BPSK devices modulate input signals by 0° and 180° phase shifts.

Both QPSK modulators and BPSK modulators are used in conjunction with demodulators that extract information from the modulated, transmitted signal. Some QPSK modulators and BPSK modulators include an integral dielectric resonator oscillator. Others are suitable for military or wireless applications. QPSK modulators and BPSK modulators with root raised cosine (RRC) and Butterworth filters are also available.

5. What are the features of FDMA and TDMA?

Frequency Division Multiple Access (FDMA)

FDMA is the basic technology for advanced mobile phone services. The features of FDMA are as follows.

1. FDMA allots a different sub-band of frequency to each different user to access the network. 

2. If FDMA is not in use, the channel is left idle instead of allotting to the other users.

3. FDMA is implemented in Narrowband systems and it is less complex than TDMA.

4. Tight filtering is done here to reduce adjacent channel interference.

5. The base station BS and mobile station MS. transmit and receive simultaneously and continuously in FDMA.

Time Division Multiple Access (TDMA)

In the cases where continuous transmission is not required, there TDMA is used instead of FDMA. The features of TDMA include the following.

1. TDMA shares a single carrier frequency with several users where each users makes use of non- overlapping time slots.

2. Data transmission in TDMA is not continuous, but occurs in bursts. Hence handsoff process is simpler.

3. TDMA uses different time slots for transmission and reception thus duplexers are not required.

4. TDMA has an advantage that is possible to allocate different numbers of time slots per frame to different users.

5. Bandwidth can be supplied on demand to different users by concatenating or reassigning time slot based on priority.

6. What are the features of CDMA and SDMA?

Code Division Multiple Access (CDMA)

Code division multiple access technique is an example of multiple access where several transmitters use a single channel to send information simultaneously. Its features are as follows.

1. In CDMA every user uses the full available spectrum instead of getting allotted by separate frequency.

2. CDMA is much recommended for voice and data communications.

3. While multiple codes occupy the same channel in CDMA, the users having same code can communicate with each other.

4. CDMA offers more air-space capacity than TDMA.

5. The hands-off between base stations is very well handled by CDMA.

Space Division Multiple Access (SDMA)

Space division multiple access or spatial division multiple access is a technique which is MIMO (multiple- input multiple-output) architecture and used mostly in wireless and satellite communication. It has the following features.

1. All users can communicate at the same time using the same channel.

2. SDMA is completely free from interference.

3. A single satellite can communicate with more satellite receivers of the same frequency.

4. The directional spot-beam antennas are used and hence the base station in SDMA, can track a moving user.

5. Controls the radiated energy for each user in space.

7. Explain the modulation of GMSK.

There are two main ways in which GMSK modulation can be generated. The most obvious way is to filter the modulating signal using a Gaussian filter and then apply this to a frequency modulator where the modulation index is set to 0.5. This method is very simple and straightforward but it has the drawback that the modulation index must exactly equal 0.5. In practice this analogue method is not suitable because component tolerances drift and cannot be set exactly.

A second method is more widely used. Here what is known as a quadrature modulator is used. The term quadrature means that the phase of a signal is in quadrature or 90 degrees to another one. The quadrature modulator uses one signal that is said to be in-phase and another that is in quadrature to this. In view of the in-phase and quadrature elements this type of modulator is often said to be an I-Q modulator. Using this type of modulator the modulation index can be maintained at exactly 0.5 without the need for any settings or adjustments. This makes it much easier to use, and capable of providing the required level of performance without the need for adjustments. For demodulation the technique can be used in reverse.

GMSK, Gaussian Minimum Shift Keying proved to be a particularly successful form of modulation for GSM and some other radio communications systems. Its combination of spectral efficiency and capability to enable efficient power amplifier operation enabled it to be an excellent choice for radio communications systems of the time.

8. Explain multicarrier modulation?

Multicarrier modulation, MCM is a technique for transmitting data by sending the data over multiple carriers which are normally close spaced. Multicarrier modulation has several advantages including resilience to interference, resilience to narrow band fading and multipath effects.

As a result, multicarrier modulation techniques are widely used for data transmission as it is able to provide an effective signal waveform which is spectrally efficient and resilient to the real world environment. Multicarrier modulation operates by dividing the data stream to be transmitted into a number of lower data rate data streams.

Each of the lower data rate streams is then used to modulate an individual carrier. When the overall transmission is received, the receiver has to then re- assembles the overall data stream from those received on the individual carriers. It is possible to use a variety of different techniques for multicarrier transmissions. Each form of MCM has its own advantages and can be sued in different applications.

There are many forms of multicarrier modulation techniques that are in use of being investigated for future use. Some of the more widely known schemes are summarised below.

1. Orthogonal frequency division multiplexing, OFDM: OFDM is possibly the most widely used form of multicarrier modulation. It uses multiple closely spaced carriers and as a result of their orthogonality, mutual interference between them is avoide

2. Generalised Frequency Division Multiplexing, GFDM: GFDM is a multicarrier modulation scheme that uses closed spaced non-orthogonal carriers and provides flexible pulse shaping. It is therefore attractive for various applications such as machine to machine communication

3. Filter Bank Multi Carrier, FBMC: FBMC is a form of multicarrier modulation scheme that uses a specialised pulse shaping filter known as an isotropic orthogonal transform algorithm, IOTA within the digital signal processing for the system. This scheme provides good time and frequency localisation properties and this ensures that inter-symbol interference and inter- carrier interference are avoided without the use of cyclic prefix required for OFDM based systems.

4. SEFDM: Spectrally efficient frequency division multiplex uses multiple carriers in the same way as OFDM, but they are spaced closer than OFDM. However it is still possible to recover the data, although with a slight power penalty.