20 GSM RF INTERVIEW QUESTIONS and Answers

GSM RF INTERVIEW QUESTIONS

1. What are the three services offered by GSM? Explain each of them briefly?

GSM services are categorized in three teleservices, bearer, and supplementary services.

A. Teleservices (communicate with other subscribers).

B. Bearer service (provides the underlaying network capacity necessary for transmission to occur between two points in the same or different networks). The bearer services describe what the network can offer (e.g. speech, data and fax).

C. Supplementary service is optional which subscriber can subscribe for free. Ex: call forwarding, call waiting,

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2. Which uplink/downlink spectrum is allocated to GSM-900?

3. Which uplink/downlink spectrum is allocated to DCS-1800?

4. How many carrier frequencies are there in GSM-900/DCS-1800? How much is the separation between the carrier frequencies?

5. What is Ciphering? Why do we need it? Name the algorithm(s) used in it?

The purpose of ciphering is to encode the burst so that it cannot be interpreted by any device other than the intended receiver. The ciphering algorithm in GSM is called the A5 algorithm. It does not add bits to the burst, meaning that the input and output to the ciphering process is the same as the input.

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6. What is Authentication? Why do we need it? Name the algorithm(s) used in it?

Authentication is the process to confirm that this user belong to the network. The Authentication algorithm in GSM is called the A8 algorithm.

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7. What is equalization? Why do we need it?

Adaptive equalization is a solution specifically designed to counteract the problem of time dispersion. It works as follows:

1. A set of predefined known bit patterns exist, known as training sequences. These are known to the BTS and the MS (programmed at manufacture). The BTS instructs the MS to include one of these in its transmissions to the BTS.

2. The MS includes the training sequence (shown in the figure as “S”) in its transmissions to the BTS. However, due to the problems over the radio path, some bits may be distorted.

3. The BTS receives the transmission from the MS and examines the training sequence within it. The BTS compares the received training sequence with the training sequence which it had instructed the MS to use. If there are differences between the two, it can be assumed that the problems in the radio path affected these bits must have had a similar affect on the non-training sequence bits.

4. The BTS begins a process in which it uses its knowledge of what happened the training sequence to correct the other bits of the transmission.

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8. What is interleaving? Why do we need it?

The aim of interleaving is to distribute subblocks of data obtained by channel coding in such a way that one data block is distributed over several TDMA frames.

The channel coder provides 456 bits for every 20 ms of speech. These are interleaved, forming eight blocks of 57 bits each, as shown in the figure below.

In any one burst, there is space for two of these blocks. Thus, if one burst transmission is lost, there is a 25% BER for the entire 20 ms of speech (2/8 = 25%).

As shown in the diagram, the front and end 3 tail bits delimit the burst; the 26 bits are training sequence bits; and the bit at both sides of the training bits are used as “bit stealing flags”.

10. Explain Speech Coding?

Speech Coding:

Instead of using 13 bits per sample as in A/D conversion, GSM speech coding uses 260 bits. This calculates as 50 x 260 = 13 kbits/s. This provides a speech quality which is acceptable for mobile telephony and comparable with wireline PSTN phones.

The voice compression coding technique is widely used in the modern digital communication systems. In this technique, a voice coder is used to set up a model to simulate the voice and noise produced by human vocal organs. The parameters to form the model will be transmitted through the TCH channels.

The voice coder is based on the residual excited linear prediction (REIP) coder. Moreover, the long term predictor (LTP) is used to enhance the compression effect. LTP can make the coding of residual data more advantageous by removing the vowels from the voice. With 20ms as the unit, the voice coder outputs 260bits after compressed coding. Therefore, the code rate is 13kbps. According to the different

classes of the importance of the information, the output bits can be classified into three categories: 50 very important bits, 132 important bits and 78 ordinary bits.

Comparing with the traditional PCM line on which the voice is coded directly and transmitted (64kbps), the 13kbps voice rate of the GSM system is much lower. The more advanced voice coder in the future can further reduce the rate to 6.5kbps (half-rate voice coding).

The coding mode is called Regular Pulse Excited-Long Term Prediction (RPE-LTP). It works as follow: 8KHZ of sampling is performed first, then divided into frames with 20ms; every frame has 4 sub-frames; the duration of every sub-frame is 5ms; and the pure bit rate is 13kbit/s.

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11. What is channel coding?

The data to be transmitted over the interface must be specially protected against corruption due to the effects of fading and co-channel interference. Additional check bits are generated for this purpose and permit the detection of transmission errors and to a certain extent the reconstruction of the original data.

The channel coding process increases the bit rate from 13 Kbit/s to 22.8 Kbit/s through adding protection to the Class-I bits. Convolution coding and addition of a CRC (Cyclic Redundancy Check) result in 456 bits coming out during the 20 ms speech data block period (= 22.8 Kbit/s).

Channel coding in GSM uses the 260 bits from speech coding as an input and outputs 456 encoded bits.

The 260 bits are split according to their relative importance:

· Block 1: 50 very important bits

· Block 2: 132 important bits and

· Block 3: 78 not so important bits

Channel Codding

12. What do you mean by Frequency re-use?

An operator purchases some frequency band. This band is divided into channels (200 KHz). To cover the whole country or city, the operator tend to reuse the channels after some distance "D" which at this distance the interference can be under control.

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13. What is Cell Splitting?

Unfortunately, the design requires cells of different sizes; one radius for urban, another for suburban, etc. There is a method for merging different sized grids, called cell splitting. Cell splitting provides a mathematical transition from one morphological region to another.

Splitting is accomplished by centering the next smaller grid on the corner or the side of the next larger hexagon. Figure below demonstrates how the side-split and corner-split are accomplished. The cell splits also help the engineer maintain the C/I ratio required by different technologies.