Ready to face interview for Gsm RF Engineer job? Do not worry, we are here to help you with job interview preparation. If you are preparing for Ionic Gsm RF Engineer interview and not sure which questions are likely asked in interview, we suggest you to go through Wisdomjobs Gsm RF Engineer interview questions and answers page to crack your job interview. GSM is Global System for Mobile communications and is a protocol for cellular networks. RF Engineer is one who optimizes the radio frequencies. It needs experience to work as RF Engineer. There are many jobs related to this across the globe. Below is the list of frequently asked Gsm RF Engineer interview questions and answers which gets you ready to face the interviews:
Question 1. What Are The Three Services Offered By Gsm? Explain Each Of Them Briefly?
Answer :
GSM services are categorized in three teleservices, bearer, and supplementary services.
Ex: call forwarding, call waiting.
Question 2. What Is Ciphering? Why Do We Need It? Name The Algorithm(s) Used In It?
Answer :
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.
Question 3. What Is Authentication? Why Do We Need It? Name The Algorithm(s) Used In It?
Answer :
Authentication is the process to confirm that this user belong to the network. The Authentication algorithm in GSM is called the A8 algorithm.
Question 4. What Is Equalization? Why Do We Need It?
Answer :
Adaptive equalization is a solution specifically designed to counteract the problem of time dispersion.
It works as follows:
Question 5. Explain Speech Coding?
Answer :
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 wire line PSTN phones.
It works as follow: 8KHZ of sampling is performed first, and 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.
Question 6. What Do You Mean By Frequency Re-use?
Answer :
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.
Question 7. Name The Interfaces Between A) Bts And Ms B) Bts And Bsc C) Bss And Msc?
Answer :
Answer :
Mobile Allocation (all frequency available for frequency hopping in the cell).
Answer :
Mobile Allocation Index offset. (The offset from the initial point in an array of frequency).
Answer :
Question 11. What Is Cycling Frequency Hopping?
Answer :
The hopping sequence occurs in a uniform manner. (Not random).
Question 12. What Is Hsn? How Do We Apply It?
Answer :
Hopping sequence number, if its value (0) cycle hopping. Otherwise it is random hopping.
Question 13. What Is Dtx? Why It Is Used?
Answer :
Discontinuous Transmission (DTX) is a mechanism which allows the radio transmitter to be switched off most of the time during speech pauses.
DTX may be applied independently to each direction, so that the control of DTX must take into account two components:
DTX can be enabled or disabled for the uplink and/or downlink mode on a per-cell basis.
Reasons for DTX When DTX is applied, actual transmission on the radio path is reduced. This will cause a decrease of the interference level in co-channel cells (using the same frequency). Another advantage will appear when using DTX in the uplink mode: it saves battery power for the mobile station. However, a disadvantage of the DTX mode is that it slightly deteriorates the quality of transmission. Note that transmitting in DTX mode does not save timeslots on the air-interface.
Question 14. What Is Drx? Why Do We Need It?
Answer :
Discontinuous reception is method used to conserve power at the MS. The paging channel, used by the BTS to signal an incoming call, is structured into sub channels. Each MS is assigned one of these sub-channels and needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into “sleep mode”, when almost no power is used.
Question 15. What Is The Gross Data Rate Of Gsm?
Answer :
270kbps.
Question 16. What Is Erlangs? What Is Meant By Gos?
Answer :
Traffic refers to the usage of channels and is usually thought of as the holding time per time unit (or the number of “call hours” per hour) for one or several circuits (trunks or channels). Traffic is measured in Erlangs (E), for example, if one subscriber is continuously on the telephone, this would generate one call hour per hour or 1 E of traffic. The traffic one cell can carry depends on the number of traffic channels available and the amount of congestion that is acceptable (to both the customer and the provider), the so-called Grade of Service (GoS).
Answer :
G900 is better. Due to path loss formula as frequency is increased, the losses which the signal will encounter will be more.
Question 18. What Is Ta? Why Do We Need Ta?
Answer :
Time advance (alignment) process The RF communication experiences a propagation delay over the distance between the BTS and the MS. In order to synchronize the MS to the BTS, a timing advance is used to align the time slots arriving at the BTS receiver:
Question 19. What Is Location Update? Why Do We Need Location Update?
Answer :
A Location Area (LA) is defined as a group of cells. Within the network, a subscriber’s location is known by the LA which they are in. The identity of the LA in which an MS is currently located is stored in the VLR. When an MS crosses a boundary from a cell belonging to one LA into a cell belonging to another LA, it must report its new location to the network1. When an MS crosses a cell boundary within an LA, it does need to report its new location to the network. When there is call for an MS, a paging message is broadcast within all cells belonging to an LA.
Question 20. What Is Meant By Imsi, Tmsi, Imei And Ms-isdn? Why They Are Needed?
Answer :
IMSI = International Mobile Subscriber Identity
TMSI = Temporary Mobile Subscriber Identity
IMEI = International Mobile Equipment Identity
MS-ISDN =Mobile Station ISDN Number
Answer :
Absolute Radio-Frequency Channel Number (ARFCN) is a code that specifies a pair of physical radio carriers and channels used for transmission and reception on the Um interface, one for the uplink signal and one for the downlink signal.
Question 22. Explain Power Control?
Answer :
Downlink measurements The mobile station measures and reports the following measurements to the BSC regarding the performance of the downlink:
Uplink measurements The BTS measures and reports the following measurements to the BSC regarding the performance of the uplink:
Periodically measuring the mobile station measures periodically the performance of the downlink, and sends the measurements in the SACCH (Slow Associated Control Channel) via the serving BTS to the BSC every SACCH multi-frame. This corresponds to the transmission of data every 104 TDMA frames or 480 Ms. The base station measures the quality of the uplink. Also, it transfers the measurements in the SACCH to the BSC every 480 ms.
Signal strength when the BSC notices that the signal strength of a particular radio link measured on the uplink becomes below the lower pre-defined threshold because the mobile station moves away from the BTS, it sends a Power Control command to the mobile station to increase its transmit power (MS_TXPWR) by a pre-defined step (typically 2 dB).
Question 23. What Is The Difference Between Fdd And Tdd?
Answer :
Question 24. What Is An Extended Cell? How Does It Impact The System? Channels And Tdma Structure?
Answer :
The current limitation on the range of a GSM cell site to 35km is mandated by the duration of the standard timeslots defined in the GSM specification. The maximum distance is given by the maximum time that the signal from the mobile/BTS needs to reach the receiver of the mobile/BTS on time to be successfully heard. At the air interface the delay between the transmission of the downlink (BTS) and the uplink (mobile) has an offset of 3 timeslots. Until now the mobile station has used timing advance to compensate for the propagation delay as the distance to the BTS changes.
This timing advance is defined in the GSM specification as 64 bits, which gives the theoretical maximum BTS/mobile separation as 35km.
With Extended Range Cell Feature, the BTS is able to receive the uplink signal in two adjacent timeslots instead of one. When the mobile station reaches its maximum timing advance, i.e. maximum range, the BTS expands its hearing window with an internal timing advance that gives the necessary time for the mobile to be heard by the BTS even from the extended distance. This extra advance is the duration of a single timeslot, a 156 bit period.
Answer :
Multiple Access schemes allow for many users to access the network.
Question 26. Which Channel(s) Is Used For Sms?
Answer :
Either SDCCH or SACCH.
Question 27. Which Channel Is Used By Ms To Request Access To The Network?
Answer :
RACH = random access channel.
Answer :
AGCH (Access Grant Channel) assigns a signaling channel (SDCCH) to the MS.
Question 29. Why Do We Need Sdcch?
Answer :
The BTS switches to the assigned SDCCH. The call set-up procedure is performed in idle mode. The BSC assigns a TCH. (SDCCH is also used to transmit text messages).
Answer :
Each time slot on a TDMA frame is called a physical channel. Therefore, there are 8 physical channels per carrier frequency in GSM.
Physical channels can be used to transmit speech, data or signaling information. A physical channel may carry different messages, depending on the information which is to be sent. These messages are called logical channels. For example, on one of the physical channels used for traffic, the traffic itself is transmitted using a Traffic Channel (TCH) message, while a handover instruction is transmitted using a Fast Associated Control Channel (SACCH) message.
Question 31. Why Do We Need Fcch, Sch And Bcch?
Answer :
FCCH: Identifies BCCH carrier by the carrier frequency and synchronizes with the frequency.
Synchronization Channel (SCH): Transmits information about the TDMA frame structure in a cell (e.g. frame number) and the BTS identity (Base Station Identity Code (BSIC)).
BCCH: Broadcasts some general cell information such as Location Area Identity (LAI), maximum output power allowed in the cell and the identity of BCCH carriers for neighboring cells.
Question 32. Why Do We Need Sacch?
Answer :
Instructs the MS the transmitting power to use and gives instructions on timing advance.
Question 33. What Is The Purpose Of Pch And Cbch?
Answer :
Paging Channel (PCH): Transmits a paging message to indicate an incoming call or short message. The paging message contains the identity number of the mobile subscriber that the network wishes to contact.
Cell Broadcast Channel (CBCH): BS uses this logical channel to transmit short message service cell broadcast.
Answer :
Traffic Multiframe Structures - The 26 traffic Multiframe structure is used to send information on the traffic channel. The 26 traffic Multiframe structure is used to combine user data (traffic), slow control signaling (SACCH), and idle time period.
The 12th frame (no. 13) in the 26-frame traffic channel Multiframe is used by the Slow Associated Control Channel (SACCH) which carries link control information to and from the MS–BTS. Each timeslot in a cell allocated to traffic channel usage will follow this format, that is, 12 bursts of traffic, 1 burst of SACCH, 12 bursts of traffic and 1 idle.
Idle Bursts: The idle time period allows a mobile device to perform other necessary operations such as monitoring the radio signal strength level of a beacon channel from other cells. The time interval of a 26 frame traffic Multiframe is 6 blocks of speech coder data (120 msec).
Question 35. How Is A Facch Formed? When Is A Facch Used?
Answer :
Fast Associated Control Channel (FACCH): Transmits handover information.
Question 36. What Do You Mean By Eirp?
Answer :
Effective isotropic radiated power (EIRP) is the amount of power that a theoretical isotropic antenna (that evenly distributes power in all directions) would emit to produce the peak power density observed in the direction of maximum antenna gain.
EIRP can take into account the losses in transmission line and connectors and includes the gain of the antenna. The EIRP is often stated in terms of decibels over a reference power emitted by an isotropic radiator with equivalent signal strength. The EIRP allows comparisons between different emitters regardless of type, size or form.
Question 37. What Is Polarization? What Are The Types Of Polarization?
Answer :
The polarization indicates the plane in which the wave is vibrating. The polarization plane is taken to be that of the electric component.
Vertical and horizontal are the simplest forms of polarization, and they both fall into a category known as linear polarization. However, it is also possible to use circular polarization. This has a number of benefits in areas such as satellite applications, where it helps to overcome the effects of propagation anomalies, ground reflections and the spin that occur on many satellites. Circular polarization is a little more difficult to visualize than linear polarization; however, it can be imagined by visualizing a signal propagation from an antenna that is rotating. Another form of polarization is known as elliptical polarization.
Answer :
Question 39. What Is Rayleigh Fading?
Answer :
Rayleigh fading is a reasonable model when there are many objects in the environment that scatter the radio signal before it arrives at the receiver.
Rayleigh fading models assume that the magnitude of a signal that has passed through such a communications channel will vary randomly, or fade, according to a Rayleigh distribution the radial component of the sum of two uncorrelated Gaussian random variables.
Rayleigh fading is most applicable when there is no dominant propagation along a line of sight between the transmitter and receiver. If there is a dominant line of sight, Rician fading may be more applicable.
Question 40. What Is Multi Path Fading?
Answer :
Multipath fading is receiving multiple copies of the signal at receiver due to reflections. The copies reach the receiver with different phases, so summation either constructive or destructive. This affects the quality of received signal in terms of BER.
Question 41. What Are The Different Types Of Diversity?
Answer :
Question 42. Explain Various Types Of Antenna Diversity?
Answer :
Antenna diversity increases the received signal strength by taking advantage of the natural properties of radio waves.
There are two primary diversity methods: space diversity and polarization diversity.
Space Diversity: Increased received signal strength at the BTS may be achieved by mounting two receiver antennae instead of one. If the two Rx antennae are physically separated, the probability that both of them are affected by a deep fading dip at the same time is low. At 900 MHz, it is possible to gain about 3 dB with a distance of five to six meters between the antennae. At 1800 MHz the distance can be shortened because of its decreased wavelength.
By choosing the best of each signal, the impact of fading can be reduced. Space diversity offers slightly better antenna gain than polarization diversity, but requires more space.
Polarization Diversity: With polarization diversity the two space diversity antennae are replaced by one dual polarized antenna. This antenna has normal size but contains two differently polarized antenna arrays. The most common types are vertical/horizontal arrays and arrays in 45 degree slant orientation. The two arrays are connected to the respective Rx branches in the BTS. The two arrays can also be used as combined Tx/Rx antennas. For most applications, the difference between the diversity gain for space diversity and polarization diversity is negligible, but polarization diversity reduces the space required for antenna.
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