2G Interview Questions – ------------------------------------------------------------Q1.Different logical channels in 2G. Control Channels-BCCH,FACCH,SDCCH,SACCH,RACCH,FCCH,SCH,PCCH,AGCH Traffic Channels-TCHF/TCHH
Q2.What is Gain of GSM antenna. Antenna is a passive device , so no gain. The gain used for Antennas is relative over isotropic source. Like 17dBi – directional gain.
Q3.How to reduce EFM radiation. Tuning power control parameters of cell and Physical Optimization by decreasing height of antenna, changing low gain antennas etc.
Q4.Coding Schemes used in GSM- GMSK & QPSK
Q5.What is frequency reuse factor. Frequency reuse factor denotes the reuse of certain frequency after count of cells. Like 3/12 reuse means a frequency used in one cell can be used again in after 12th that is 13th cell. This helps in minimizing C/I of network.
Q6.How to Improve network network Accessibility. Network accessibility can improved by minimizing by improving congestion (RACH, SDCCH and TCH).
Q7.How to resolve indoor customer complaints. Indoor complaints can be resolved by installing indoor boosters, optimizing neighboring cells, using high gain antennas and planning new sites and IBS.
Q8.New Site planning criteria. Coverage and Capacity site planning.
Q9.Knowledge of 3G and 4G. 3G or UMTS is evolution over 2G.Data rates are 2Mbps in UL and 14 Mbps in DL. 4G is LTE, mobile broadband speeds can be achieved.
Q10.High Gain antenna uses. On highways mainly.
Q11.What is Combiner. Hardware which combines two frequencies in one output. Two trxs can be combined.
Q12. Benefits of TMA
Provides a gain of 12 dB Improves System sensitivity Reduces noise figure of equipment Increase mobile battery
Q13. BSIC NCC+ BCC Used for BTS identification for initial network access and Neighbor identification in dedicated mode.
Q15.What is Mobile Originated Call setup basic steps.
RACCH request Access grant SDCCH Allocate Authentication Ciphering Alerting Call proceeding Connect
Q16. Handover Reasons
Bad Coverage Bad Quality High Distance High Power Traffic based.
Q17. What is AMR Adaptive Multi Rate Coding States- Half Rate AMR & Full Rate AMR Code- 12.2 ,10.2,7.95,7.40,6.70,5.90,5.15, 4.7 Kbps
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Decoding the UMTS/UTRAN Specifications The purpose of this wiki area is to provide a clear and concise presentation of the information given in the 3GPP UMTS/UTRAN specifications, specifically on the "Uu" interface. Because
UMTS/UTRAN inherit many concepts and subprotocols from GSM and GPRS (" Um"), some understanding of those technologies is probably a prerequisite for understanding this wiki.
General Comments on the Wiki One of the problem with UMTS complexity is that the standard supports a lot of deployment variations. For simplicity, this wiki should probably focus on the most commonly deployed style of UMTS/UTRAN worldwide: FDD radio interface and GSM-MAP core network. Please do not just copy-paste large sections of text from the specifications themselves into the wiki. First, it's a copyright violation. Second, it defeats the purpose of the wiki. So if you can't/won't rewrite something to make it more understandable, just make a reference to the specification document and section/paragraph, like this: "3GPP 25.331 10.5.1". Small quotations are good, but please put them in italics like this and note the document and paragraph from which the text was quoted. To edit this wiki, you can login as user "guest" with the password "guestpass". If you are a regular editor of this wiki, please request a personal account from Kurtis (log in to see contact info) so that your changes can be tracked. If you have a specific question about a specific part of the specification, please put the question into the wiki using italicized text like you see here. You can also subscribe to the UMTS discussion list here.
Other Web Resources for Understanding UMTS
A good tutorial on UMTS.
General Comments on UMTS Like GSM/GPRS, UMTS defines logical and physical channels, but there is a greater variety of physical channel types and there's is a new intermediate sublayer, the transport layer, between the logical and physical channel. The existence of transport channels permits the grouping of logical channels with similar bearer performance (e.g., error rate) requirements.
UMTS Channels Logical Channels
BCCH (Broadcast Control Channel) - Downlink only. Broadcast. Functionally similar to GSM BCCH. Carries a cycle of repeating System Information Blocks (SIBs).
PCCH (Paging Control Channel) - Downlink only. Unicast. Functionally similar to GSM Paging Channel (PCH). Carries paging requests. CCCH (Common Control Channel) - Downlink/uplink. Unicast. Functionally similar to GSM Access Grant Channel (AGCH) and Random Access Channel (RACH). Primarily carries messages related to RRC connection setup. DCCH (Dedicated Control Channel) - Downlink/uplink. Functionally similar to GSM SDCCH, SACCH, or FACCH, depending on the context. Carries user-specific control plane traffic. DTCH (Dedicated Traffic Channel) - Downlink/uplink. Functionally similar to the GSM PDTCH or TCH. Carries user plane traffic.
Transport Channels
BCH (Broadcast Channel) - Downlink only. Carries the BCCH. PCH (Paging Channel) - Downlink only. Carries the PCCH. FACH (Forward Access Channel) - Downlink only. Carries the downlink CCCH. May carry one or more downlink DCCH and/or DTCH depending on the context. May carry the BCCH. DCH (Dedicated Channel) - Downlink/uplink. Carries the DCCH and/or the DTCH. RACH (Random Access Channel) - Uplink only. Carries the uplink CCCH. May carry the uplink DCCH and/or DTCH depending on the context.
Physical Channels The physical channels use different slot formats, documented in 3GPP 25.211 Section 5. The mapping of transport channels onto physical channels is defined in 3GPP 25.211 Section 6.
P-CCPCH (Primary Common Control Physical Channel) - Carries the BCH. S-CCPCH (Secondary Control Control Physical Channel) - Carries one or more FACHs and the PCH. PICH (Paging Indication Channel) - Carries a bit mask of reduced paging information. Its existence allows the handset to sleep more of the time thereby conserving battery. Does not carry any higher-layer data. P-SCH (Primary Synchronization Channel) - Supports the first stage of cell synchronization: word sync. S-SCH (Secondary Synchronization Channel) - Supports the second stage of cell synchronization: frame sync. PRACH (Physical Random Access Channel) - Carries the RACH. AICH (Access Indication Channel) - Indicates the base station's reception of a PRACH preamble transmission, which is the go-ahead signal for the handset to transmit the PRACH burst. Does not carry any higher-layer data. The UMTS access procedure is more complex to insure proper initialization of uplink power control. P-CPICH (Primary Common Pilot Channel) - Channel state reference and physical cell identifier. Does not carry any higher-layer data. Similar in purpose to the GSM FCCH/SCH logical channels.
DPDCH (Dedicated Physical Data Channel) - Carries one or more DCH. DPCCH (Dedicated Physical Control Channel) - Carries physical link control information such as power control bits and the transport format combination indicator. Does not carry any higher-layer data.
UMTS Layers Layer 1 - PHY The UMTS PHY is based on WCDMA. Key features of CDMA:
A single company has a strangle-hold on the IP. In the real world, with Doppler and multipath and varying propagation delays, nothing is really orthogonal, so you usually get well less than half the theoretical capacity. It requires you to control uplink power to within about 3 dB to prevent the near-far effect. It requires you to process large swaths of bandwidth continuously even for low-rate signals. The main noise source that degrades receiver performance is other users, not thermal noise, so per-user performance varies wildly as the traffic mix changes. Since the UMTS CDMA signal has a high crest factor (similar to Gaussian noise), the amplifiers require a lot of extra headroom, making them much less efficient than the constant modulus amplifiers used in GSM/GPRS.
Most UMTS deployments utilize frequency-division duplexing (FDD). Unlike GSM, in which transmit and receive bursts are separated in time, UMTS-FDD requires simultaneous transmission and reception. Thus, instead of a simple switch, the RF front-end in a UMTS handset utilizes a duplexer. Small duplexers are typically quite lossy, requiring the (already inefficient) PAs in handsets to produce even more power.
UMTS Frequency Bands The primary UMTS operating frequency band is the IMT-2000 band at 2100 MHz. This band is used for UMTS throughout most of the world including Asia, Europe, Australasia, Africa, and parts of South America. In the United States, the IMT-2000 band was not (entirely) available at the time UMTS was being rolled out. Therefore, the 1900 MHz "PCS" band and the 850 MHz "cellular" bands are used for UMTS in the U.S. and countries that typically follow U.S. frequency band planning in North America and parts of South America. The 3GPP technical specifications permit UMTS to be operated in numerous, often regionspecific frequency bands - a total of 14 as of R8 of the specs. As traffic has grown, UMTS has been deployed in in more and more of these operating bands - a process colloquially referred to as spectrum "re-farming" - including 900 MHz (GSM), 1800 MHz (DCS), and 1700/2100 MHz (U.S. AWS).
The 3GPP technical specifications support the use of virtually any downlink/uplink frequency pairing in FDD mode, though it is extremely rare for networks to utilize non-standard operating bands and duplex spacings.
FDD Radiomodem This is everything from the formatted radio frame data bits to the antenna, including:
spreading scrambling pulse shaping radio frequency tuning and power control
Forward Error Correction From 3GPP 25.212 4.2. This is the encoding/transmission path between a transport block and a radio frame:
transport block concatenation parity word append code block segmentation convolutional or turbo encoding rate-matching (puncturing or repeating channel bits in coded blocks to match input and output rates) interleaving segmentation of coded blocks into radio frames more interleaving
Special Cases
PRACH processing
Layer 2 Layer 1-2 Interface Data structures exchanged between L1 and L2 are called transport blocks. UMTS defines concatenation and rate-matching mechanisms in L1 that allows great flexibility in transport block size on most L1-L2 interfaces. However, there is a maximum code block size, defined in 3GPP 25.212 4.2.2.2 that determines the maximum available data rates for given channel types. See DecodingUMTS/FEC for more details. https://wush.net/trac/rangepublic/wiki/DecodingUMTS/RRC#PRACHProcedure
