3G 1) Explain what is CQI ? What are the general values for CQI ? CQI (Channel Quality Indicator) is the feedback which the system receives from the UE and it mainly indicates the radio condition of the UE. Depending on the CQI values, NodeB scheduler allocates resources to the UE. Higher the CQI, better the network. An average CQI value of about 22 and above, indicates a reasonably good network. CQI values less than 17, indicates a low quality network and optimization is required. 2) What modulations are used in HSDPA ? a. QPSK b. 16 QAM c. 64 QAM 3) What is TTI ? a. Transmission time interval – 2ms : The TTI is related to the size of the data blocks passed from the higher network layers to the radio link layer. To combat errors due to fading and interference on the radio link data is divided at the transmitter into blocks and then the bits within a block are encoded and interleaved. The length of time required to transmit one such block determines the TTI. At the receiver all bits from a given block must be received before they can be deinterleaved and decoded. Having decoded the bits the receiver can estimate the bit error rate (BER). And because the shortest decodable transmission is one TTI the shortest period over which BER can be estimated is also one TTI 4) What is HSDPA code Multiplexing? HSDPA code multiplexing is a complimentary method to time multiplexing that gives the possibility to share the HS-DSCH resource among users in both code and time domain, thereby improving the overall performance in terms of system throughput and number of users served at a given delay HS-SCCH power control reduces the RBS output power needed for the HS-SCCHs and thereby increases the system capacity 5) What are Ec/Io levels and what does it represent? Ec/No signifies the level difference between received pilot signal and the overall noise floor. No is the noise floor, which signifies all the signals (useful and interfering) present at the receiver side.
It represents the quality of the 3G network For example: A value of Ec/No= -8dB tells us that the spread signal is 8 dB below the noise floor Higher the Ec/No value, the better it is….
Ec/No = RSCP/RSSI 6) Name various 3G and HSDPA KPIs a. b. c. d. e. f.
RRC Success rate RAB Success rate ( Packet switched , High speed , Voice , Video) RAB Drop rate ( Packet switched , High Speed , Voice , Video) HS Throughput HS Number of users Data Payload
7) What are the types of handover in 3G ? a. Soft Handover ( Intra-RNC , Inter- RNC ) b. Softer Handover ( HO between different sectors of same site) c. Inter- RAT Handover ( UMTS - > GSM) 8) Explain Pilot channel Ec/Io and Pilot SIR. According to 3GPP Def- Ec/Io = RSCP/RSSI (Ratio of the Received signaling code power to the power spectral density in the band and measurement shall be performed on CPICH) while SIR = RSCP/ISCP (Ratio of the Received signaling code power to Interference signaling code power) ISCP is the non-orthogonal part of RSSI Pilot SIR considers orthogonality and Pilot power is not included as Interference power, So we have to take Orthogonality factor @ into account.@ = 0.4 (Macro cell) and @ = 0.06 (Micro cell) 9) Difference between Layer 3 and Layer 2 messages. a. Layer-3 is messaging is responsible to translate messages related to Logical channels (mapped on transport channels) with respect to signaling, control and user information in a simple transparent directional way (UL/DL), However Layer 2 messaging is addressing all information related to internal transport channels and most of them are non transparent to users, that why first we consider Layer 3 for our analysis 10)
What is pilot pollution and what are its effects?
a. Too many Pilots of similar strength in one location causes the best server to swap rapidly and Active set keeps updating during this
fluctuation. UE cannot sustain the call because of poor Ec/Io ,also UE cannot hand over and it affects to the hand over algorithm, 11) a. b. c. d. e. 12)
Name few features available in P7 SW for Ericsson CS traffic over IP ( IUB over IP) Support for 64 QAM Enhanced Layer 2 Multiple HS PS interactive RAB Combination Number of HS Users per cell What is flexible scheduler and where is it used?
a. Scheduling decides which terminals, the shared transmission should be directed to any given moment ( TTI). Ericsson offers three different scheduling mechanisms: Round Robin, Proportional Fair and Flexible. Flexible scheduler enables different levels of fairness vs throughput b. Scheduling strategy is of fundamental importance when using a shared channel in HSDPA. The HSDPA Flexible Scheduler allows for operator specific trade of system capacity and fairness among users on cell level. 13)
What is dynamic code resource allocation?
a. The operator can configure the number of HS-PDSCH codes (SF=16) that should be reserved for a HSDPA capable cell by setting numHsPdschCodes. The maximum number of HS-PDSCH codes is dependent on license level and parameter maxNumHsPdschCodes; up to 15 codes may be allocated. With a high number of HS-PDSCH codes allocated, the risk for code blocking will however increase and allocating 15 HS-PDSCH codes from the RNC is not advised. The number of HS-PDSCH codes in use for HS-DSCH transmission may also be dynamically adapted by the HSDPA Dynamic Code Allocation feature. With this feature it is possible to borrow codes from the DCH domain when those codes are not used for DCH traffic. 14)
Benefits of HSUPA
a. EUL – Enhanced Uplink or HSUPA provides speed upto 1.4Mbits/s b. Higher Capacity – 50 – 100 % c. Reduced Latency – 50 – 75 ms 15) What are the different optimization tools available in Ericsson OSS ?
a. MRR , NCS , FAS , FOX ( Frequency Optimization Expert) , NCS ( Neighbor Cell Support) , TET 16) Describe briefly GPEH , UETR, CTR, RNO . a. GPEH – General Performance Event Handling b. UETR - enables the operator to record important events and measurements for a selected UE, traveling through a network c. CTR - collects data for a number of UE connections within a certain recording area. The recording area is defined as a specific cell in which the UEs are going to be followed. CTR can record up to a maximum of 16 simultaneous connections. d. RNO – Radio Network Optimization. 17) Describe the requirement of Second Carrier a. If Accessibility , Retain ability and integrity of cell KPIs are affected b. Monitoring the Downlink Transmitted carrier power and relating it to Capacity Management thresholds c. Traffic Volume in total is high per cell d. HS Throughput performance is poor cell-wise and number of users are high e. Monitoring the Downlink channelization codes and relating it to Capacity Management thresholds 18)
What is the strategy followed in deploying second carrier?
a. Single cell or hot-spot b. Cluster wise where the second carrier clusters cover only a part of the area covered by the first carrier. Field experiences have proven that both scenarios can be safely and effectively implemented in live loaded network without degrading performance. 19)
How is traffic steered in second carrier?
a. With the help of qoffset2n parameter traffic can be pushed from f1 to f2 and vice versa b. IF LS – Inter frequency Load sharing
2G 20)
What is C/I and C/A
a. C/I – Carrier to Interference Ratio is the ratio between cochannels reused b. C/A – Carrier to Adjacent Ratio is the ratio between adjacent channels reused. 21)
What is Frequency Hopping and What are the types
Frequency hopping is not timeslot hopping A connection is not transmitted using only one frequency in a cell, but bursts of consecutive TDMA frames of that connection are transmitted using certain frequencies of a specified frequency set which is defined by the MA (Mobile Allocation) list. Without using frequency hopping, the speech quality of a connection may either be good or bad. There are calls in a cell which will suffer under bad speech quality; other connections will have a quite good speech quality. The reason is that: Rayleigh fading (short term fading) is different for different frequencies; the interference level is different on different frequencies. Frequency hopping will average the quality for all connections: For a certain connection, the link quality may now change from burst to burst. Nevertheless, due to interleaving, not only 1 but 8 consecutive bursts as a whole must be successfully decoded. Therefore, even if there are some bad quality bursts in these 8 consecutive bursts a speech frame may be still successfully decoded. Frequency hopping is most effective in case of slow moving or static mobile stations. Different types of hopping :
Baseband hopping In Baseband hopping the call is hopped between TRX and each TRX is having fixed frequency. For freq planning the number of frequencies required has to be equal to the TRX planned. Combiner limitation is not there as such any combiner can be used for this hopping system
Synthesizer hopping In Synthesizer hopping the call remains on the same TRX but the
TRX changes frequencies in each frame based on allocated freq list. For freq planning the number of frequencies required is more than the TRX per cell. Fractional loading is considered for the freq planning. Combiner limitation is there as such this hopping cannot be used with filter combiners. Need hybrid or special combiners. 22)
What is CBCCH and its advantages
CBCCH (Common BCCH) Each band has its own BCCH, this feature enables possibility to combine multi- frequency bands into one cell having one BCCH ( Broadcast common control Channel). Normally lower frequency bands are chosen for more coverage area. Advantages: Combining signaling channels Combine Traffic channels and thereby increasing Trunking gain Tighter re-use of Non-BCCH frequency band Quality improvement due to decreased number of handovers
23) What are the ways to steer traffic between different bands within GSM One way to steer traffic between different bands in Ericsson network would be by changing the threshold values for path loss (LOL) and Hysteresis (LOLHYST) and TAOL (Timing advance Threshold) and Hysteresis (TAOLHYST). This way traffic can be steered between Under laid (UL – 900 Band) and Overlaid (1800 Band) and vice versa
24)
What are the KPIs for GPRS/Edge
Few of the GPRS/Edge KPIs are Total Data Volume ( Downlink and Uplink) GPRS/Edge IP/LLC Throughput ( Downlink and Uplink) Accessibility Total established TBF Retainability – TBF Drop rate
25)
PDCH Failure rate Packet Channel Allocation Attempts PDCH Utilization
What is TBF ?
Temporary Block Flow (TBF) is a logical connection between the BSS and the MS. A TBF is set up when there is data to send at the BSS or MS side. 26)
What are the various reasons for low Throughput?
Simultaneous TBF allocation on a PDCH due to high subscriber usage. Interference in the cell due to which there is more retransmission. Subscriber is served at the boundary of the cell and is reselected often. Reselection reduced the overall data throughput Due to high Circuit switch traffic on demand PDCH are pre-empted from the cell as such throughput goes down. Capacity limit in the PCU. Number of devices available is less due to dimensioning and high data capacity in the BSC. Problem in the Gb interface. Subscriber is in low coverage area.
General Knowledge 27)
What is the difference between Electrical and Mechanical Tilt? Mechanical tilt is achieved by corresponding mounting of the antennas using special mounting devices. Electrical tilt is a built in function of an antenna which can be either fixed or adjustable. Either an antenna has or does not has this function. The effective tilt is the sum of the both tilts.
28) What is Path-loss and Link Budget Path loss (or path attenuation) is the reduction in power density (attenuation) of a wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system. Path loss normally includes propagation losses caused by the natural expansion of the radio wave front in free space, absorption losses (sometimes called penetration losses), when the signal passes through media not transparent to electromagnetic waves, diffraction losses when part of the radiowave front is obstructed by an opaque obstacle, and losses caused by other phenomena.
Link Budget o Link budget analysis will figure out the maximum allowable path loss or the idea of the variation in signal field strength which varies mostly due to the distance between Base station and mobile, and applying maximum path loss value to the prediction model where we obtain the Cell coverage range for each services. Eventually we are tailoring the Cell radius thru Link budget o Before dimensioning the radio network, the link budget for different environments (indoor, outdoor, in-car) must be considered. From the link budget, the maximum allowable path loss can be derived. The various terms used in Link Budget ( 2g) calculation are :
o
MS: Maximum output power9dBm), Feeder loss(dB), Antenna gain(dB), EiRP(dBi), Receiver sensitivity(dBm) BTS: Rx-diversity gain(dB), Antenna gain(dB), Head amplifier gain(dB), Jumper, feeder, connector losses(dB), Duplexer losses(dB), Receiver sensitivity (dBm), Duplexer losses(dB), Receiver sensitivity(dBm) Environment: Body loss(dB), Building (indoor) penetration loss(dB), Path loss(dB), Fading margin(lognormal and Rayleigh)(dB), interference margin(dB), Frequency hopping gain(dB)
Link Budget (3g) - For 3g/UMTS , we calculate only reverse link or uplink. In the downlink we wish to find out the maximum cell range, as with the uplink. In the down link we share the available power of the base station(let say 20W) amongst all the users, but really we don’t know the amount of available power at the base station for another user. This available power is {20WSummation (Pi) } where Pi is the power consumed at the base station to maintain a link with user i. But we don’t know the position of UE, we don’t know what is Pi –This is downlink problem. In uplink each user has his own power amplifier and therefore the uplink is not a power limited
29) What is the difference between GSM and UMTS Link Budget calculations?
Interference Margin is new term in UMTS: Interference level varies due to loading of the cell. Load factor affects the coverage. More loading allowed in the system, larger interference margin and smaller coverage areas where in GSM, Noise level with MS and Base station remains constant.
30)
In UMTS we have a variable Processing Gain (Gp),so many services having different data rates and different processing Gain where in GSM no real concept of variable processing gain. Soft Hand over: It gives rise to MACRO diversity gain which reduces the influences of fading in UMTS where in GSM no real concept of soft hand over (only HHO possible) Fast power control system is utilized in UMTS,so fast fading margin is added as a limitation where GSM uses slow power control. What is the difference between dB, dBm and dBi ?
dB ( Decibel) is a ratio and is a relative measure of two values ; As a unit for the strength of a signal, dB expresses the ratio between two power levels. To be exact, dB = log (P1/P2). The unit dBm denotes an absolute power level measured in decibels and referenced to 1 milliwatt (mW). To convert from absolute power "P" (in watts) to dBm, use the formula dBm = 10*log (P/1 mW). This equation looks almost the same as that for the dB. However, now the power level "P" has been referenced to 1 mW. It turns out that in the practical radio world, 1 mW is a convenient reference point from which to measure power. dBi is a ratio, measured in decibels, of the effective gain of an antenna compared to an isotropic antenna. The greater the dBi value, the higher the gain and, as such, the more acute the angle of coverage.
