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10 WiMAX interview questions and answers

This page covers RF questionnaire set prepared by experienced specialists in RF domain. This domain has always demand right from development,testing and installation. These top 10 WiMAX interview questions and answers help engineers seeking WiMAX technology job to crack the interview.These questions are very useful as viva questions also.

Question-1. Why was the WiMAX system introduced?
Answer-1:WiMAX system has been introduced as a replacement of DSL technologies. WiMAX is a broadband wireless technology used mainly for providing internet connectivity over the air. There are different physical layers and frequency bands of operation based on country and applications. It utilizes concept of OFDM modulation technique to achieve the higher data rate by transmitting data over multiple subcarriers closely packed without leaving much subcarrier spacing in between. Read more.

2. How many physical layers are there as per WiMAX standard? Explain the functions of modules used in WiMAX OFDM physical layer?
Answer-2: There are five physical layers as mentioned in the wimax standard. It includes OFDM, OFDMA, SC, SCa and Wireless HUMAN. The most commonly employed in wimax devices are OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access). OFDM and OFDMA differ in the way data is mapped onto the resource. In OFDM all the 256 subcarriers are utilized by the subscribers (SSs) and they have been shared based on time interval between SSs. In OFDMA there are different FFT sizes supported. In case of 1024 point FFT, SSs are allocated few of the subcarriers and further shared based on time between them. Refer OFDM physical layer and OFDMA physical layer articles.

3. What is the difference between OFDM and OFDMA?
Answer-3: As mentioned in the interview question 2, OFDM and OFDMA differ in the way resources are allocated to the subscribers. Also in OFDM FFT size is fixed to 256 subcarriers. In OFDMA FFT size is variable and it can take any value from 128, 512, 1024 and 2048. Frame structure is also different. There is also difference in the physical layer modules. Refer OFDM vs OFDMA for more information.

WiMAX Frame,WiMAX Interview questions and answers

4. Explain WIMAX frame structure and various MAC fields such as DLMAP, DCD, ULMAP and UCD and their significance.
Answer-4: In order to understand wimax frame structure interviewee need to understand and explain frame structure of both OFDM and OFDMA physical layers. Basic OFDM wimax frame consists of preamble part, header part (FCH) and downlink bursts (1 to 4). Preamble is used for synchronization purpose. FCH is always BPSK1/2 and carry modulation-code rate information of consecutive downlink bursts as well as their lengths in number of symbols. Downlink burst no.1 carry DLMAP, DCD, ULMAP, UCD. DLMAP and ULMAP specify the position of downlink bursts and uplink bursts in the entire wimax frame. DCD and UCD specify downlink channel descriptor and uplink channel descriptor which in turn map to modulation-code rate for the bursts i.e. DIUC and UIUC. Read more.

5. How is the data rate calculated in Fixed WiMAX based on OFDM physical layer?
Answer-5: Data rate is the rate at which data is transmitted over the air and is measured in Mbps.
Raw data rate = Nsc * bm * Cr /Ts;
Where Nsc is the number of data subcarriers(here 192 for OFDM),
bm is coded bits per subcarriers
Cr is ratio of input bits to output coded bits
Ts is the symbol duration
For BPSK 1/2, Maximum data rate will be (192*1*1/2)/(11.8 microsec) = 8.13 Mbps
For 64QAM 3/4, Maximum data rate will be (192 * 6 * 3/4)/(11.8) =73.2 Mbps

6. What is the unit of resource allocation in fixed wimax i.e. OFDM system?
Answer-6: The basic unit of resource allocation in fixed wimax is physical slot (PS). Physical slot is defined as 4/(sampling frequency) The sampling frequency = Nfft * ΔF
Here, Nfft is the FFT size and ΔF is the subcarrier spacing

7. What is the unit of resource allocation in mobile wimax i.e. OFDMA system?
Answer-7: The unit of resource allocation in mobile wimax (OFDMA) is slot. The slot definition varies based on zone type. There are different zones supported in the mobile wimax frame. In the downlink the most common used zone types are PUSC, FUSC and AMC. In the uplink PUSC and AMC are used.
In downlink PUSC, slot is 1 sub-channel X 2 symbols
In Uplink PUSC, slot is 1 sub-channel X 3 symbols
In downlink FUSC, slot is 1 sub-channel X 1 symbol
AMC is 2 symbols X 3 sub-channels

8. What is the difference between WiMAX system and LTE system?
Answer-8: WiMAX and LTE both are used for providing broadband internet access services. Voice services are also planned to provide over wimax and LTE networks using Voice over IP protocols. WiMAX stands for World Wide Interoperability for Microwave Access and LTE stands for Long Term Evolution. WiMAX fall back to non-cellular technologies such as WLAN. LTE fall back to cellular technologies such as GSM, UMTS etc. In wimax both uplink and downlink use symmetric modulation schemes such as OFDM or OFDMA. In LTE downlink uses OFDMA and uplink uses SC-FDMA. Read more.

9. What is the difference between TDD and FDD?
Answer-9: Both TDD and FDD are duplexing topologies used mainly to share the common central resources. In TDD, frequency is shared among subscribers time slot wise. In FDD, one pair of frequency is assigned for one connection one for downlink and one for uplink and hence transmission will happen simultaneously in both the directions. Read more.

10. What is the advantage and disadvantage of using QAM modulation over QPSK?
Answer-10:QAM involves both amplitude and phase variation to map the digital data over the subcarriers. QPSK involves only phase variation while amplitude remains constant. 16QAM for example maps 4 bits of data on one single carrier and QPSK maps 2 bits of data on one single carrier. Hence 16QAM help increase data rate over QPSK but receiver will become complex in order to retrieve the modulated symbols if it is passed through the noisy channel environment. Refer basics of QPSK and QAM to understand more concepts.

Links to WiMAX MAC Layer resources

WiMAX Ranging Procedure
WiMAX Bandwidth Request procedure
WiMAX Network Entry Procedure
WiMAX FCH header format
WiMAX TLV Format basics
WiMAX Timers
WiMAX Network Interface types
WiMAX QoS Classes

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