WiMAX Interview Questions And Answers

WiMAX Technology is an IP based, wireless broadband access technology that provides performance similar to 802.11/Wi-Fi networks with the coverage and QOS (quality of service) of cellular networks. WiMAX is also an acronym meaning "Worldwide Interoperability for Microwave Access (WiMAX).

The WiMAX Forum and to the founding members of the WiMAX Technology Forum, which committed themselves early to the process of creating a collaborative standards body. As a founding member of the WiMAX Technology Forum, Intel recognized that a well developed ecosystem was necessary to drive adoption and thereby drive lower hardware costs. Intel was also instrumental in getting other silicon chip manufacturers involved whose products would form the core of WiMAX technology.

WiMAX Technology is arguably even more important for the fixed broadband wireless segment than mobile broadband, at least internally to that industry. It seems clear that mobile broadband wireless holds the loftier long term monetary and customer growth potential. However, the fixed wireless segment has been fragmented essentially since its inception. There are no cohesive standards for outdoor metropolitan area networks beyond the adapted Wi-Fi technologies. Wi-Fi Technology as a standard has been accepted in broad strokes by the industry and the public. However, it is not a well conceived citywide technology.

Since much of the technology being utilized in the IEEE 802.16 standard (WiMAX Technology standard) is widely deployed, there is a historical body of evidence supporting the safety of technologies used in upcoming WiMAX Technology and WiMAX products. Microwave and other spectrum technologies enjoy over a hundred years of historical evidence of safety when prudently handled and configured. The amount of power allowed to deliver broadband wireless signal varies from frequency to frequency, however, most are modest topping out at around 40 watts at the tower relay site. While certain basic precautions need to be taken when onsite at communications towers (i.e. standing directly in front of active microwave links at essentially zero range) the configurations for public use are understood and safe. Customer premise equipment is even safer.

It is important to remember that WiMAX Technology is a global broadband wireless standard. The question of whether or not it could replace either DSL or Cable will vary from region to region. Many developing countries simply do not have the infrastructure to support either cable or DSL broadband technologies. In fact, many such countries are already widely using proprietary broadband wireless technologies. Even in such regions however, it is very unlikely that either Cable or DSL technologies would disappear. The business case and basic infrastructure often dictates that the cheapest solutions will predominate. In many areas in developing nations, it may be cheaper to deploy Cable and DSL in the cities at least for fixed applications, whereas WiMAX Technology will dominate outside of major towns.

To answer this question it is important to understand the state of technical fragmentation experienced in the past by the mobile wireless and fixed broadband wireless industry. Early broadband wireless systems began as extensions of indoor local area network (LAN) technology known as Wi-Fi Technology or the 802.11b protocol. This standard has evolved into a ubiquitous and widely available standard used in short range hotspots all over the globe. However, the media access controller (MAC) and physical layer (PHY) specifications for this protocol are suboptimum for outdoor citywide wireless networks or metropolitan area networks (MAN). Recent updates and new standards such as 802.11g, 802.11a and 802.11n have improved these elements. However, once again these technologies are configured for best performance in small venues and at short range.

Mobile broadband wireless or 3G has enjoyed two largely consistent standards, those being the code division multiple access (CDMA) based approach with its evolution data only (EVDO) and the universal mobile telecommunications system (UMTS) and its faster upgrade high speed downlink packet access (HSDPA), which in particular has gained some deployments in the past year. However, these technologies were slow to mature into economically viable and affordable iterations. The EVDO schema is now in a Revision A version which improves bandwidth considerably. Verizon and Sprint are the first US based carriers to begin wide deployment. Sprint currently has deployed most of its markets with 3G as has Verizon. The bandwidth limitations have been significant and the adoption by carriers, particularly those utilizing GSM technology here in the US has been very slow (as they are essentially incompatible technologies).

Clearly, WiMAX Technology and Wi-Fi Technology are complementary technologies and will remain so for the foreseeable future. The widely available Wi-Fi technology used in hotspots in hotels, restaurants, airports and even larger Wi-Fi zones in some cities will continue to grow for many years. The recent flurry of municipal Wi-Fi mesh networks has only served to cement the technology into the wireless equation. Wi-Fi is not going away any time soon.

The answer to this question probably generates more confusion than any other single aspect of WiMAX Technology. In the early days of WiMAX it was common to see statements in the media describing WiMAX multipoint coverage extending 30 miles. In a strict technical sense (in some spectrum ranges) this is correct, with even greater ranges being possible in point to point links. In practice (and especially in the license-free bands) this is wildly overstated especially where non line of sight (NLOS) reception is concerned.

The short answer is yes, as never before with broadband wireless systems. However, this area appears to be early ground that vendors are staking out to differentiate their products and philosophies. The WiMAX standard itself incorporates much better and more flexible security support than the Wi-Fi standard. It can be sometimes confusing when industry pundits and detractors talk of standards such as WiMAX Technology and then in the same breath describe ways in which vendors will be "different" or that WiMAX Technology security might be weak. At first glance, these comments on the part of some vendors zealous to promote the added capabilities of their products can leave one feeling uncertain about the quality and reliability of the product.

WiMAX Technology supports very robust data throughput. The technology at theoretical maximums could support approximately 75 Mbps per channel (in a 20 MHz channel using 64QAM ¾ code rate). Real world performance will be considerably lower---perhaps maxing out around 45 Mbps/channel in some fixed broadband applications. Remember however, that service across this channel would be shared by multiple customers. Actual transmission capabilities on a per customer basis could vary widely depending on the carrier's chosen customer base, which is actually an inherent strength because it can be defined by QOS in a deliberate fashion to offer different bandwidth capabilities to customers with different needs (and different budgets). Mobile WiMAX capabilities on a per customer basis will be lower in practical terms, but much better than competing 3G technologies. WiMAX is often cited to possess a spectral efficiency of 5 bps/Hz, which is very good in comparison to other broadband wireless technologies, especially 3G.

As of last count, the WiMAX Forum membership included over 500 companies with many other firms evaluating the technology as a possible fit for their business and technical models. A complete list of WiMAX Forum members can be found on its site. WiMAX Forum ™ Members.

The most recent versions of both WiMAX Technology standards in 802.16 cover spectrum ranges from at least the 2 GHz range through the 66 GHz range. This is an enormous spectrum range. However, the practical market considerations of the Forum members dictated that the first product profiles focus on spectrum ranges that offered Forum vendors the most utility and sales potential.

Determining whether or not a service provider needs a license to deliver service is not a question answerable with a simple yes or no. Many elements must be factored into the decision of whether or not to attempt to obtain a license.

The first frequency with interoperable certified product to ship was in the International 3.5 GHz bands. Today products are available at 2.5 GHz, 5.8 GHz, 2.3 GHz and 3.65 GHz. Allthough no formal product profile has been set, likely the Forum will attempt to address the 700 MHz range. There are no plans for Mobile WiMAX Technology at 5.8 GHz.

WiMAX subscriber decodes the DLFP as it is transmitted at BPSK1/2. DLFP tells position of DLMAP,ULMAP,UCD and DCD which are transmitted in downlink burst 1. Once ULMAP/UCD is decoded subscriber comes to know its uplink transmission frame position and modulation-coding scheme it need to use in the wimax frame. Refer WiMAX MAC layer inside on RFWireless-world.com.

The main difference is that fixed wimax uses OFDM technique while mobile wimax uses OFDMA technique. Also subscriber is stationary or moving at pedestrian speed in fixed wimax while in mobile wimax it moves at higher speed.

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.

There are five physical layers as mentioned in the wimax standard.
It includes:
★ OFDM
★ OFDMA
★ SC
★ SCa
★ 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.

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.

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.

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.

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 sub-carriers
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.

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.