WiMAX Interview Preparation Guide

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WiMax combines the advantages of favorite Wi-Fi and a wide coverage of cellular network. It takes a best part of wi-fi networks, the fast speed and broadband internet experience. WiMax is a WAN technology, service providers will deploy a wimax network that enables access over long distance. Coverage for a geographical area is divided into a series of overlapping areas called cells. Each cell provides coverage for users within that immediate vicinity. When you travel from one cell to another, the wireless connection is handed off from one cell to another.

The central radio transmitter/receiver installed by service provider to broadcasts WiMax signals. These transmitters are typically mounted on towers or tall buildings. Cellular networks are based on the concept of cells (a logical division of geographical area), each such cell is allocated a frequency and is served by a base station.
Base station consists of a receiver, transmitter and a control unit. Adjacent base stations use different frequencies to avoid cross-talk.

The procedure helps wimax subscriber camp on to wimax compliant base station. Following are the summarized steps for network entry. Please note that this procedure is as per IEEE 802.16-2004 OFDM PHY and MAC layer specifications. This version of wimax is also referred as fixed wimax due to non-mobility of subscriber stations. The procedure for mobile version of wimax i.e. mobile wimax is similar to this but there are few changes to MAPs/channel descriptors and header format as well as addition in MAC messages.

In wimax system, ranging procedure is initiated by SS(Subscriber Station) to establish connection with BS(Base Station). This ranging procedure are of many types mainly available for synchronization and maintenance of RF link. After Ranging is completed Base station waits for SBC REQ message transmitted by the Wimax SS. Using SBC REQ message SS informs BS of its basic capabilities. Unlike ranging request(RNG REQ) there is no dedicated slot for bandwidth request and SBC REQ. Bandwidth request can be transmitted any where in the uplink subframe except the reserved ranging slot.

WiMAX supports the following for encryption:
★Advanced encryption standard
★Triple data encryption standard

The following modulation schemes are supported by WiMAX:
★ Binary phase shift keying modulation
★ Quadrature phase shift keying modulation
★ Quadrature amplitude modulation

Standards does not define any uniform global licensed spectrum for WiMax, however the WiMax forum has published 3 licensed spectrum profiles 2.3 GHz, 2.5 GHz and 3.5 GHz, in an effort to drive standardization and decrease cost. Also plans for use of analog TV spectrum (700 MHz) await the complete deployment of digital TV.

Yes, WiMax is a standard based design, and WiMax standards are well defined to provide much better and flexible security than wi-fi networks.
★ WiMax security stack supports two encryption standards one is popular DES3 (Data Encryption Standard) and other is AES (Advanced Encryption Standard).
★ Additionally it requires dedicated security processor for base station.
★ It also defines minimum encryption requirements for the traffic and for end to end authentication.

We need WiMAX technology for high speed broadband like internet access on the move. We have broadband connections that provide high speed networks, but are attached to LAN systems i.e. not portable. Wi-fi provides access to such systems but limited to a much shorter distance.
Then we have cellular networks which provide internet access but their speed is limited and they are relatively costly. To overcome these problems we need WiMax or comparable technology.

WiMax can not provide highest performance over 50 kilometers. As the distance increases, bit error rate thus reducing performance. Reducing distance to less than 1km allows a device to operate at higher bit rate. A user closer to base station gets better speed at around 30 mbps.
Also as an available bandwidth is shared between no of users, performance depends on number of active users connecting to that base station. So this needs a use of Quality of Service (QOS) mechanism to provide a minimum guaranteed throughput.

A service flow in wimax is partially characterized by the following attributes:
★ A 32-bit Service Flow ID (SFID) is assigned to all existing service flows. The SFID serves as the principal identifier for the Service Flow and has an associated direction.
★ A 16-bit Connection ID (CID) is associated with each active SFID (connection active).
★ A set of QoS parameters specifying the required resources. The principal resource is bandwidth, but the specification may also include latency requirements.
★ A set of QoS parameters defining the level of service being provided.

It is a stand alone indoor device which is installed at a good reception area. It acts as a wi-fi access point for other devices such as home pc, VOIP handset etc. WiMax operators generally provide this gateway device or subscriber unit which communicates with base station and provides wi-fi access within home or office for device like laptop, Smartphone.

In this class, BW is granted to mobile subscriber if and only there will be left over bandwidth from other QoS classes. This QoS class guarantess neither delay nor throughput. It allows minimum reserved traffic rate and maximum sustained traffic rate.

This type is used for non real time Variable bit rate traffic with no delay guarantee. But min. rate is guaranteed. FTP falls under this QoS type.

This QoS type is developed to support VOIP along with silence suppression. There will be no traffic transmission during silence time. QoS parameters are same as UGS type. Check table-1 for application of this type. ertPS is similar to UGS in which base station assigns MST on active mode and no BW is allocated during silent time period. Here BS need to poll mobile subscriber to know whether silent period has been ended or it is continuing.

It is used for VBR real time traffic for example MPEG video. Unlike UGS where in fixed allocation is made by BS, here BS regularly polls MS to find out allocation need. Hence bandwidth is allocated on need basis and is adaptive in nature. For this wimax QoS type Min. reserved traffic rate and MST need to be mentioned separately. For UGS and ertPS Min. reserved traffic rate and MST are both same. QoS parameters are same as UGS.

This Wimax QoS class provides fixed bandwidth allocation on periodic basis. Once the connectivity is established, no more requests are needed. For application of this type. QoS parameters for this type are MST, tolerated jitter and max. latency.

Following are the WiMAX QoS classes:
★ UGS
★ rtPS
★ ertPS
★ nrtPS
★ BE

Both WiMAX and LTE are used for providing broadband wireless internet services.
WiMAX is the completely new standard developed for providing high data rate internet services. WiMAX is designed for only data applications. As it is entirely new technology without any legacy support,it is very costly and requires service provider to install all the equipments.
LTE follow cellular standards such as HSPA,WCDMA,GSM etc but it is designed mainly for data applications. It is also referred as all IP data network. Voice over LTE is also possible using legacy fall backs and using VOIP protocols. Due to predecessors existing equipments can be make use of and hence it is less costly and requires minimum initial investment.

EXAMPLE:
FEC code type for downlink burst
150(0x96), 0x01(msb:0),0x01(QPSK 1/2)
★ Here first byte is type.
★ As value is less than 127 length field in second byte is one byte with msb set to zero and its remaining 7 bytes indicates that size of value field is one(1).
★ The value field is mentioned as 0x01 representing as QPSK 1/2 modulation-code rate.
If the size of 'value' field is more than 127 bytes(>127) then
★ Size of length field shall be one byte more than what is actually used to mention size of 'value' field in bytes.
★ MSB of first byte of 'length' field is set to one
★ The remaining 7 bits of first byte of 'length' field indicates additional bytes of 'length' field
★ The bytes other than first byte of 'length field' is used to mention size of the 'value field'.

Following rules apply for this. Size of value field depends on length field specified.
If the actual size of 'value' field is less than or equal to 127 bytes(length<=127) then
★ Size of 'length' field will be 1 byte
★ MSB of 'length' field is set to zero
★ The rest of 7 bits of length field indicates actual size of 'value' field in number of bytes.

In wimax TLV formats are used for parameters in both mac management messages as well configuration file. TLV stands for Type, length and value. Size of type field will be always 1 byte. Size of length field is either 1 byte or more than 1 byte.

The modulation-code rate of wimax FCH or DLFP is fixed to BPSK-1/2. It has total size of about 88 bits as mentioned below.
WiMAX FCH/DLFP: { BS ID(4 bits), Frame No.(4 bits), CCC(4 bits), Reserved(4 bits),
Rate ID(4 bits), Preamble(1bit), Length(11 bits), DIUC(4bits), Preamble(1bit), Length(11 bits),
DIUC(4bits), Preamble(1bit), Length(11 bits), DIUC(4bits), Preamble(1bit), Length(11 bits),
HCS(8 bits)

As we know wimax frame consists of downlink subsubframe and uplink subframe. Downlink Subframe consists of preamble(2 OFDM symbols), header, downlink bursts(1, 2, 3 or 4). After downlink subframe, uplink subframe starts with some gap used for TTG and contention slots. The transmissions from BS to SSs is referred as downlink and transmissions from SSs to BS is referred as uplink subframe. Header mentioned above is known ad FCH (Frame Control Header) or DLFP (Downlink Frame Prefix). On this page we will see header used in fixed wimax as per OFDM specifications for 256 point FFT.

As we know wimax frame consists of downlink subsubframe and uplink subframe. Downlink Subframe consists of preamble(2 OFDM symbols), header, downlink bursts(1, 2, 3 or 4). After downlink subframe, uplink subframe starts with some gap used for TTG and contention slots. The transmissions from BS to SSs is referred as downlink and transmissions from SSs to BS is referred as uplink subframe. Header mentioned above is known ad FCH (Frame Control Header) or DLFP (Downlink Frame Prefix). On this page we will see header used in fixed wimax as per OFDM specifications for 256 point FFT.