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WiMAX PHYSICAL LAYER as per OFDM specifications

WiMAX Introduction:

This paper covers fixed wimax frame and symbol structure and describes fixed wimax Physical layer or OFDM physical layer modules and functions of each in brief as per IEEE 802.16-2004. IEEE 802.16-2004 defines five physical layers namely SC, SCa, OFDM, OFDMA and WirelessHuman. OFDM Physical layer is used in the broadband technology namely 'fixed wimax' and OFDMA is used in Mobile wimax.

Description:

In WiMAX Frame is of either TDD type or FDD type. For Fixed wimax case,TDD has been choosen and the same is adopted by wimax forum.



TDD Frame has two parts downlink subframe and Uplink(UL) subframe. DL-subframe or downlink subframe is transmitted by Base station and received by Subscriber station, while UL subframe is transmitted by more than one SSs and received by Base Station.
DL subframe comprise of Preamble, FCH followed by Downlink Bursts. Preamble consists of two symbols for the case of DL-subframe and one symbol for the case of UL-subframe with the exception in Ranging Request frame. Preamble is the known pattern and is mainly used in the receiver for synchronization purpose (time, frequency and channel).

FCH is Frame Control Header (DLFP-Downlink Frame Prefix) and consists of 88 bits which carries mainly lengths (in unit of symbols) and modulation-code rates of Downlink bursts. Other than this it has BSID and Frame Numbers which is used in seed calculation. It is always BPSK-1/2, as the mod-code rate is known it can be easily demodulated-decoded on subscriber side and data can be derived and the same is consecutively used for demodulation-decoding of the Downlink Bursts. All the Downlink and Uplink Bursts carried MAC PDU comprise of GMH, payload and CRC. GMH is the short form of Generic MAC Header and CRC is of Cyclic Redundancy Check. UL subframe or uplink subframe consist many uplink bursts. Each Burst is transmitted by Subscriber station (SS), which comprise of Preamble followed by burst, which carries MAC PDU.

WiMAX Physical layer

The physical layer mainly composed of scrambler,Reed solomon encoder,convolution encoder,puncturing, interleaver,data mapping(data modulation), OFDM symbol formation, 256 point IFFT , cyclic prefix(CP) insertion before baseband data is inserted to DAC and RF part to pump into the air.

Following figure describes wimax Physical layer modules used to convert raw data of MAC layer into complex data. These modules are used for forming one OFDM symbol. If the Burst is more than one symbol long the same modules are used repeatedly. This complex data passes through DAC and RF Frequency converter before transmission by Antenna. The reverse will take place in reception.


wimax physical layer transmitter

Let us analyze what happens to the MAC layer PDU as it is passes through these modules of wimax physical layer as per OFDM specifications. IEEE standard 802.16-2004 defines modulation-code rate table. In the same for QPSK 3/4 parameters are mentioned as below.
Modulation(QPSK)- Uncoded block size(36 bytes)-coded block size(48 bytes)-Overall coding rate(3/4)-RS code(40,36,2)-CC code rate (5/6)

For example if the PDU has 20 bytes and modulation-code rate is BPSK1/2 then PDU is fragmented into 12 and 8 bytes to be carried over 2 OFDM symbols over the air. In the second symbol necessary padding will be incorporated. We will take example of QPSK 3/4 as mentioned above to understand the modules in OFDM Physical layer as per wimax physical layer standard IEEE 802.16-2004 which is also referred as 802.16d.

Step 1: 35 bytes will be fed to the input of scrambler which is a generic linear feedback shift register with EX-OR gate. The output is randomized data of 35 bytes to which hex byte of zero is added.

Step 2: The 36 bytes will be fed as input to the RS encoder which will give 40 bytes of data with redundancy added for error correction at the receiver on the other side. RS Encoder is bypassed for BPSK1/2.

Step 3: The Convolution Encoder will double this input bytes and will produce 80 bytes of data as the rate is ½ of this encoder.

Step 4: Puncturing will remove bits as per order mentioned in the standard hence the data rate will increase. For QPSK3/4 puncturing configuration is 5/6, which produces total of 48 bytes as mentioned in the block Diagram.

Step 5: The Interleaver consists of permutation equations which help in error correction as it spreads the data over distributed carriers in bunch of 192 data carriers of OFDM symbol of total 256 carriers.

Step 6: This 48bytes (i.e. 384bits) will be fed to QPSK data mapping which will produce 192 complex data which will be carried over 192 data carriers. In QPSK 2 bits are converted to one complex data as per the constellation diagram.

Step 7: The symbol is formed after embedding pilot, DC and guard carriers. WiMAX consists of OFDM symbol of 256 carriers in total consisting of 192 data, 8 pilots, 1 DC and rest of the carriers as guard carriers.

Step 8: These 256 values are fed to the IFFT module which will produce time domain complex data.

Step 9: Cyclic prefix is added which helps to overcome effect of the delay spread is added to OFDM symbol.

Step 10: After CP is added to each symbol, packet formation take place. For the Downlink subframe, Preamble and FCH is appended to the Downlink bursts after they have passed through all above physical layer modules. For the Uplink Burst, only preamble is appended to the uplink burst after burst is formed by concatenating all the symbols.

The reverse process takes place in the wimax physical layer receiver with the exception that time, frequency and channel estimation and equalization modules are incorporated before it passes through the FFT, de mapping, de-interleaving, decoding and descrambling blocks.

REFERENCE:

IEEE 802.16-2004 Standard Section 8.3 OFDM wimax Physical layer

Links to WiMAX MAC Layer resources

REG REQ and REG RSP
SBC REQ and SBC RSP
DSA REQ and DSA RSP
UCD,UIUC,DCD,DIUC
DLMAP and ULMAP
WiMAX Ranging Procedure
WiMAX Bandwidth Request procedure
WiMAX Network Entry Procedure
WiMAX CID vs SFID
WiMAX FCH header format
WiMAX TLV Format basics
WiMAX Timers
WiMAX Network Interface types
WiMAX QoS Classes

RELATED LINKS

Fixed wimax MAC
WiMAX-OFDM PHY
wimax network architecture
WiMAX protocol stack

               

Other Standard Physical Layers

•  Wireless physical layer overview
•  11b physical layer
•  11a physical layer
•  fixed wimax physical layer-OFDM
•  mobile wimax physical layer-OFDMA
•  11n physical layer
•  GSM Physical layer
•  TD-SCDMA Physical layer
•  GPRS physical layer
•  LDACS1 Physical layer
•  10,40,100 Gigabit Ethernet Physical layer
•  Zigbee Physical layer
•  WCDMA Physical layer
•  Bluetooth Physical layer
•  WLAN 802.11ac Physical layer
•  WLAN 802.11ad Physical layer
•  LTE Physical layer

RF and Wireless Terminologies


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