Wimax Physical layer | OFDM Physical layer of WiMAX

The Wimax Physical layer article describes OFDM physical layer of wimax is as per IEEE 802.16-2004 OFDM physical layer specifications. The physical layer block diagram of wimax transmitter has been described.

Introduction:In this article we will cover wimax TDD frame structure, wimax transmitter Block diagram, fixed wimax modulation-code rate table and functions of each transmitter modules. It covers how data pass through physical layer modules which include 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.

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.

WiMAX TDD Frame Structure

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.

Figure-1 : WiMAX OFDM Frame Structure

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 i.e. time, frequency and channel synchronization.

In the wimax frame, FCH is Frame Control Header i.e. 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.

Block diagram of WiMAX Physical layer as per OFDM modulation

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.

Figure-2 : WiMAX Physical layer block diagram for transmitter part

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)

Physical layer modulation code rate table

Table-1 : Physical layer Modulation code rate as defined in wimax standard

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 wimax block Diagram above.
➨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

Other Standard wireless physical layers

RF and Wireless Terminologies

SATELLITE RF Antenna Avionics Wireless LiFi vs WiFi MiFi vs WiFi BPSK vs QPSK BJT vs FET PDH vs SDH CS vs PS MS vs PS

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