WiMAX

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.

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.

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.

Initial raging acquires correct transmission parameters (Power adjustment, timing offset estimation and synchronization) to communicate with the BS.

Periodic raging is to maintain uplink communication with the BS and to adjust transmission parameters.

Contention raging procedure is required by the SS to access the system for the first time and no dedicated connection resources assigned to the SS.

Non-contention raging is regulated by the BS to allow the SS to finish system access earlier when dedicated channel is provided and polled initial ranging.

There are various wimax ranging procedures as mentioned below.

★ Initial Ranging

★ Periodic Ranging

★ Contention Ranging

★ Non-Contention Ranging

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. 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.

RNG RSP decoded by SS will analyze following:

★ 'Timing Adjust' field (signed 32 bit) and will drive PHY to advance frame Transmission accordingly.

★ 'Power level Adjust' field (signed 8-bit, 0.25dB) and will drive RF attenuators at RF layer accordingly.

★ 'Offset Frequency Adjust' field (signed 32 bit, Hz units) and will drive RF synthesizer/Ref. OCXO accordingly.

★ Ranging Status (if status is 're range') will indicate whether SS need to do retransmit RNG REQ or (if status is 'Success') start BW REQ transmission for determining slot for SBC REQ.

RNG REQ is transmitted by SS to BS. After performing Synchronization (Time, Freq, Channel) RNG REQ Frame is decoded to bits at BS. Type field just after GMH will describe message type which is '0x04' for RNG REQ, which tells BS what need to be done after receiving the message.

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.

★ Frequency selective fading will be able to affect few of the subchannels/subcarriers and not entire band.

★ OFDM overcomes effect of ISI occuring mostly in multipath channel environment.

FDM is the short form of frequency division multiplexing and TDM is the short form of time division multiplexing.

★ TDM is employed in PCM transmission to obtain T1 at the rate of 1.544Mbps.

★ FDM is employed in satellite,Radio,HF and other wireless technologies.

★ Both TDM and FDM is employed in GSM cellular technology.

BW REQ Header requests refer to the mechanism that SSs use to indicate to the BS that they need uplink BW allocation. Because the uplink burst profile can change dynamically, all requests for BW shall be made in terms of the number of bytes needed to carry the MAC Header and payload but not the PHY overhead. BW REQ message may be transmitted during any uplink allocation, except during any initial ranging interval. BW is always requested on a CID basis and BW is allocated on an SS basis.