Elements of 802.16 - 1 [Under WiMAX]
Before we have a look at 802.16 layers and their functions, let us have a look at what 802.16 solution looks like. We will mainly concern ourselves with NLOS frequencies for mobility operations (Mobile WiMAX).
The terms BS stand for Base Station and MS/SS imply user equipment (SS - Subscriber Station is fixed one whereas MS - Mobile Station is mobile one).
» OFDMA/S-OFDMA PHY
As a refresher, please look at articles on OFDM and OFDMA. In brief, OFDM transmits individual bits of data over number of orthogonal frequencies in parallel. Due to orthogonality, the subcarriers could be close to each other and still have theoretically zero interference. In addition, the individual subcarriers can be modulated differently. OFDMA specifies way to use OFDM for multiple access. OFDMA create logical subchannels, each subchannel contain number of subcarriers (selected randomly or contiguously). Question to ask here is why not use CDMA: inherent limitation to CDMA is processing gain (CDMA article). To have higher data rate, with limitation of processing gain, we will need higher bandwidth. In comparison, due to orthogonality property, OFDM provide much better spectral efficiency.
OFDM/OFDMA has number of other inherent advantages - obvious being: with OFDMA, randomly selected subcarriers provide consistent performance to all users, different modulation schemes can be adopted. Adoption of modulation schemes is important to mobile environment as channel conditions differ for each user and so appropriate modulation/coding scheme can be used.
802.16 uses S-OFDMA. In S-OFDMA, subcarrier spacing remain same, making it scalable and easing the hardware design.
MS PHY layer will typically look for (known/standardised) pilot signals. MS PHY layer will then estimate certain PHY parameters (possibly includes FFT size, BW etc.), which will then allow it to look for subcarriers. MS PHY will then look for Preamble (corresponding to FFT) for time and frequency synchronisation. Preambles are standard specified PN sequences (each subcarrier is 1 out of 3 consecutive subcarriers). From layman's point, Preambles are paricular waveforms that helps synchronising. Next procedure is to read FCH and DL-MAP as described below.
» Frame structure
802.16 MAC uses "frames" for operation. Frames for DL and UL are called DL subframe and UL subframe respectively. DL subframe contain FCH and DL/UL Maps as shown in diagram below: Note that UL-map need not be in next numbered symbol time; both DL/UL maps are variable in length and so accommodated accordingly.
FCH is a Frame Control Header. FCH contain information about DL-MAP coding, length etc. FCH is the first part/message that MS need to be able to decode. FCH will be using preset (QPSK for OFDMA PHY) modulation and coding scheme (specified in 802.16).
DL/UL Maps are MAC (management) messages with DL-map describing current DL subframe and UL-map describing usage of relevant UL subframe. Below diagram shows the way DL/UL subframes organised for TDD/FDD and their map relevance.
The usage of subframes is in terms of OFDM symbol. In essence, we have a burst based time division multiplex operation. Below is an example of OFDMA frame structure for TDD.
The DL bursts meant for certain logical MAC functions (channels), data transfer etc. UL bursts are meant to be used by MSs based on UL-map. Some (both DL/UL) bursts are meant to be used by individual MSs or groups of MSs or all MSs.
In next article, we will look at how MAC manages the medium access.
References: WiMAX Handbook (edited) by Ahson and Ilyas, Fundamentals of WiMAX by Andrews, Ghosh, and Muhamed, WiMAX forum web site, Latest 802.16-2009 (covering amendments 802.16e-2005, 802.16-2004/Cor1-2005, 802.16f-2005 & Std 802.16g-2007 made on 802.16-2004).
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