OFDMA Physical Layer Basics: WiMAX Example

Introduction

This article describes OFDMA basics, OFDMA frame structure, OFDMA symbol, and the OFDMA Physical layer as per the IEEE 802.16e mobile WiMAX standard. It also explains the OFDMA TDD frame, which consists of PUSC, FUSC, and AMC Zones in the downlink, and PUSC/AMC zones in the Uplink.

We will explore the Mobile WiMAX OFDMA physical layer, including the scrambler, convolution coding, interleaving, repetition coding, modulation, subchannel formation, OFDMA data mapping, subframe formation, IFFT, and guard insertion. We will illustrate the physical layer with the example of 1024 FFT for the Downlink chain, which is used by the Base Station to send information to Mobile subscribers.

OFDMA Basics

OFDMA stands for Orthogonal Frequency Division Multiple Access. It is a modulation and multiple access technique for next-generation wireless networks such as Mobile WiMAX and LTE. OFDMA is an extension of Orthogonal Frequency Division Multiplexing (OFDM), the current modulation of choice for high-speed data access systems like IEEE 802.11a/g wireless LAN (WiFi) and IEEE 802.16a/d/e wireless broadband access systems (WiMAX).

In current OFDM systems, only one user can transmit on all subcarriers at a given time, and time division or frequency division multiple access supports multiple users. OFDMA, however, allows multiple users to transmit simultaneously on different subcarriers per OFDM symbol. Hence, it is often referred to as Multiuser-OFDM.

Mobile WiMAX IEEE 802.16e-2009 defines the OFDMA Physical layer and MAC layer, popularly known as Mobile WiMAX. Mobile WiMAX is used for broadband data communication, similar to cellular technologies. Base Station and Subscriber station equipment for Mobile WiMAX technology have been developed for various RF frequencies, including 2.3-2.4GHz, 2.5-2.7GHz, and 3.3-3.8GHz, as required for spectrum allocations in different countries. Commonly used beam widths range from 1.25 MHz to 20 MHz in OFDMA. It supports FFT sizes of 128, 512, 1024, and 2048, but 512 and 1024 are commercialized by most equipment vendors and certified by the WiMAX Forum.

The following diagram explains the TDD frame structure of OFDMA.

Frame Structure-TDD

As depicted in the frame structure below, resource allocations occur in two dimensions: time and frequency. The X-axis represents Time (OFDM Symbols), and the Y-axis represents Frequency (Sub-channels). Mobile WiMAX supports PUSC, FUSC, and AMC Zones in the Downlink and PUSC and AMC zones in the Uplink as mandatory zones. In this example, we will discuss the PUSC zone.

Downlink Burst 1 and 2 in the downlink subframe part occupy both time and frequency resources for data transmission over the air. Similarly, Uplink Burst 1 and 2 in the uplink subframe part also occupy both time and frequency resources.

For more information on mobile WiMAX terms, visit our page on terminology.

OFDMA Symbol Description

As per mobile WiMAX, a 1024-point FFT consists of 30 subchannels, with each subchannel composed of 24 data subcarriers. Hence, one symbol of 1024 subcarriers will have 720 data subcarriers. It also has 120 pilot subcarriers, 92 left guard subcarriers, 91 right guard subcarriers, and 1 DC subcarrier, totaling 1024 (720+120+1+92+91).

Remember that PUSC will have 2 symbols in a slot and will carry 48 data subcarriers in a subchannel. A renumbering sequence permutes data subcarriers in a subchannel across 1 slot (i.e., 2 symbols).

Mobile WiMAX Physical Layer as per OFDMA Specifications

Many software applications, known as vector signal generators from R&S, Agilent, and Anritsu, generate 16e-compliant Baseband signals for Base Stations. Common inputs for generating IQ are BW, Cyclic prefix, FFT size, DL_IDcell or DL_Perm_Base (0 to 31), Sector (0,1,2), Frame duration, DL Ratio, Repetition coding for DL MAP, Subchannel Bit map, No. of downlink Bursts, and information fields for each downlink burst. These Information fields for each burst are DIUC, OFDMA Symbol offset, Sub-channel offset, boosting, Number of OFDMA Symbols, Number of Sub-channels, and Repetition coding factor (0, 2, 4, and 6). Sometimes, the preamble sequence number (range 0 to 113) replaces IDcell and sector as input to generate the baseband IQ signal.

As mentioned in the Block diagram, MAC data will go through the following modules:

1. Scrambler: A combination of LFSR (Linear Feedback Shift Registers) and Ex-OR gates. It randomizes data and removes long strings of 0s and 1s, reducing the PAPR, a major problem of OFDMA-based Physical layers. The structure follows a Pseudo-Random Binary Sequence (PRBS) generator with the polynomial 1+ X¹⁴+ X¹⁵.

2. Convolution Coding and Puncturing: Before feeding the MAC data to PHY modules, the FEC size is calculated based on data to be carried over the burst based on slot concatenation rules mentioned in the standard and mod-code rate table. Each FEC block is encoded by a binary convolution encoder with a native rate of 1/2 and a constraint length equal to 7. The puncturing module removes bits from the convolution encoder’s output, increasing the bit rate.

3. Interleaving: There are two permutation equations performed in this block. These equations depend on ncpc and ncbps. Ncpc is the number of coded bits per subcarrier, i.e., 2, 4, or 6 for QPSK, 16-QAM, or 64-QAM, respectively. Ncbps is the number of coded bits per slot. Ncbps= (Number Of uncoded bits per FEC block*(1/Rate)). Example: For QPSK ½, Repetition Factor=1 (no repetition), 2 subchannels and 2 Number Of OFDMA symbols used. It has one FEC block with 12 Un-coded bytes and 24 coded bytes.

4. Repetition Coding: Repetition coding further increases the signal margin over the modulation and FEC mechanisms. For the downlink, the number of allocated slots (Ns) should be in the range of R*K to R*K + (R-1), where K is the number of required slots before applying the repetition scheme. For example, when the required number of slots before repetition is 10 (Equal K) and the repetition of R Equal to 6 is applied for the burst transmission, the number of allocated slots (Ns) for the burst can range from 60 slots to 65 slots.

5. Digital Modulation: After interleaving and choosing the appropriate repetition coding, the data bits are entered serially to the constellation mapper. Gray mapped QPSK and 16-QAM are supported, while 64-QAM support is optional. The constellations are normalized by multiplying the constellation point with the factor ‘c’ to achieve equal average power as described in the Equation below: (I + j*Q) ×c, where c is the normalization factor.

6. Subchannel Formation: For DL Partially Used Subchannelization (PUSC) and all UL modes, the set of used subcarriers, that is, data and pilots, is first partitioned into subchannels. Then, the pilot subcarriers are allocated from within each subchannel. In PUSC, each subchannel contains its own set of pilot subcarriers. Follow the steps mentioned below:

   1. Dividing the total subcarriers into the number of clusters (Nclusters) containing 14 adjacent subcarriers each (starting from carrier 0). The number of clusters, Nclusters, varies with FFT sizes. These are called Physical clusters.

   2. Renumber the physical clusters into logical clusters using the following formula:

      RenumberingSequence = PhysicalCluster (First DL zone, or Use All SC indicator = 0 in STC_DL_Zone_IE)

      RenumberingSequence = PhysicalCluster + 13* DL_PermBase mod Nclusters, otherwise

      NOTE: The renumbering sequence has been provided in the symbol table.

   3. Allocate logical clusters to groups. The allocation algorithm varies with FFT sizes. For 1024 FFT size, divide the clusters into six major groups: Group 0 includes 12 clusters 0-11, group 1 includes 8 clusters 12-19, group 2 includes 12 clusters 20-31, group 3 includes 8 clusters 32-39, group 4 includes 12 clusters 40-51, group 5 includes 8 clusters 52-59. Note that these groups may be allocated to segments. By default, group 0 is allocated to sector 0, group 2 is allocated to sector 1, and group 4 is allocated to sector 2.

   4. Allocating subcarriers to the subchannel in each major group is performed separately for each OFDMA symbol by first allocating the pilot carriers within each cluster and then taking all remaining data carriers within the symbol. The parameters vary with FFT sizes. For 1024 FFT size, use the parameters from the Symbol Table, with basic permutation sequence 6 for even-numbered major groups (G0, G2, G4) and basic permutation sequence 4 for odd-numbered major groups (G1, G3, G5) to partition the subcarriers into subchannels containing 24 data subcarriers in each symbol.

7. OFDMA data mapping

   A slot in the OFDMA PHY requires both a time and subchannel dimension for completeness and is the minimum possible data allocation unit. The definition of an OFDMA slot depends on the OFDMA symbol structure, which varies for uplink and downlink and depends on zone type. Data outputted from the modulator block is broken into size (1 slot equal to Number of subchannels by Number of OFDMA symbols, depending on DL FUSC, DL PUSC, or UL PUSC. (Somewhat similar to FTDMA))

   DL PUSC Slot structure: 1 sub-channel by 2 OFDMA symbols.

   OFDMA Data Mapping in downlink should be done as per the following steps:

   1. Segment the data after the modulation block into blocks sized to fit into one OFDMA slot.

   2. Each slot should span one subchannel in the subchannel axis and one or more OFDMA symbols in the time axis, as per the slot definition in 8.4.3.1. Map the slots such that the lowest numbered slot occupies the lowest numbered subchannel in the lowest numbered OFDMA symbol.

   3. Continue the mapping such that the OFDMA subchannel index is increased. When the edge of the Data Region is reached, continue the mapping from the lowest numbered OFDMA subchannel in the next available symbol.

8. DL Subframe Formation: The DL Subframe is composed of a Preamble, FCH, Maps (DLMAP, ULMAP), and one or more Downlink bursts. These fields are formed and mapped as per the TDD frame structure outlined above based on FFT size and other useful parameters.

9. IFFT: There are IFFT sizes of 2048, 1024, 512, and 128 mentioned in the standard, but only 512 and 1024 have been adopted by most vendors as per WiMAX forum-published specifications.

10. Guard Insertion: One of the main advantages of OFDM is its effectiveness against the multi-path delay spread frequently encountered in mobile communication channels. Reducing the symbol rate by N times results in a proportional reduction of the relative multi-path delay spread relative to the symbol time. To completely eliminate even the very small ISI that results, a guard time is introduced for each OFDM symbol. The guard time must be chosen to be larger than the expected delay spread, such that multi-path components from one symbol cannot interfere with the next symbol. The value of CP, i.e., Tg, can be T/4, T/8, T/16, T/32.

Please note that all figures are taken from the IEEE standard and WiMAX forum-released documents to demonstrate the concept of the OFDMA Physical layer.

References

1. IEEE standard 802.16-2004 OFDMA
2. IEEE standard 802.16e-2005 OFDMA
3. wimax_forum_mobile_system_profile_v1_40