OFDMA sub-channelization

In order to create the OFDM symbol in the frequency domain, the modulated symbols are mapped on to the subchannels that have been allocated for the transmission of the data block.

Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called subchannels. The minimum frequency-time resource unit of sub-channelization is one slot.

The number and exact distribution of the subcarriers that constitute a subchannel depend on the subcarrier permutation mode. The number of subchannels allocated for transmitting a data block depends on various parameters, such as the size of the data block, the modulation format, and the coding rate.

In the time and frequency domains, the contiguous set of subchannels allocated to a single user—or a group of users, in case of multicast—is referred to as the data region of the user(s) and is always transmitted using the same burst profile. In this context, a burst profile refers to the combination of the chosen modulation format, code rate, and type of FEC: convolutional codes, turbo codes, and block codes.

Two types of sub-carrier permutations: Diversity & Contiguous

The subcarriers that constitute a subchannel can either be adjacent to each other or distributed throughout the frequency band, depending on the
subcarrier permutation mode. A distributed subcarrier permutation provides better frequency diversity, whereas an adjacent subcarrier distribution is more desirable for beamforming.

Downlink Full Usase of Sub-Carrers (DL-FUSC)

In the case of DL FUSC, all the data subcarriers are used to create the various subchannels. Each subchannel is made up of 48 data subcarriers, which are distributed evenly throughout the entire frequency band, as depicted in Figure. In FUSC, the set of the pilot subcarriers is divided in to two constant sets and two variables sets. The variable sets allow the receiver to estimate the channel response more accurately across the entire frequency band.

==== Parameters of FUSC subcarrier permutation(1024 FFT) ====
Subcarriers per subchannel : 48
Number of subchannels : 16
Data Subcarriers used : 768
Fixed Pilot Subcarriers : 11
Variable Pilot Subcarriers : 71
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Downlink Partial Usase of Sub-Carrers (DL-PUSC)

DL PUSC is similar to FUSC except that all the subcarriers are first divided into six groups. Permutation of subcarriers to create subchannels is performed independently within each group, thus, in essence, logically separating each group from the others. In the case of PUSC, all the subcarriers except the null subcarrier are first arranged into clusters. Each cluster consists of 14 adjacent subcarriers over two OFDM symbols, as shown in Figure. In each cluster, the subcarriers are divided into 24 data subcarriers and 4 pilot subcarriers. The clusters are then renumbered using a pseudorandom numbering scheme, which in essence redistributes the logical identity of the clusters. After renumbering, the clusters are divided into six groups, with the first one-sixth of the clusters belonging to group 0, and so on. A subchannel is created using two clusters from the same group, as shown in Figure.

Frequency reuse is much simple in PUCS. E.g consider the BS with 3 segments, here it is possible to allocate all or only a subset of the six groups to a given transmitter. By allocating disjoint subsets of the six available groups to neighboring transmitters, it is possible to separate their signals in the subcarrier space, thus enabling a tighter frequency reuse at the cost of data rate.

==== Parameters of PUSC subcarrier permutation(1024 FFT) ====
Subcarriers per cluster: 14
Number of subchannel : 30
Data Subcarriers used : 720
Pilot Subcarriers : 120
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Uplink Partial Usage of Subcarriers (UL-PUSC)

In UL PUSC, the subcarriers are first divided into various tiles, as shown in Figure. Each tile consists of four subcarriers over three OFDM symbols. The subcarriers within a tile are divided into eight data subcarriers and four pilot subcarriers. The optional UL PUSC mode has a lower ratio of pilot subcarriers to data subcarriers, thus providing a higher effective data rate but poorer channel-tracking capability. The two UL PUSC modes allow the system designer a trade-off between higher data rate and more accurate channel tracking depending on the Doppler spread and coherence bandwidth of the channel. The tiles are then renumbered, using a pseudorandom numbering sequence, and divided into six groups. Each subchannel is created using six tiles from a single group. UL PUSC can be used with segmentation in order to allow the system to operate under tighter frequency reuse patterns.

Band Adaptive Modulation and Coding (B-AMC)

Unique to the band AMC permutation mode, all subcarriers constituting a subchannel are adjacent to each other. Although frequency diversity is lost to a large extent with this subcarrier permutation scheme, exploitation of multiuser diversity is easier. Multiuser diversity provides significant improvement in overall system capacity and throughput, since a subchannel at any given time is allocated to the user with the highest SNR/capacity in that subchannel.

In this subcarrier permutation, nine adjacent subcarriers with eight data subcarriers and one pilot subcarrier are used to form a bin, as shown in Figure. Four adjacent bins in the frequency domain constitute a band. An AMC subchannel consists of six contiguous bins from within the same band. Thus, an AMC subchannel can consist of one bin over six consecutive symbols, two consecutive bins over three consecutive symbols, or three consecutive bins over two consecutive symbols.

OFDM symbol structure

In an OFDM system, a high-data-rate sequence of symbols is split into multiple parallel low-data rate-sequences, each of which is used to modulate an orthogonal tone, or subcarrier.

WiMAX has three classes of subcarriers:

1. Data subcarriers are used for carrying data symbols.
2. Pilot subcarriers are used for carrying pilot symbols. The pilot symbols are known a priori and can be used for channel estimation and channel tracking.
3. Null subcarriers have no power allocated to them, including the DC subcarrier and the guard subcarriers toward the edge. The DC subcarrier is not modulated, to prevent any saturation effects or excess power draw at the amplifier. No power is allocated to the guard subcarrier toward the edge of the spectrum in order to fit the spectrum, of the OFDM symbol within the allocated bandwidth and thus reduce the interference between adjacent channels

Above figure shows a typical frequency domain representation of an IEEE 802.16e-2005 OFDM symbol containing the data subcarriers, pilot subcarriers, and null subcarriers.

Multiple Access Schemes

Multiple-access strategies typically attempt to provide orthogonal, or noninterfering, communication channels for each active link. The most common way to divide the available dimensions among the multiple users is through the use of frequency, time, or code division multiplexing.

In frequency division multiple access (FDMA), each user receives a unique carrier frequency and bandwidth.

In time division multiple access (TDMA), each user is given a unique time slot, either on demand or in a fixed rotation.

In code division multiple access (CDMA) systems allow each user to share the bandwidth and time slots with many other users and rely on orthogonal binary codes to separate out the users.

OFDMA is essentially a hybrid of FDMA and TDMA:

Users are dynamically assigned subcarriers (FDMA) in different time slots (TDMA).In OFDMA, the subcarrier and the power allocation should be based on the channel conditions in order to maximize the throughput.

Multiuser diversity and Adaptive modulation are the two key principles that enables high performence in OFDMA. Multiuser diversity describes the gains available by selecting a user or sub-set of users having "good" conditions. Adaptive modulation is the means by which good channels can be exploited to achieve higher data rates.

OFDMA Pros and Cons

In OFDMA multiple access is two dimensional (time and frequency)

Multiple users use separate subchannels for multiple access

- Improved capacity

- Improved scheduling and QoS support

- Reduced interference (no intra-cell interference)

- Improved link margin (subchannelization gain)

- High spectral efficiency

Flexible subchannelization

- Pseudo-random permutation (PUSC) for frequency diversity, or

- Contiguous assignment (AMC) to enable beamforming

- Scalable structure to support variable bandwidths

- Allocation of subcarriers to multiple SS's (Subscriber Stations) in an OFDM symbol time- Group of M subcarriers as a unit of allocation - Subchannel

- Narrow subcarrer spacing is sensitive to carreer frequency error

- high PAR ratio (Peak to Average Power Ratio), which reduces the eficiency and hence increases the cost of the power amplifier, which is one of the most expesive component in Radio

OFDM Basics

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation technique that has recently found wide adoption in a widespread variety of high-data-rate communication systems, including digital subscriber lines, wireless LANs (802.11a/g/n), digital video broadcasting, and now WiMAX and other emerging wireless broadband systems.

The basic idea of Multicareer modulation is quite simple. The ISI would be zero only if symbol time of the siganl is larger than the channel delay. But the modern wireless networks with broadband links providing several mega-bits per second (e.g. 802.11a promises 54 Mbps), which has very less symbol time than channel dealy causes ISI. But it may not be practical to implement an equalizer at all because of overwhelming complexity caused by the high speed link.

On the other hand, if we could somehow reduce the symbol rate so that ISI becomes negligible, while still maintaining the required information bit rate, equalization becomes unnecesssary.

One way to do this is simply to increase the level of modulation in an M-ary pulse modulation scheme but there is a limit on how large M can be. Because as M increases, the spacing between each sample decreases and so it would be difficult at the doecoder side to decoded the weak signal.

The other way to increase the symbol interval is through parallel transmission over many orthogonal channels. This will widen the symbol time larger than channel delay and ISI can be avoided. These individual substreams can then be sent over parallel subchannels, maintaining the total desired data rate, so the subchannels experience relatively flat fading. Thus, the ISI on each subchannel is small. Moreover, in the digital implementation of OFDM, the ISI can be completely eliminated through the use of a cyclic prefix. Such orthogonal carriers can be easily generated using IFFT operation.

In the above figure Tb is useful OFDM simbol time and Tg is cyclic prefix, which is nothing but replica of last few bits in the symbol