Overview of OFDMA

OFDM modulation offers an attractive technique for high-speed data transmission in mobile communication since it can effectively combat frequency-selective multipath fading using relatively simply frequency-domain equalization. OFDM has been widely adopted in several standards, e.g. digital video broadcasting for terrestrial (DVB-T) [22], high-speed WLAN and WMAN such as IEEE 802.11b/g (Wi-Fi) and IEEE 802.16 (WiMAX). Moreover, the high computational complexity of OFDM implementation has became possible due to the success development of digital signal processing (DSP) and very large scale integrated circuit (VLSI).

The concept of OFDM is coming from frequency division multiplexing (FDM).

In FDM, the entire signal bandwidth is divided into several non-overlapping sub-bands as shown in Fig. 2.1 (a). The conventional parallel data transmission modulates each independent data on different sub-bands, and then these sub-bands are frequency-multiplexed. In order to prevent from the adjacent channel interference, the spectrum spacing allocated between sub-bands leads to inefficient utilization of the signal bandwidth. This problem is overcome by employing overlapping sub-bands as shown in Fig. 2.1 (b), and the signal bandwidth can be effectively utilized.

Furthermore, OFDM technology is invented to divide the entire signal bandwidth is into mutual orthogonal overlapping subcarriers, and the orthogonality guarantees subcarrier transmission without interference from each other. As a result, the OFDM technology can achieve high spectral efficiency. Each transmitted signal in each narrow band subcarrier experiences flat channel fading; thus, the channel equalization can be performed by a simple one-tap frequency-domain equalizer. However, the channel delay spread causes the inter-symbol interference (ISI) which destroys the orthogonality of subcarriers in OFDM. In order to avoid ISI effect, a guard interval with the length longer than the maximum channel delay spread is inserted to the frond of an OFDM symbol. Although the guard interval can be used to resolve ISI problem, the loss of orthogonality among subcarriers causes inter-channel interference (ICI). A cyclic prefix (CP), which is equal to a part of the OFDM symbol tail, is widely adopted in current standards to maintain the subcarrier orthogonality and avoid ICI effect as shown in Fig. 2.2.

OFDMA technology is a multi-user version of OFDM modulation, and it is a major multiple access method considered for future wireless systems. IEEE 802.16e-2005 PHY layer provides OFDMA specification for multi-user communication. Multiple access is achieved in OFDMA by assigning mutually exclusive subsets of subcarriers to individual users, which allows simultaneous low data rate transmission from several users. The all available subcarriers of uplink and downlink in OFDMA are divided into several subsets of subcarriers termed as sub-channels as shown in Fig. 2.3.

OFDMA can be seen as an alternative to combining OFDM with time division Fig. 2.2 OFDM symbol with cyclic prefix.

(a)

(b)

Fig. 2.1 (a) Conventional non-overlapping sub-bands. (b) Overlapping sub-bands.

multiple access (TDMA) or time-domain statistical multiplexing, i.e. packet mode communication. As shown in Fig. 2.4, the resources of OFDMA transmission are partitioned in the time-frequency space, and time slots are assigned along the OFDM symbol index as well as OFDM subcarrier index. OFDMA is considered as highly suitable for broadband wireless networks due to the advantages including scalability, MIMO easy applying, and the multi-user diversity ability to take advantage of channel frequency selectivity. OFDMA has another advantage of that low-data-rate users can send continuously with low transmission power. Constant delay and shorter delay can be achieved. In IEEE 802.16e OFDMA specification, there are two types of data allocation for sub-channelization, contiguous and diversity. As shown in Fig. 2.5, the

Fig. 2.4 Two-dimensional resources of OFDMA transmission.

Fig. 2.3 OFDMA subcarrier allocation.

contiguous permutation collects contiguous subcarriers to form a sub-channel. On the contrary, the diversity permutation pseudo-randomly spread out the subcarriers of sub-channel over the entire bandwidth and brings the benefit of frequency diversity and robustness against the frequency select fading channel.

A concept of scalable OFDMA (S-OFDMA) [23] is also introduced to the IEEE 802.16e OFDMA specification, which supports for 128, 512, 1K, and 2K fast Fourier transform (FFT) sizes to address a variable bandwidth sizes from 1.25MHz to 20MHz for NLOS operations as shown in Table 2.1.

Several basic term definitions should be established to help us to follow IEEE 802.16e OFDMA specification.

● Subcarrier: An OFDM symbol is made up of subcarriers as shown in Fig.

2.6. There are three subcarrier types: data subcarriers for data transmission, pilot subcarriers for channel estimation purposes and null subcarriers for no transmission, guard bands, non-active subcarriers and

Fig. 2.5 Adjacent and distributed subcarrier allocations.

the DC-subcarrier. Subcarrier spacing is 10.9375 KHz.

● Sub-channel: It is a set of subcarriers. IEEE 802.16 OFDMA systems define two modes of sub-channel building method. In the distributed subcarrier permutation mode, subcarriers of a sub-channel are not contiguous, and their distribution is determined by the permutation types of Partial Usage of Sub-channels (PUSC) and Full Usage of Sub-channels (FUSC). In adjacent subcarrier mode, sub-channels are constituted of bins and determined the distribution by the permutation type of AMC.

IEEE 802.16e OFDMA specification supports multiple schemes for dividing the frequency-domain and time-domain resources between users, which is called sub-channelization. The relationship between the basic terms of the two-dimensional units is shown in Fig. 2.7 and brief introduced as follows.

Fig. 2.6 An OFDM symbol.

TABLE 2.1

SCALABLE OFDMA PARAMETERS

Parameters Values

System channel bandwidth (MHZ) 1.25 5 10 20

FFT Size 128 512 1024 2048

Sampling Factor 28/25

Sampling Frequency 1.4 5.6 11.2 22.4

CP Ratio 1/32, 1/16, 1/8, and 1/4

Modulation Mode QPSK, 16QAM, and 64QAM

Subcarrier Frequency Spacing 10.94 kHz

Frame Duration 5 ms

● Frame: It is an essential packet format of transmitted data sequence. A frame may contain both an uplink sub-frame and a downlink sub-frame.

● Sub-frame: It is a component to make up a frame and identified as an uplink sub-frame and a downlink sub-frame.

● Zone: A zone is the region of contiguous OFDMA symbols which are applied with the same permutation scheme (i.e., PUSC, FUSC or AMC).

It is allowed to have different zones in a sub-frame.

● Segment: The set of available sub-channels form a segment which is applied with the same MAC definition. There can be three segments in a zone.

● Burst: It is a region which includes the contiguous sub-channel and OFDMA symbol to transmit the broadcast or unique data for Fig. 2.7 Two-dimensional basic terms in the OFDMA data structure.

corresponding users.

● Slot: It is the minimum possible data allocation unit and spans both the time domain (OFDM symbol) and frequency domain (sub-channel). It contains 48 data subcarriers for all sub-channelization schemes, but their arrangement is different in different schemes [24].

● Cluster: It contains 14 adjacent subcarriers over 2 contiguous symbols with 4 pilot subcarriers in PUSC permutation scheme.