Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN---3

Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN
MULTICARRIER DESIGN REQUIREMENTS AND TRADEOFFS
A typical early step in the design of an Orthogonal Frequency Division Multiplexing (OFDM)-based system is a study of subcarrier design and the size of the Fast Fourier Transform (FFT) where optimal operational point balancing protection against multipath, Doppler shift, and design cost/complexity is determined. For this, we use Wide-Sense Stationary Uncorrelated Scattering (WSSUS), a widely used method to model time varying fading wireless channels both in time and frequency domains using stochastic processes. Two main elements of the WSSUS model are briefly discussed here: Doppler spread and coherence time of channel; and multipath delay spread and coherence bandwidth.
A maximum speed of 125 km/hr is used here in the analysis for support of mobility. With the exception of high-speed trains, this provides a good coverage of vehicular speed in the US, Europe, and Asia. The maximum Doppler shift [15] corresponding to the operation at 3.5 GHz (selected as a middle point in the 2-6 GHz frequency range) is given by Equation (1).
Equation (1)
The worst-case Doppler shift value for 125 km/hr (35 m/s) would be ~700 Hz for operation at the 6 GHz upper limit specified by the standard. Using a 10 KHz subcarrier spacing, the Inter Channel Interference (ICI) power corresponding to the Doppler shift calculated in Equation (1) can be shown [16] to be limited to ~-27 dB.
The coherence time of the channel, a measure of time variation in the channel, corresponding to the Doppler shift specified above, is calculated in Equation (2) [15].
Equation (2)
This means an update rate of ~1 KHz is required for channel estimation and equalization.
The maximum delay spread for fixed broadband wireless is specified by the Stanford University Interim (SUI) channel model [17]. The worst-case rms delay spread corresponding to SUI-6 (Terrain Type A: hilly terrain with moderate-to-heavy tree densities) channel is 5.24 µs. The International Telecommunications Union (ITU-R) Vehicular Channel Model B [18] shows delay spread values of up to 20 µs for mobile environments. The subcarrier spacing design requires a flat fading characteristic for worst-case delay spread values of 20 µs with a guard time overhead of no more than 10% for a target delay spread of 10 µs. The coherence bandwidth of the channel (50% frequency correlation) corresponding to the 20 µs delay spread, given by Equation (3) [15], is shown to be approximately 10 KHz.
Equation (3)
This means that for delay spread values of up to 20 µs, multipath fading can be considered as flat fading over a 10 KHz subcarrier width.
An OFDM system is also sensitive to phase noise and the negative impact of impairment increases for narrower subcarrier spacing, which makes the design more expensive and complex.
The above rationale, based on the coherence time, Doppler shift, and coherence bandwidth of the channel, is the basis for the consideration of a scalable structure where the FFT sizes scale with bandwidth to keep the subcarrier spacing fixed.
Simulation results generated in [6] for a 2.5 MHz channel bandwidth when the FFT size is kept at 2048 shows a considerable amount of degradation in performance plot (Bit Error Rate vs. Signal to Noise Ratio) which is clearly recognizable for 64-QAM and high mobility.
Table 1: OFDMA scalability parameters Parameters Values
System bandwidth (MHz) 1.25 2.5 5 10 20
Sampling frequency
(Fs,MHz) 1.429 2.857 5.714 11.429 22.857
Sample time
(1/Fs,nsec) 700 350 175 88 44
FFT size
(NFFT) 128 256 512 1024 2048
Subcarrier frequency spacing 11.16071429 kHz
Useful symbol time
(Tb=1/Δƒ) 89.6 µs
Guard time
(Tg=Tb/8) 11.2 µs
OFDMA symbol time
(Ts=Tb+Tg) 100.8 µs
Without scalability, performance is reduced or cost is increased for low- and mid-size channel bandwidths.
Table 1 summarizes the main scalability parameters as recommended for adoption in the standard.
Note that in Table 1, the over-sampling factor used is 8/7 (Fs = floor(8/7 BW/0.008)x0.008) as globally specified in the standard for all OFDMA operations. The guard time can attain any of the four possible values 1/4, 1/8, 1/16 and 1/32. By setting the value to 1/8 of an OFDM symbol, a maximum of 11.2 µs delay spread can be tolerated with an overhead of around 10%.
WirelessMAN OFDMA supports a wide range of frame sizes (see Table 2) to flexibly address the need for various applications and usage model requirements. With a 2048 FFT size, the number of OFDM symbols in the short frame size, (e.g., 2 ms), will be very small for narrow bandwidths (less than 2 OFDM symbols for 1.25 MHz band) which makes the short frame sizes practically unusable (due to high overhead). Another advantage of scalability is to guarantee a lower bound on the number of OFDM symbols per frame (particularly a problem for small bandwidth and frame sizes).
Table 2: Scalable OFDMA frame sizes Frame Sizes
(msec) Frame Sizes
(OFDM symbols)
2 19
2.5 24
4 39
5 49
8 79
10 99
12.5 124
20 198
In the remainder of this paper, the following items are emphasized as the drivers of scalability and are revisited frequently.
Subcarrier spacing is independent of bandwidth.
The number of used subcarriers (and FFT size) should scale with bandwidth.
The smallest unit of bandwidth allocation, specified based on the concept of subchannels (to be defined later), is fixed and independent of bandwidth and other modes of operation.
The number of subchannels scales with FFT size rather than with the capacity of subchannels.
Tools are provided to trade mobility for capacity.
Note that fixing the capacity of the subchannel may not be the best choice especially for low-bandwidth systems where typical applications are different in nature.