Scalable OFDMA selected by WiMax Forum

在802.16-2005 section 8.4.2.3中規定Channel Bandwidth與sampling factor之間的關係, 由Channel Bandwidth(BW), number of used sub-carrier(Nused)與sampling factor可決定sub-carrier spacing, 在相同的sampling factor之下, sub-carrier spacing是固定的
WiMAX forum 為了提高設備實作時的互連性, 特別依據802.16-2004及802.16-2005再明確規範出各項必要及選項功能, 完整描述於WiMAX_Forum_Mobile_System_Profile Release 1.0的文件中,
OFDMA with TDD是System Profile Release 1的基本要求, 台灣選用2.5GHz屬於WiMAX forum定義的2.496GHz~2.69GHz Band Class 3, 移動台MS必須支援整個Band Class的frequency range, 基站台BS則可依BS Vendor宣告支援特定Band Class的部分frequecy range, 但至少需支援到最大channel bandwidth的3倍

Channel Bandwidth為1.25MHz, 5MHz, 10MHz, 20MHz時, 皆為1.25Hz的倍數, 查表得sampling factor n=28/25
Channel Bandwidth為1.75MHz, 7MHz, 8.75MHz時, 皆為1.75MHz的倍數, 查表得sampling factor n=8/7

在802.16-2004 Section 8.4.2.4提到sampling frequency Fs=floor(n*BW/8000)*8000
BW為5MHz的sampling frequency Fs=floor((28/25)*5/8000)*8000=5.6MHz
BW為10MHz的sampling frequency Fs=floor((28/25)*10/8000)*8000=11.2MHz

又sub-carrier frequency spacing可由此推算 deltaF=Fs/Nfft
所以在保持sub-carrier frequency spacing不變的條件下, 只要變動FFT size即可支援不同的channel bandwidth具OFDMA功能, 也就是scalable OFDMA
BW為5MHz若使用Nfft=512, 則BW為10MHz時, Nfft=1024, 而sub-carrier frequency spacing皆維持為deltaF=5.6/512=10.94kHz

Useful symbol time Tb= 1/deltaF, 所以BW為5MHz及10MHz的useful symbol time Tb=1/10.94=91.4usec

在Mobile WiMAX System Profile release 1的section 4.1.1.4中選定cyclic prefix(CP)的必要項目為1/8, 以保護傳送中因multipath fading造成的影響, 所以Guard Time Tg=Tb/8=91.4/8=11.4usec, 加上cyclic prefix 以後的OFDMA symbol duration Ts=Tb+Tg=91.4+11.4=102.9usec, 以Mobile WiMAX System Profile release 1的section 4.1.1.5規定frame length的必要項目為5ms, 因此一個frame可以有5/0.129=48個OFDMA symbol
The guard time can protect the multipath delay spread whose delay time is smaller than guard time. The common pratice about the delay spread in home is 50nsec and the delay spread in outdoors is 300nsec. The guard time in WiMAX is much larger than those common pratice delay spread values.

公式摘要:
Nused= number of used sub-carrier
Nfft= smallest power of 2 greater than Nused
Sampling Frequency Fs=floor(n*BW/8000)*8000
Sub-carrier frequency spacing deltaF=Fs/Nfft
Useful symbol time Tb=1/deltaF
Guard Time Tg=Tb*CP
OFDMA symbol duration Ts=Tb+Tg

使用OFDMA時, 當channel bandwidth變大時只要改變FFT size, 仍可維持相同的Sub-carrier frequency spacing及OFDMA symbol time
In other words, the basic principles of Scalable OFDMA are outlined as:

[1]Sub-carrier spacing is independent of channel bandwidth
[2]The number of sub-carrier scales with channel bandwidth
[3]The smallest unit of bandwidth allocation, based on the concept of subchannels, is fixed and indepedent of bandwidth and other modes of operation.
[4]The number of subchannels scales with bandwidth and the capacity of each individual subchannel remains constant.