WiMAX Physical Layer (PHY)




Download 343,31 Kb.
Pdf ko'rish
bet49/55
Sana24.05.2024
Hajmi343,31 Kb.
#252539
1   ...   45   46   47   48   49   50   51   52   ...   55
Bog'liq
garg2007

21.15.1 WiMAX Physical Layer (PHY)
The 802.16 PHY supports TDD and full and half duplex FDD operations; how-
ever, the initial release of mobile WiMAX only supports TDD. Other advanced 
PHY features include adaptive modulation and coding (AMC), hybrid automatic 
repeat request (HARQ) and fast channel feedback (CQICH) to enhance coverage 
and capacity of WiMAX in mobile applications.
For the bands in the 10–66 GHz range, 802.16 defi nes one air interface called 
Wireless MAN — SC. The PHY design for the 2–11 GHz range (both licensed and 
unlicensed bands) is more complex because of interference. Hence, the standard 
supports burst-by-burst adaptability for modulation and coding schemes and spec-
ifi es three interfaces. The adaptive features at the PHY allow trade-off between 
robustness and capacity. The three air interfaces for the 2–11 GHz range are:
Wireless MAN — SCa uses single carrier modulation.
Wireless MAN — OFDM uses a 256-carrier OFDM. This air interface provides 
multiple access to different stations through time-division-multiple access.
Wireless MAN — OFDM uses a 2048-carrier OFDM scheme. The interface 
provides multiple access by assigning a subset of the carriers to an individual 
receiver.







Ch21-P373580.indd 770
5/3/07 10:58:50 PM


Support for QPSK, 16-QAM, and 64-QAM are mandatory in the downlink 
with mobile WiMAX. In the uplink 64-QAM is optional. Both convolutional 
code and turbo code with variable code rate and repetition coding are supported. 
The combinations of various modulation and code rates provide a fi ne resolution 
of data rates. The frame duration is 5 ms. Each frame has 48 OFDM symbols with 
44 OFDM symbols available for data transmission.
The base station (BS) scheduler determines the appropriate data rate for 
each burst allocation based on the buffer size, channel propagation conditions 
at the receiver, etc. A channel quality indicator (CQI) channel is used to provide 
channel state information from the user terminals to the BS scheduler. 
WiMAX provides signaling to allow fully asynchronous operation. The 
asynchronous operation allows variable delay between retransmissions which 
gives more fl exibility to the scheduler at the cost of additional overhead for each 
retransmission. HARQ combined with CQICH and AMC provides robust link 
adoption in the mobile environment at vehicular speeds in excess of 120 km/h.

Download 343,31 Kb.
1   ...   45   46   47   48   49   50   51   52   ...   55




Download 343,31 Kb.
Pdf ko'rish