Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN---7

Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN

RANGING IN OFDMA

The OFDMA PHY specifies a ranging allocation that can be used for ranging as well as bandwidth request. Initial and periodic ranging processes are supported to synchronize the SS’s with the BS at the initial network entry and also periodically during the normal operation. Bandwidth request mechanism is supported so that SS’s can request UL allocations for transmission of data to the BS. A set of 256 special pseudo-noise 144 bit-long ranging codes are divided into three groups for Initial Ranging, Periodic Ranging, and Bandwidth Requests, such that the BS can determine the purpose of the received code by the subset to which the code belongs. One or more groups of six adjacent subchannels are allocated to ranging where the ranging codes are BPSK modulated to the allocation. The SS randomly selects one code from the allocated set of codes and transmits back to the BS through ranging allocation. Different SS’s can collide on their ranging and/or bandwidth requests and the BS is still able to receive simultaneous requests.

To process an Initial Ranging request, a ranging code is repeated twice and transmitted in two consecutive OFDM symbols with no phase discontinuity between the two OFDM symbols (see Figure 8). This way, the BS can properly receive the requests from un-ranged SS’s with a larger value of synchronization mismatch when the first attempt is made to enter the network. The SS can optionally use two consecutive ranging codes transmitted during a four-OFDM symbol period (see Figure 9). This option decreases the probability of failure and increases the ranging capacity to support larger numbers of simultaneous ranging SS’s while at the same time it further increases the capability of the system to support larger numbers of synchronization mismatches.

Figure 8: Initial ranging transmission
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Figure 9: Initial ranging using two ranging codes
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Figure 10: Periodic ranging and bandwidth request transmission
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Figure 11: Periodic ranging and bandwidth request transmission using three codes
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For Periodic Ranging or Bandwidth Requests, the options are either to use one or three consecutive ranging codes transmitted during a one or three OFDM symbol period (see Figure 10 and Figure 11). In the case of three ranging codes, the probability of failure decreases at the same time as the ranging capacity increases, to support larger numbers of simultaneous ranging SS’s.