Two methods are used for OTDM: one involves bit interleaving and the second uses packet interleaving.

Bit interleaving:

i. Bit interleaving is performed across the bits within an OFDM symbol and across at most three OFDM symbols.

a) Exploits frequency diversity.

b) Randomizes any interference, interference looks nearly white.

c) Latency is less than 1µs.

ii. Bit interleaving is performed in three stages:

a) First, $3N_{CBPS}$ coded bits are grouped together.

b) Second, the coded bits are interleaved using a N_CBPS block symbol interleaver.

c) Third, the output bits from 2nd stage are interleaved using a $(N_{CBPS}/10) \times 10$ block tone interleaver.

d) The end results are that the $3N_{CBPS}$ coded bits are interleaved across 3 symbols and within each symbol.

iii. If there are less than $3N_{CBPS}$ coded bits, which can happen at the end of the header or near the end of a packet, then the second stage of the interleaving process is skipped.

Packet interleaving:

i. Optical signals representing data streams from multiple sources are interleaved in time to produce a single data stream.

ii. The interleaving can be done on a packet by packet basis as shown in the Figure 5.5.

iii. If the packets are of fixed length, the recognition of packet boundaries is much simpler i.e. fixed length packets are used.

iv. In packet-interleaved case, framing pulses can be used. These framing pulses mark the boundary between the packets.

Fig5.4: An Optical Multiplexer to create the bit interleaved TDM stream

Fig5.5: Function of a packet-interleaved optical multiplexer

v. Assume if n data streams are to be multiplexed and the bit period of each these streams is T. Also framing pulses are used then the inter pulse width is $τ = T/n+1$ because n+1 pulses must be transmitted in each bit period.

vi. There is a guard time between the successive pulses. One purpose of this guard time is to provide some tolerance in the multiplexing and demultiplexing operations.

vii. In this packet interleaved case, the data stream externally modulates a periodic stream of narrow pulses. If the bit interval is T the separation between successive pulses is also T.

viii. In this way, we have to reduce the interval between successive pulses to τ, a high-rated multiplexed data signal. This is done by passing through the initial sequence through a series of compression stages as shown in Figure 5.6.

Fig5.6: An Optical Multiplexer to create the packet interleaved TDM stream