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TDMA |
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| Aperto® uses an advanced version of the TDMA protocol, the closest relative of which is the IEEE 802.16 MAC protocol. The protocol makes extremely efficient use of the available bandwidth, which is responsible for Aperto’s higher spectral efficiency when compared to the competition. |
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| The structure of the TDMA/TDD framing used is shown in the figure. An unique feature of the frame structure is that it operates in the burst mode in both upstream and downstream directions. This allows the system to individually tailor and control the link parameters of each burst, on a SU by SU basis and thus enable Aperto’s OptimaLink® technology. For example: A packet can be transmitted to SU1 using QPSK modulation, more powerful FEC coding, vertical polarization and high transmit power level, while the very next packet can be transmitted to SU2 using 16QAM modulation, less powerful FEC, horizontal polarization and Lower transmit power. |
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| All data is carried in MAC layer packets called WPDUs (Wireless Protocol Data Units). Higher layer packets are concatenated and/or fragmented when they are encapsulated into the WPDUs for over-the-air transmission. This enables the MAC to transport multiple small packets, such as TCP ACKs, in a single WPDU burst, rather than spread over multiple bursts. The size of the WPDU is dynamically adapted as a function of current link conditions (smaller WPDUs for high error rate links). WPDUs are also the units used for re-transmission, in case of link errors. There are a number control packets shown in the figure: |
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- MAP packets: These occur in every frame, and carry information regarding the contents of the frame. The SUs use the MAP in the downstream direction to figure out which bursts are meant for them, so that they have to turn on their receivers only at burst start time. They use the MAP in the upstream direction to figure out when they are allowed to transmit.
- REQ packets: These are used in the contention mode by the SUs in the upstream direction, to request bandwidth. These slots are very small in size, so that even if collisions occur, not much bandwidth is wasted. Note that upstream data transmissions are never sent in the contention mode, they are always in the reserved mode.
- Link Maintenance packets: These are used by the BSU to periodically monitor the state of the link, and also for initial SU registration and ranging.
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| Multiple access in the upstream is provided by means of a request grant mechanism in which the SUs transmit in the special REQ slot to request upstream bandwidth from the BSU. If more than one SU transmits at the same time, then they collide, and then re-transmit after a random amount of backoff. The BSU controls the amount of backoff dynamically as a function of contention activity. When the BSU receives a REQ packet, it grants data slots to that SU in subsequent frames. The SU also piggybacks additional requests in data slots, to reduce the amount of contention traffic. Tests have shown that single REQ is all that it takes to transfer an entire file containing thousands of packets. |
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| The optimizations done in the Aperto MAC protocol have created a very robust and efficient upstream transport mechanism. This is crucial for deployments serving the SME market, where in some cases there is More Upstream Traffic as Compared to Downstream Traffic. |
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