Backward compatibility with the legacy devices (IEEE 802.11b/g)
The typical 802.11n network consists of legacy 802.11b/g enabled systems and new 802.11n enabled MIMO-OFDM systems. The channel access scheme used in both the systems is CSMA/CA which mandates the interoperability and backward compatibility across these standards.
Based on the transmission from/to the MIMO-OFDM system, we can arrive at two challenges in achieving this goal.
Case 1: MIMO-OFDM transmission and legacy reception:
When the receiver is a legacy station, then the MIMO-OFDM system will utilize only one transmit antenna and uses the same burst structure as 802.11a. The other MIMO stations uses its multiple receive antennas efficiently by exploiting receive diversity. Then the packet is decoded and the channel is deferred for the MIMO-legacy transmission to progress without collisions. If there is a transmission from the legacy systems, the MIMO stations can be efficiently used as mentioned above.
Case 2: MIMO-OFDM transmission and reception: :
When the intended receiver is a MIMO station in the presence of legacy station, then the transmitted signal should be in such a way that the legacy stations should understand and defer the channel. To achieve this objective, either the same legacy frame format can be used for MIMO transmission or a special preamble structure can be used.
The challenge here is that a simple extension of the legacy preambles to MIMO system may be helpful in achieving backward compatibility but may not provide good performance because of the beamforming effects.
Similarly, the use of special preambles designed for MIMO systems can work well for MIMO stations but fails with respect to backward compatibility. Hence, preamble design is an important challenge to achieve interoperability with legacy stations and better performance in MIMO stations.
From the above discussions, we see that to provide backward compatibility and protection with the use of new preamble, new receiver algorithms have to be defined.
Since the typical 802.11n network has legacy stations and new MIMO stations, and CSMA/CA MAC is used, there should be certain ways for these stations to understand each other and protect themselves from the interference created by each other. The 3 main proposals for the 802.11n standard had specified different protection mechanisms at the PHY and MAC level. In the PHY layer level, a special preamble and header is sent when MIMO-OFDM transmission happens in the presence of the legacy stations.
This makes the legacy station to defer the medium for MIMO-OFDM traffic. In the MAC layer, protection is done in two ways. One way is to provide protection using conventional network allocation vector (NAV) mechanism and the other way is to provide protection using spoofing wherein the PHY layer convergence function (PLCP) header part of the frame is modified.
The length field which gives the length of the payload in octets and the rate field which specifies the rate at which the payload is transmitted are changed suitably. One can use the rate and length field to calculate the duration of the packet. To provide protection, the rate field is kept at the lowest rate (say 6 Mbps in OFDM mode).
The legacy stations receiving this packet calculate the duration field and tunes the RF to receiver mode and receives till this duration. So there will be no interference from the legacy stations for the MIMO-OFDM traffic. When legacy stations transmit, the MIMO stations can receive the signal through different receive antennas and can easily decode the header to defer the channel access.
From the above discussions, we see that to provide backward compatibility and protection with the use of new preamble, new receiver algorithms have to be defined. Also, the receiver algorithms should be robust and simple to implement.