MIMO in 802 11n Wireless Networks Part-2

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In addition to MIMO in 11n, comparison with 11ac has been discussed in this article.

(D) Spatial Division Multiplexing (SDM)

One of the cornerstone features of 802.11n is Spatial Division Multiplexing:

• The transmitter sends different spatial streams simultaneously from each antenna
• Designed to optimize performance without requiring feedback from the receiver
• Channel state is inferred by assuming channel reciprocity (the channel behaves similarly in both directions)

(E) Cyclic Shift Diversity (CSD/CDD)

Another important technique in 802.11n:

• Implements transmit diversity by transmitting from each antenna with fixed phase shifts
• The receiver selects the best signal from among these transmissions
• Can be combined with Maximum Ratio Combining (MRC) for improved performance
• Also known as Cyclic Delay Diversity when used in this context

Feedback Mechanisms

Different MIMO techniques employ varying feedback requirements:

• SDM: No feedback required
• Beamforming (BF): Uses either implicit or explicit feedback
• MRC: No feedback required
• Space-Time Block Coding (STBC): No feedback required

Key MIMO Implementation Points

• MRC can be combined with CSD, SDM, or STBC
• SDM can be combined with STBC
• STBC has specific constraints:
  • Only works with even numbers of antennas (two per spatial stream)
  • All-or-nothing implementation – if any spatial stream uses STBC, all must use it
  • When combined with SDM, STBC halves the effective data rate
  • CSD can be combined with MRC

Beamforming Techniques

802.11n introduced two primary beamforming approaches:

Implicit Beamforming:

(Only in 802.11n, not carried forward to 802.11ac)

1. Beamformer requests: “Send me a sounding frame”
2. Beamformee responds with the sounding frame
3. Beamformer precodes transmissions assuming channel reciprocity

Explicit Beamforming

(Available in both 802.11n and 802.11ac)

1. Beamformer sends a sounding frame
2. Beamformee responds with channel state information (how it heard the frame)
3. Beamformer precodes transmissions based on this explicit feedback

802.11ac Advancements

• Operates exclusively in the 5GHz band (unlike 802.11n which supports both 2.4GHz and 5GHz)
• Builds on the same OFDM (Orthogonal Frequency Division Multiplexing) foundation as 802.11n
• Supports wider channel bandwidths: 20MHz → 40MHz → 80MHz → 160MHz or 80+80MHz
• Introduces 256QAM (Quadrature Amplitude Modulation) for higher data rates
• Theoretically supports up to 8×8 MIMO configurations, though practical implementations typically use 4×4 for APs and fewer for clients
• Uses VHT (Very High Throughput) MCS index 0-9
• Not all rate options are available for every MIMO combination
• Downlink Multi-User MIMO (DL MU-MIMO) capability
• Every packet uses A-MPDU (Aggregated MAC Protocol Data Unit) framing, even for single packet
• Common implementation of A-MSDU (Aggregated MAC Service Data Unit) within A-MPDU
• New encryption method: Galois/Counter Mode Protocol (GCMP)
• Introduced VHT TXOP (Transmission Opportunity) power save
• Added Extended Basic Service Set load element
• Supports dynamic bandwidth operation

Features Removed from 802.11n

Several features were deprecated in the transition to 802.11ac:

• Green Field preamble mode replaced with Legacy_VHT preamble
• Implicit beamforming removed in favor of explicit beamforming
• Reduced Interframe Space (RIFS) eliminated

Conclusion

The evolution from 802.11n to 802.11ac represents significant advancements in wireless networking technology. While 802.11n introduced crucial MIMO techniques and basic beamforming, 802.11ac built upon this foundation with wider channels, higher-order modulation, more sophisticated beamforming, and multi-user capabilities. Understanding these technologies is essential for network professionals designing, deploying, and troubleshooting modern Wi-Fi networks.