A Joint Design of PHY and MAC layers for MIMO multi-hop networks

Just as MIMO can benefit cellular networks, multi-hop networks can also benefit from MIMO technology. One attractive aspect of MIMO communication is that the spatial degrees of freedom can be leveraged to support point-to-multipoint and multipoint-to-point transmission. This can be exploited in both physical layer and higher layer protocols in multi-hop networks. MIMO can be used to help the medium access control protocol (MAC) avoid the hidden node problem. It can also be used to support multiple simultaneous connections through space-time interference cancellation or multicast, thereby facilitating spatial reuse.

Fully exploiting MIMO capability in a mobile multi-hop network requires a flexible physical layer to provide various modes of operation to exploit the diversity and capacity advantages as well as a MAC protocol that can efficiently choose from the multitude of modes of operation. More critically, interaction between those two layers for better choice is the key for performance enhancement benefited from MIMO technology. Conventional approaches for designing MAC protocol, however, cannot achieve this goal. Therefore, a cross-layer approach of jointly design physical layer MIMO transceiver and MAC layer protocol must be taken.

Select Results

We have developed a new MAC protocol for MIMO-OFDM multi-hop networks. The protocol supports simultaneous communication by allowing users to contend for one of several spatial transmission slots. It leverages the previously developed MIMA-MAC and exploits the ability of multiple receive antennas to cancel multiple data streams, even when the streams are not synchronized. Joint physical and higher layer simulations confirm good error rate performance and throughput versus similar non-MIMO 802.11 style MAC protocols.

We have developed a space-time receiver structure that allows us to cancel asynchronous MIMO-OFDM interference. This is important because MAC protocols that vary the number of streams sent to different users need a physical layer that can cancel asynchronous interference with different frame timing and frequency offsets.

We have developed a protocol that employs spatial multiplexing with antenna subset selection for data packet transmission, while using the Alamouti space-time code for control of packet transmission. The proposed protocol outperforms the previously developed MIMA-MAC protocol, which does not use an antenna selection scheme, as well as a hybrid of the IEEE 802.11 MAC protocol with Alamouti encoding.

Future Research

We are aggressively researching several aspects of multi-hop networking with an emphasis on how to measure network performance and how to leverage MIMO communication in practical protocols.

• What could conventional ad-hoc networking problems be translated into MIMO multi-hop networking?
  - Enhancing end-to-end low throughput by MIMO-aware MAC protocols
  - Suppression of the hidden terminal using adaptive stream control
  - Alleviation of fading to make communication more reliable thanks to adaptive MIMO
  - Efficient handshaking procedures for incoming/outgoing nodes via out-band control channels
   
• How can state-of-the-art MIMO technologies be exploited to achieve higher performance?
  - Enhancement of the communication link using antenna selection and multi-mode adaptation to provide additional spatial diversity in the communication link.
  - Mechanisms for utilizing spatial reuse that enables concurrent transmissions
  - Limited feedback precoding and efficient handshaking method for feedback information required for closed-loop MIMO
  - Exploitation of MIMO broadcast for multiple access control
  - Adaptive precoding to achieve higher spatial reuse and channel adaptation    

Select Publications

T. Tang and R. W. Heath, Jr. ``A Space-Time Receiver for MIMO-OFDM Ad Hoc Networks,'' Proc. of the Military Communications Conference, pp. 1-6, Atlantic City, NJ, Oct. 17-20, 2005.

T. Tang, M. Park, R. W. Heath, Jr., and S. Nettles, ``A Joint MIMO-OFDM Transceiver and MAC Design for Mobile Ad Hoc Networking,'' Proc. of the International Workshop on Wireless Ad-Hoc Networks, pp. 315-319, Oulu, Finland, May 31 - June 3, 2004.

M. Park, S. Nettles and R. W. Heath, Jr., ``Improving Throughput and Fairness for MIMO Ad Hoc Networks Using Antenna Selection Diversity,'' Proc. of the IEEE Global Telecommunications Conf., vol. 1, pp. 3363-3367, Dallas, TX, Nov. 29 - Dec. 3, 2004.

T. Tang and R. W. Heath, Jr., ``Space-Time Interference Cancellation in MIMO-OFDM Systems,'' IEEE Trans. on Veh. Tech., vol. 54, no. 5, pp. 1802-1816, September 2005.