High-speed Wireless LAN for Broadband Wireless Home Networks

3.1 Interference control between home networks

Problems caused by interference between home networks due to the spread of WLAN is becoming apparent in public areas such as train stations and in dense residential areas such as areas with many apartments, where neighboring APs use the same channel because of the paucity of channels. In the IEEE 802.11ac specifications, as mentioned above, the number of channels will be reduced to 2 or 4 because the channel width is 80 MHz (mandatory) or 160 MHz (optional), which means that interference between home networks is likely to occur in the 5-GHz band in the near future. Given that two APs using the same channel are adjoining, each home network will have only half the average throughput of conventional WLAN because the channel is shared by time division multiple access based on the carrier sense multiple access with collision avoidance (CSMA/CA) protocol (Fig. 1).

Fig. 1. Interference control between home networks.

We are developing an interference control technique as one solution to the interference problem. This technique links multiple APs by network transmission signals to achieve the cooperation of the APs via the beamforming technology of multi-user multiple-input multiple-output (MU-MIMO) transmission.

MU-MIMO, which is a new technology for wireless broadband communication, is expected to be adopted in IEEE 802.11ac as an option (Fig. 2) [1]. It achieves high frequency-utilization efficiency by sending data to multiple stations (STAs) over a single channel at the same time by controlling the directional characteristics of the radio signals by using beamforming control technology so the signals do not interfere with each other. We developed a wireless transmission prototype that employs MU-MIMO transmission, and succeeded in transmitting data in real time to six STAs at more than 1 Gbit/s for the first time in the world (Fig. 3) [2]–[4].

Fig. 2. Multi-user MIMO technique.

Fig. 3. Multi-user MIMO transmission prototype.

The interference control between home networks is an extension of the beamforming technology of MU-MIMO. An AP sends data to multiple STAs in its home network without letting signals interfere with the neighboring home network by controlling the directional characteristics of the radio signals, so that two APs in each home network can send data to their STAs over the same channel at the same time without interference. As a result, the throughput is approximately double that of the conventional CSMA/CA method (Fig. 4) [5].

Fig. 4. Prototype of interference control system between home networks.

3.2 Multi-user multi-channel control

APs specified in IEEE 802.11ac can perform high-speed communication using the 160-MHz-bandwidth channel as an option. In addition, to ensure compatibility with the conventional IEEE 802.11a and 802.11n protocols, the APs communicate with IEEE 802.11a-specified STAs using 20-MHz-bandwidth channels and with IEEE 802.11n STAs using channels up to the 40-MHz-bandwidth channel. Therefore, when an IEEE 802.11ac-specified AP uses a 20-MHz-bandwidth channel to transmit data to an IEEE 802.11a STA, the frequency utilization efficiency degrades because the 140-MHz band is unused.

As one solution, we are developing a multi-user multi-channel control technique in which the AP allocates a communication channel to each STA using 20-MHz-bandwidth units and transmits data to each STA simultaneously by frequency division multiplexing (Fig. 5). This new technique achieves efficient use of frequency resources by transmitting data to each STA using 20-MHz- and 120-MHz-bandwidth channels at the same time. If there is only one active STA in a home network, the AP may use just the 20-MHz-bandwidth channel to communicate with the STA. In this case, by synchronizing the timing of data transmission with the neighboring AP, the AP and the neighboring AP can transmit data via the 20-MHz- and 120-MHz-bandwidth channels at the same time.

Fig. 5. Multi-user multi-channel control.

3.3 Network control for wireless LAN

As mentioned above, the WLAN uses the CSMA/CA method as the medium access control scheme. Because CSMA/CA is based on autonomous distributed control, WLAN users can build a wireless home network simply by using the AP; the channels are selected freely by the users. However, in dense residential areas such as neighborhoods with many apartment buildings, a problem may occur where optimal allocation of channels used by each AP is not performed, and frequency resources are not used effectively.

One solution that we are working on is a network control technique for WLAN in which an interference management engine is set on the broadband network. This engine collects information on the radio environment such as channel utilization rates from each AP, calculates the optimal allocation of channels, and informs each AP of the appropriate channel (Fig. 6). This technique enforces optimal channel usage automatically. In addition, techniques to provide a synchronization signal from the Ethernet-based network, called synchronous ethernet (SyncE), and precision time protocol (PTP), have been standardized. These techniques can achieve frequency, phase, and time synchronization between APs, which is expected to make interference control between home networks more accurate by synchronizing the frequency and phase of radio signals transmitted by the APs.

Fig. 6. Network control for wireless LAN.