Connect to 802.11n but Tx rate only 54mbps - Macbook Air mid 2012

My MBA connect to 5G wifi with 802.11n mode, but the transmission rate is only 54mbps. speedtest tells the same.

It happened after I reset network preference.

Anyone knows how to fix this problem?

Thanks.

en0:

Card Type: AirPort Extreme (0x14E4, 0xE9)

Firmware Version: Broadcom BCM43xx 1.0 (7.21.72.0.6a2)

MAC Address: 98:fe:94:3d:2a:3e

Locale: APAC

Country Code: X1

Supported PHY Modes: 802.11 a/b/g/n

Supported Channels: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, 161, 165

Wake On Wireless: Supported

AirDrop: Supported

Status: Connected

Current Network Information:

ROMA-5G:

PHY Mode: 802.11n

BSSID: e4:f4:c6:00:dd:a6

Channel: 161,-1

Country Code: X1

Network Type: Infrastructure

Security: WPA2 Personal

Signal / Noise: -29 dBm / -85 dBm

Transmit Rate: 54