IEEE 802.11n is the expansion of the WLAN 802.11a/g standards for peak data rates up to 600Mbit/s.

IEEE 802.11n, like 802.11a/g, uses OFDM modulation but also MIMO (multiple-input and multiple-output) technology and also a special coding method are employed up to a bandwidth of 40MHz to increase the data rate.

This expansion providing higher data rates is also known as High Throughput (HT) mode. While the non-HT mode ensures backward compatibility with IEEE 802.11a/g.

It is useful to describe how 802.11n signals are generated.

In HT mode, the user data to be transmitted is specifically encoded and subsequently radiated via up to four transmitting antennas.

In the first step, the data is scrambled, encoded and split into a maximum of four spatial streams. Each stream is interleaved and then separately modulated by means of the BPSK, QPSK, 16QAM or 64QAM method.

Subsequently, space time block coding (STBC) may optionally be used to provide redundancy and an increased immunity of transmissions to interference. A cyclic shift to decorrelate the space-time streams is followed by spatial mapping, i.e. the distribution of the pre-encoded data to the respective OFDM carriers.

Channel sounding

The transmitter tries to optimise the spatial mapping with respect to the current transmission conditions, i.e. MIMO channel. To this end, channel sounding is first performed on the transmission channel. Now an inverse fast fourier transform (IFFT) follows to convert the signal from the frequency domain back into the time domain, and a guard interval (GI) is added to protect the signal from inter symbol interference (ISI) in case of multipath propagation. Finally, the signals are allocated to the different transmitting antennas.

The development and test of modules and equipment for compliance with IEEE 802.11n therefore requires, on the one hand, that all new functions provided in addition to conventional WLAN are tested in detail while, on the other, the correct implementation of backward compatibility must be ensured.

A few operating steps

The use of a dedicated IEEE 802.11n option for the signal generators can mean that only a few operating steps are necessary to prepare the generators for receiver tests - irrespective of whether frames with HT mode (up to 600Mbit/s) or without HT mode (up to 54Mbit/s) are to be generated. The information data to be transmitted (PPDU) may be voice or data, possible physical modes include legacy (backward compatibility with 802.11a/g), HT-mode mixed and HT-mode greenfield.

Available transmission modes include 20MHz, 40MHz, duplicate, upper and lower.

In the duplicate mode, the same carriers, with 90 degree phase shift, are transmitted on the two upper and lower 20MHz channels.

Spatial streams, space-time and extended spatial streams may be varied. Conventional PPDUs have a maximum length of 4095 bytes and HT PPDUs 65535 bytes. The raw data rate can be set between 6Mbit/s and 600Mbit/s depending on the spatial stream setup and data settings such as modulation, guard and channel coding. PLCP preamble, interleaver and time domain windowing may be enabled or disabled.

Considering the backward compatibility of IEEE 802.11n and this standard's manifold options for variation, the capability of generating signals that reflect this variety is of paramount importance.

The frame block configuration provides this capability. The user may cascade up to 100 frame blocks, i.e. groups of equally configured frames that will be continuously transmitted in sequence. This makes it possible to combine different configurations such as the HT mode and the conventional modes of 802.11a/g in order to scrutinise the receiver under any aspect.

By Simon Ache, an applications engineer from Rohde & Schwarz