Objectives of 802.11n standard
This section briefly discusses the objectives of the 802.11n system and the challenges in attaining them are discussed. They are:
A. Achieving higher data rate
B. Backward compatibility with the legacy devices (IEEE 802.11b/g)
A. Achieving higher data rate
The main objective of the 802.11n system is to achieve higher data rates in a multipath fading channel. One of the ways suggested in the 802.1ln standard is the use of MIMO-OFDM technology.
The increase in demand on the data rate capabilities of wireless systems necessitated an increase in bandwidth and signaling rate. As the bandwidth increases, the multipath distortion or frequency-selective fading caused by the physical medium becomes worse.
The multipath channel causes a time dispersion of the transmitted signal resulting in the overlap of the various transmitted symbols at the receiver. This is referred to as intersymbol interference (ISI), which, if left uncompensated, causes high error rates.
One of the solutions to the ISI problem is the use of the OFDM technique proposed by J. A. C, Bingham 1990. In OFDM systems:
- The high rate transmit signal is divided into many lower rate sub streams and each sub stream is modulated by orthogonal carriers.
- Then all are added to obtain a serial stream and transmitted.
- Due to this division, the bandwidth occupied by each sub stream will be less compare to the total bandwidth and this converts the frequency selective fading channel to flat fading channel. Hence, an ISI free scenario is obtained.
- Moreover, the signal of duration equal to delay spread of the channel is taken from the last part of modulated signal and appended in front to the modulated signal. This is called cyclic prefix (CP). Therefore the ISI between the OFDM symbols can be completely eliminated through the use of a cyclic prefix. Cyclic prefix also helps in maintaining the orthogonality between the carriers at the receiver in multipath channel.
- The whole system can be realized using an Inverse fast Fourier transform (IFFT) block at the transmitter and Fast Fourier transform (FFT) at the receiver. At the receiver, FFT reduces the multipath channel impulse response into a multiplicative constant with the transmit signal on a tone-by-tone basis. So each tone can be equalized independently and the complexity of equalizer is eliminated.
A typical OFDM transmitter and OFDM receiver is shown in Figure 1 and in Figure 2. OFDM has been adopted as the modulation scheme in 802.11a and 802.11g systems to achieve the maximum rate of 54 Mbps.
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Figure 1:Typical OFDM transmitter.
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Figure 2:Typical OFDM receiver.
To increase the data rate further in the multipath channel, MIMO technology is used along with OFDM. This is called as MIMO-OFDM technology and is used in 802.11n system. Multiple independent streams are transmitted simultaneously to increase the data rate. A typical MIMO system is shown Figure 3.
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Figure 2:Diagram of a MIMO system.
In a MIMO-OFDM transmitter, a vector is transmitted in each tone with multiple transmit antennas. At the receiver, the signal at each RX antenna will have signal from all the transmit signal coming from different channels.
After FFT, the channel frequency response will be a matrix in each tone. The receive vector in each tone vector will be the matrix multiplied by the transmit vector. Then spatial detection is performed on the receive vector of each tone to equalize for the channel and separate the transmit signals.
Multipath remains an advantage for a MIMO-OFDM system since frequency selectivity caused by multipath improves the rank distribution of the channel matrices across frequency tones, thereby increasing capacity. A typical MIMO-OFDM transmitter and receiver are shown in Figures 4 and 5, respectively.
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Figure 4: MIMO-OFDM transmitter.
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Figure 5: MIMO-OFDM receiver.
Ultimately, the increase in number of antennas in the transmitter and at the receiver creates many system implementation issues. In the system implementation side, the use of MIMO-OFDM technology in 802.11n system requires multiple radio frequency (RF) and baseband (BB) chains.
There must be at least as many chains as independent data steams at the transmitter and at the receiver. The introduction of parallelism in the data streams to increase the data rate requires parallel blocks in each BB and RF for each stream to be processed.
This complexity in turn increases the power consumption and area. Apart from these hardware and cost complexities, there is a need for low complexity and robust receiver's tasks due to the introduction of the new preamble which operates for both the legacy and MIMO stations.
This is because the initial receiver tasks such as estimation of the receive power for automatic gain control (AGC), start of packet (SOP) detection, coarse time offset, coarse frequency offset, and fine time offset depends on the structure of the preamble.
The other implementation issue in MIMO-OFDM systems is the spatial detection algorithm to be used at the receiver. The receive signal in each RX antenna is a superposition of signals coming from all the transmit antennas. To separate them at the receiver, a spatial detector is employed.
The spatial detection is done in subcarriers and the complexity increases as the number of subcarrier increase. There are different types of spatial detectors with different computational complexity and performance. There exists a tradeoff between the complexity of the detection technique and its performance.
About the author
Sathish Viswanathan is working for an MNC in the area of HSUPA and HSDPA (NodeB L1). He completed his master's in wireless communication with a specialization in PHY layer of WLAN at AU-KBC reserach centre, India. There, he worked in time and frequency synchronization aspects of WLAN systems and MIMO-OFDM.
Part 2: Backward compatibility with the legacy devices (IEEE 802.11b/g).