Dealing with Interference with WiMAX

Interference—Some Assumptions

The primary objection to wireless systems is the concern that there are or will soon be too many operators on the same frequency, which will cause so much interference that the technology will become unusable. This issue is not that simple.

Such an assumption relies largely on the use of unlicensed spectrum, where, according to Larry Lessig's "tragedy of the commons" scenario, multiple operators broadcast on the same unlicensed (read "free") spectrum, ultimately rendering it useless. Although this scenario may already be evident in the case of Wi-Fi variants (largely limited to the 2.4 GHz range), WiMAX is considerably different. WiMAX currently has no problems, only solutions.

Since 1927, interference protection has always been at the core of federal regulators' spectrum mission. The Radio Act of 1927 empowered the Federal Radio Commission to address interference concerns. This act primarily focused on three parameters: location, frequency, and power. The technology of the time did not permit consideration of a fourth element: time. In the modern sense, one might consider that a spectrum used by cell phones in a metropolitan area (dense population with millions of users) would command a very high price at a spectrum auction. At the other end of the "spectrum," a frequency band, say 2.5 GHz, in an exurban or rural market may go for very little money at an auction or at resell by a spectrum broker. It is entirely possible that the wireless service provider may find a very low cost licensed spectrum and enjoy a protected spectrum, which will largely negate the concern over interference from other broadcasters (the purpose of the Radio Act of 1927 in the first place).

What OFDM Means to WiMAX

To the telecommunications industry, an WiMAX OFDM-based system can squeeze a 72 Mbps uncoded data rate (~100 Mbps coded) out of 20 MHz of channel spectrum. This translates into a spectrum efficiency of 3.6 bps per Hz. If five of these 20 MHz channels are contained within the 5.725 to 5.825 GHz band, giving a total band capacity of 360 Mbps (all channels added together with 1 ×frequency reuse). With channel reuse and through sectorization, the total capacity from one BS site could potentially exceed 1 Gbps.

OFDM has manifold advantages in WiMAX, but among the more notable advantages is greater spectral efficiency. This is especially important in licensed spectrum use, where bandwidth and spectrum can be expensive. Here, OFDM delivers more data per spectrum dollar. In unlicensed spectrum applications, OFDM mitigates interference from other broadcasters due to its tighter beam width (less than 28 Mhz) and guardbands, as well as its dispersal of the data across different frequencies so that if one flow is "stepped on" by an interfering signal, the rest of the data is delivered on other frequencies.

QoS: Error Correction and Interleaving

Error correcting coding builds redundancy into the transmitted data stream. This redundancy allows bits that are in error or even missing to be corrected. The simplest example would be to simply repeat the information bits. This is known as a repetition code. Although the repetition code is simple in structure, more sophisticated forms of redundancy are typically used because they can achieve a higher level of error correction. For OFDM, error correction coding means that a portion of each information bit is carried on a number of subcarriers; thus, if any of these subcarriers has been weakened, the information bit can still arrive intact.

Interleaving is the other mechanism used in OFDM systems to combat the increased error rate on the weakened subcarriers. Interleaving is a deterministic process that changes the order of transmitted bits. For OFDM systems, this means that bits that were adjacent in time are transmitted on subcarriers that are spaced out in frequency. Thus errors generated on weakened subcarriers are spread out in time; that is, a few long bursts of errors are converted into many short bursts. Error correcting codes then correct the resulting short bursts of errors.

QPSK Versus QAM

Rather than attempting to be all things to all subscribers, WiMAX delivers a gradation of QoS dependent on distance of the SS from the BS: The greater the distance, the lower the guarantee of QoS. WiMAX utilizes three mechanisms for QoS; from highest to lowest, these mechanisms are 64-QAM, 16-QAM, and QPSK. Figure 1 illustrates modulation schemes.

Figure 1: Modulation schemes focus the signal over distance.

By using a robust modulation scheme, WiMAX delivers high throughput at long ranges with a high level of spectral efficiency that is also tolerant of signal reflections. Dynamic adaptive modulation allows the BS to trade throughput for range. For example, if the BS cannot establish a robust link to a distant subscriber using the highest order modulation scheme, 64-QAM, the modulation order is reduced to 16-QAM or QPSK, which reduces throughput and increases effective range. Figure 2 demonstrates how modulation schemes ensure throughput over distance.

Figure 2: Modulation schemes ensure a quality signal is delivered over distance by decreasing throughput.

QPSK and QAM are the two leading modulation schemes for WiMAX. In general the greater the number of bits transmitted per symbol, the higher the data rate is for a given bandwidth. Thus, when very high data rates are required for a given bandwidth, higher-order QAM systems, such as 16-QAM and 64-QAM, are used. 64-QAM can support up to 28 Mbps peak data transfer rates over a single 6 MHz channel. However, the higher the number of bits per symbol, the more susceptible the scheme is to intersymbol interference (ISI) and noise. Generally the signal-to-noise ratio (SNR) requirements of an environment determine the modulation method to be used in the environment. QPSK is more tolerant of interference than either 16-QAM or 64-QAM. For this reason, where signals are expected to be resistant to noise and other impairments over long transmission distances, QPSK is the normal choice.

Multiplexing in OFDM

As shown in Figure 3, an efficient OFDM implementation converts a serial symbol stream of QPSK or QAM data into a size M parallel stream. These M streams are then modulated onto M subcarriers via the use of size N (N ≤ M) inverse FFT. The N outputs of the inverse FFT are then serialized to form a data stream that can then be modulated by a single carrier. Note that the N-point inverse FFT could modulate up to N subcarriers. When M is less than N, the remaining N−M subcarriers are not in the output stream. Essentially, these have been modulated with amplitude of zero.

Figure 3: Block diagram of a simple OFDM transmitter

Although it would seem that combining the inverse FFT outputs at the transmitter would create interference between subcarriers, the orthogonal spacing allows the receiver to perfectly separate out each subcarrier. Figure 4 illustrates the process at the receiver. The received data is split into N parallel streams that are processed with a size N FFT. The size N FFT efficiently implements a bank of filters, each matched to N possible subcarriers. The FFT output is then serialized into a single stream of data for decoding. Note that when M is less than N, in other words fewer than N subcarriers are used at the transmitter, the receiver only serializes the M subcarriers with data.

Figure 4: Block diagram of a simple OFDM receiver