Modulation | Technologies

Modulation is the process of changing the amplitude, frequency, or phase of a radio frequency carrier signal with the information signal (such as voice or data). The 802.16 system uses different types of digital modulation depending on a variety of transmission factors. The modulation types used in 802.16 systems include binary phase shift keying (BPSK), quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM).

Binary Phase Shift Keying (BPSK)

Binary phase shift keying (BPSK) is a modulation process that converts binary bits into phase shifts of the radio carrier without substantially changing the frequency of the carrier waveform. The phase of a carrier is the relative time of the peaks and valleys of the sine wave relative to the time of an unmodulated “clock” sine wave of the same frequency. BPSK uses only twophase angles, corresponding to a phase shift of zero or a half cycle (zero or 180 degrees of angle). WiMAX uses BPSK modulation when a very robust signal is required.

Quadrature Phase Shift Keying (QPSK)

Quadrature phase shift keying (QPSK) is a type of modulation that uses 4 different phase shifts of a radio carrier signal to represent the digital information signal. These shifts are typically +/- 45 and +/- 135 degrees.

Quadrature Amplitude Modulation (QAM)

QAM is a combination of amplitude modulation (changing the amplitude or voltage of a sine wave to convey information) together with phase modulation. There are several ways to build a QAM modulator. In one process, two modulating signals are derived by special pre-processing from the information bit stream. Two replicas of the carrier frequency sine wave are generated; one is a direct replica and the other is delayed by a quarter of a cycle (90 degrees). Each of the two different derived modulating signals are then used to amplitude modulate one of the two replica carrier sinewaves respectively. The resultant two modulated signals can be added together. The result is a sine wave having a constant unchanging frequency while having an amplitude and phase that both vary to convey the information. At the detector or decoder the original information bit stream can be reconstructed. QAM conveys a higher information bit rate (bits per second) than a BPSK or QPSK signal of the same bandwidth, but is also more affected by interference and noise.

Figure 1 shows that amplitude and phase modulation (QAM) can be combined to form an efficient modulation system. One digital signal changes the phase and another digital signal changes the amplitude.

Figure 1: Quadrature Amplitude Modulation (QAM)

Adaptive Modulation

Adaptive modulation is the process of dynamically adjusting the modulation type of a communication channel based on specific criteria (e.g. interference or data transmission rate). WiMAX systems use adaptive modulation to ensure the modulation type matches the channel characteristics (signal quality level).

In general, the more efficient (data transmission capacity) the modulation type, the more complex or precise the modulation process is. The more precise the modulation process (smaller changes represent digital bits), the more sensitive the modulation is to distortion or interference. This usually means that as the data transmission rate increases, the sensitivity to interference intensifies. To help manage this process and ensure the maximum data transmission rate possible, 802.16 systems automatically change their data modulation types and data transmission rates (Autorate) based on the ability of the channel to transfer data. The 802.16 systems will usually try to send information at the highest data transmission rate possible. If the data transmission rate cannot be maintained, the 802.16 systems will attempt to transmit at the next lower data transmission rate. Lower data transmission rates generally use a less complex (more robust) modulation type.

Frequency Reuse | Technologies

Frequency reuse is the process of using the same radio frequencies on radio transmitter sites within a geographic area that are separated by sufficient distance to cause minimal interference with each other. Frequency reuse allows for a dramatic increase in the number of customers that can be served (capacity) within a geographic area on a limited amount of radio spectrum (limited number of radio channels). Frequency reuse allows WiMAX system operators to reuse the same frequency at different cell sites within their system operating area.

The number of times a frequency can be reused is determined by the amount of interference a radio channel can tolerate from nearby transmitters that are operating on the same frequency (carrier to interference ratio).

Carrier to interference (C/I) level is the amount of interference level from all unwanted interfering signals in comparison to the desired carrier signal. The C/I ratio is commonly expressed in dB. Different types of systems can tolerate different levels of interference dependent on the modulation type and error protection systems. The typical C/I ratio for narrowband mobile radio systems ranges from 9 dB (GSM) to 20 dB (analog cellular). WiMAX systems can be much more tolerant to interference levels (possibly less than 3 dB C/I) when OFDM and adaptive antenna systems are used.

WiMAX systems may also reuse frequencies through the use of cell sectoring. Sectoring is a process of dividing a geographic region (such as a radio coverage area) where the initial geographic area (e.g. cell site coverage area) is divided into smaller coverage areas (sectors) by using focusing equipment (e.g. directional antennas).

Figure 1.20 shows how radio channels (frequencies) in a WiMAX communication system can be reused in towers that have enough distance between them.

Figure 1.20: WiMax Frequency Reuse

The radio channel signal strength decreases exponentially with distance. As a result, mobile radios that are far enough apart can use the same radio channel frequency with minimal interference.

Orthogonal Frequency Division Multiple Access (OFDMA)

Orthogonal frequency division multiple access is the process of dividing a radio carrier channel into several independent sub-carrier channels that are shared between simultaneous users of the radio carrier. When a mobile radio communicates with an OFDMA system, it is dynamically assigned a specific sub-carrier channel or group of sub-carrier channels within the radio carrier. By allowing several users to use different sub-carrier channels, OFDMA systems increase their ability to serve multiple users and the OFDMA system may dynamically allocate varying amounts of transmission bandwidth based on how many sub-carrier channels have been assigned to each user.

As demonstrated in Figure 1, the WiMAX system allows more than one simultaneous user per radio channel through the use of orthogonal frequency division multiple access (OFDMA). The WiMAX radio channel can be divided into multiple sub-carriers and that the sub-carriers can be dynamically assigned to multiple users who are sharing a radio carrier signal. Finally, the data rates that are provided to each user can vary based on the number of subcarriers that are assigned to each user.

Figure 1: Orthogonal Frequency Division Multiple Access (OFDMA)