Modulation vs. Multiplexing
Wireless communication spectrum is limited and strictly allocated, so the same bandwidth cannot be reused without coordination. To increase spectral efficiency, engineers developed various modulation and multiplexing techniques. These techniques enabled successive mobile generations such as 3G, 4G, and 5G. Which components inside a smartphone handle these functions?
Modulation and multiplexing are distinct concepts.
Digital signal modulation (ASK, FSK, PSK, QAM)
Digital modulation maps binary data (0 and 1) onto different analog waveforms. ASK uses amplitude to represent bits, FSK uses frequency, PSK uses phase, and QAM combines amplitude and phase to represent multiple bits per symbol.
Once each phone's antenna transmits waveforms carrying 0s and 1s, the next problem is how to distinguish which signals belong to which user in the shared air interface. That is the role of multiplexing.
Multiplexing (TDMA, FDMA, CDMA, OFDM)
Multiplexing separates electromagnetic waves among users. TDMA separates users by time slots, FDMA by frequency bands, CDMA by orthogonal spreading codes, and OFDM by orthogonal subcarrier frequencies.
Note that modulation and multiplexing are applied during digital signal processing. They are performed together as part of the same digital processing chain.
Digital Modulation and the Signal Chain
Modern mobile communication is digital. Analog speech or audio is first converted into binary data, which is then digitally modulated onto analog electromagnetic carriers for transmission. The simplified signal flow is shown in Figure 1.
Digital Communication System Architecture
The architecture of a digital communication system is outlined in Figure 2. A user may make voice calls or perform data communication. The main processing stages are described below.
Voice uplink (transmit)
Microphone captures analog low-frequency audio. An analog-to-digital converter (ADC) samples it into digital bits. The baseband processor (BB) performs encoding, CRC, channel coding, interleaving, ciphering, formatting, multiplexing, and modulation along with protocol control and I/O management.
After baseband processing, digital-to-analog conversion and RF upconversion produce a high-frequency analog RF signal. The RF chip then conditions and transmits the electromagnetic wave via the antenna.
Voice downlink (receive)
The antenna receives RF signals. The RF chip amplifies and downconverts the received RF to an intermediate or baseband analog signal, which is sampled by an ADC. The baseband processor performs demodulation, de-multiplexing, de-formatting, deciphering, de-interleaving, channel decoding, CRC check, and decompression. A final DAC converts the processed digital audio back to analog for playback by the earpiece.
Data communication
Data sessions are primarily digital, so signals typically enter the baseband processor directly for the same kinds of processing steps described above.
Note: Communication theory relies heavily on mathematics such as Fourier transforms, Laplace transforms, and discrete-time analysis. The system descriptions above are simplified schematic representations intended to convey core concepts; detailed implementations involve many additional complexities.
Communication-related Integrated Circuits: Baseband, IF, and RF
Baseband (BB)
The baseband is a digital integrated circuit responsible for digital signal processing: compression/decompression, channel coding/decoding, interleaving/de-interleaving, ciphering/deciphering, formatting, multiplexing/demultiplexing, modulation/demodulation, protocol management, and I/O control. These functions are typically integrated into a single system-on-chip (SoC). Major mobile baseband suppliers include Qualcomm, Broadcom, Marvell, and MediaTek.
Intermediate frequency (IF)
Because RF carrier frequencies are very high, earlier systems used an intermediate frequency stage to process signals at a lower frequency before final conversion to RF. Advances in direct-conversion (zero-IF) techniques have reduced the need for IF stages by allowing RF signals to be converted directly to baseband while meeting sensitivity and noise requirements, saving space and cost.
Radio Frequency (RF)
RF integrated circuits (RFICs) handle the high-frequency analog front end. Typical RF functions include the transceiver, low-noise amplifier (LNA), power amplifier (PA), band-pass filter (BPF), frequency synthesizer, and mixer. Implementations may use GaAs-based MESFET or HEMT, SiGe BiCMOS, or silicon CMOS technologies. Gallium nitride (GaN) is also used for high-power PAs in some designs. RF functionality can be spread across multiple ICs or integrated into a single SoC depending on design choices.
