Amplitude Modulation

Also known as “AM”, it is a method of adding information to an electronic signal in which the signal is varied by its height to impose information on it. The information being carried causes the amplitude (height of the sine wave) to vary. In the case of LANs, the change in the signal is registered by the receiving device as a 1 or a 0. A combination of these conveys different information, such as letters, numbers, punctuation marks, or control characters. In the world of modems, digital bit stream can be transmitted over an analog network by amplitude modulation, with the carrier frequency being modulated to reflect a 1 bit by a high amplitude sine wave (or series of sine waves) and a 0 bit with a low amplitude sine wave or (series of sine waves). The principal forms of Amplitude Modulation are:

· QDM: Double-band Amplitude Modulation
· QAM: Quadrature Amplitude Modulation
· SSB: Single-sideband Modulation
· VSB: Vestigal Sideband Modulation

Modulation

Now that we’ve seen a few of the multiplexing technologies which help divide wireless spectrum, let’s take a deeper look at how the spectrum is put to use via modulation techniques (overlaying voice for communication).

*Note: CDMA, TDMA, etc are typically looked upon as “channel access methods” but actually employ modulation techniques to transmit data (e.g. a CDMA signal being put onto a carrier)

Modulation: The process of varying some characteristic of the electrical carrier wave as the information to be transmitted on that carrier waves varies. In the wireless world, we first take a signal, such as a telephone conversation, and impress it upon a constant radio wave called a carrier. Once this is done, the voice signal varies (or modulates) this radio wave. The two (voice signal and carrier wave) go together over the air. A voice frequency in the audible range (which we can hear) thus modulates or varies a constant frequency in the radio range (which we cannot hear). Essentially, modulation makes voice band and radio band frequencies work together. Three types of modulation are commonly used for communications:

- Amplitude Modulation
- Frequency Modulation
- Phase Modulation
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And there are variations on these themes called Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). In the next few days, we will cover all of these types of modulation and their associated themes.

Frequency Division Multiple Access (FDMA): One of several technologies used to separate multiple transmissions over a finite frequency allocation. FDMA refers to the method of allocating a discrete amount of frequency bandwidth to each user to permit many simultaneous conversations. In cellular telephony, for example, each caller occupies approximately 25kHz of frequency spectrum. The cellular telephony frequency band, allocated from 824 MHz to 840 MHz and 869 MHz, consists of 416 total channels, or frequency slots, available for conversations. Within each cell, approximately 48 channels are available for mobile users. Different channels are allocated for neighboring cell sites, allowing for re-use of frequencies with a minimum of interference. This technique of assigning individual frequency slots, and re-using these frequency slots throughout the system, is known as FDMA.


The below picture depicts a visual comparison of how FDMA, TDMA, and CDMA allocate bandwidth.

*Note how FDMA follows the “Frequency” axis, TDMA the “Time” axis, and CDMA the “Code” axis.

Source: http://www.ce-mag.com/archive/2000/sepoct/0009c30a.jpg

Time Division Multiple Access (TDMA)

Now that we’ve seen how CDMA uses what can be referred to as a “spread spectrum” technology to allow users to share a bandwidth of frequencies, we will begin to explore other multiplexing concepts. Today’s term divides bandwidth access by time rather than a special coding scheme (CDMA).

Time Division Multiple Access (TDMA): A technique for transmitting a number of separate data, voice and/or video signals one after another. With Time Division multiplexing, you “sample” each voice conversation, interleave the samples, send them on their way, then reconstruct the several conversations at the other end. There are several ways to do the sampling. You can sample eight bits (one byte) of each conversation, or you can sample one bit. The former is called “word interleaving”; the latter “bit interleaving”. The basic goal of multiplexing – whether it be time division or any other form – is to save money, to cram more conversations (voice, data, video or facsimile) onto fewer phone lines. Basically, to substitute electronics for copper.




Picture Source: http://en.wikipedia.org/wiki/File:Tdma-frame-structure.png

The CDMA Development Group (CDG)

To wrap-up our section on CDMA, here is a little more explanation of the CDG:

The CDMA Development Group (CDG) is an industry consortium of companies who have come together to develop the products and services necessary to lead the adoption of CDMA wireless systems around the world. In working together, the 100 member companies will help ensure interoperability among systems, while expediting the availability of CDMA technology to consumers. The CDMA Development Group is committed to the definition of CDMA features, services, technical requirements and other activities that promote the availability and evolution of CDMA (IS-95 based) wireless systems worldwide. Specific objectives include:

· Leading the adoption of CDMA based systems around the world
· Maintaining a forum to address issues impacting manufacturers and carriers actively involved in CDMA deployments
· Developing next-generation CDMA systems
· Minimizing the time required to implement CDMA services and features

For more information on the CDG, you can visit http://www.cdg.org

CDMA2000 IXEV-DO (Evolution-Data Optimized)

A third-generation wireless protocol that is a stepping stone in the evolution of cdma2000. 1 XEV-DO "data optimized" is a data only overlay that uses a 1.25 MHz channel to provide a peak rate data throughput of 2.4 Mbps. EV-DO was designed as an evolution of the CDMA2000 (IS-2000) standard that would support high data rates and could be deployed alongside a wireless carrier's voice services. The back-end network is entirely packet-based, and thus is not constrained by the restrictions typically present on a circuit switched network and while the EV-DO feature of CDMA2000 networks provides access to mobile devices with forward link air interface speeds of up to 2.4 Mbit/s with Rev. 0 and up to 3.1 Mbit/s with Rev. A, the reverse link rate for Rev. 0 can operate up to 153 kbit/s, with Rev. A operating at speeds up to 1.8 Mbit/s. EVDO was designed to be operated end-to-end as an IP based network, and so it can support any application which can operate on such a network and bit rate constraints. Several Korean mobile operators have rolled out 1XEV-DO networks.

cdma2000

Code Division Multiple Access 2000: A third generation (3G) wireless system, cdma2000 is a trademark of Qualcomm, the company that commercialized CDMA. cdma2000 essentially is the CDMA approach to IMT-2000, the ITU's concept for a single, global standard for 3G wireless technology. Based on earlier CDMA versions (also known as TIA/EIA IS-95a and IS-95b), cdma2000 (also known as IS-2000) has been approved by the ITU. The initial version, known as cdma2000 1xMC (one times Multi-channel), offers 3G capabilities within a single standard 1.25 MHz channel, effectively doubling the voice capacity of the predecessor cdmaOne systems and offering data speeds up to 307 Kbps. The high-speed version is known officially as cdma2000 3xMC (Three times Multi-Carrier), as it makes use of three standard 1.25 MHz channels within a 5 MHz band to deliver data speeds up to 2 Mbps in support of integrated voice, data and video. cdma2000 runs in the 800 MHz and 1.8-2.0 GHz spectrum.