An introduction to the access technologies
that allow multiple users to share a common communications channel.
Access
methods are multiplexing techniques that provide communications services to
multiple users in a single-bandwidth wired or wireless medium. Communications
channels, whether they’re wireless spectrum segments or cable connections, are
expensive. Communications services providers must engage multiple paid users
over limited resources to make a profit. Access methods allow many users to
share these limited channels to provide the economy of scale necessary for a
successful communications business. There are five basic access or multiplexing
methods: frequency division multiple access (FDMA), time division multiple
access (TDMA), code division multiple access (CDMA), orthogonal frequency
division multiple access (OFDMA), and spatial division multiple access (SDMA).
HDMI
Encoder Modulator,16in1 Digital Headend, HD RF Modulator at Soukacatv.com
SKD3013 3 Channel HD Encode Modulator
SKD121X Encoding & Multiplexing Modulator
Household Universal Encoding & Modulation Modulator
SKD19 Series 1U Rack 12CH Encode Modulator
FDMA
FDMA is the process
of dividing one channel or bandwidth into multiple individual bands, each for
use by a single user (Fig. 1). Each individual band or channel is
wide enough to accommodate the signal spectra of the transmissions to be
propagated. The data to be transmitted is modulated on to each subcarrier, and
all of them are linearly mixed together.
1. FDMA divides the
shared medium bandwidth into individual channels. Subcarriers modulated by the
information to be transmitted occupy each subchannel.
The best example of
this is the cable television system. The medium is a single coax cable that is
used to broadcast hundreds of channels of video/audio programming to homes. The
coax cable has a useful bandwidth from about 4 MHz to 1 GHz. This bandwidth is
divided up into 6-MHz wide channels. Initially, one TV station or channel used
a single 6-MHz band. But with digital techniques, multiple TV channels may
share a single band today thanks to compression and multiplexing techniques
used in each channel.
This
technique is also used in fiber optic communications systems. A single fiber
optic cable has enormous bandwidth that can be subdivided to provide FDMA.
Different data or information sources are each assigned a different light
frequency for transmission. Light generally isn’t referred to by frequency but
by its wavelength (λ). As a result, fiber optic FDMA is called wavelength
division multiple access (WDMA) or just wavelength division multiplexing (WDM).
One of the older FDMA systems is the original
analog telephone system, which used a hierarchy of frequency multiplex
techniques to put multiple telephone calls on single line. The analog 300-Hz to
3400-Hz voice signals were used to modulate subcarriers in 12 channels from 60
kHz to 108 kHz. Modulator/mixers created single sideband (SSB) signals, both
upper and lower sidebands. These subcarriers were then further frequency
multiplexed on subcarriers in the 312-kHz to 552-kHz range using the same
modulation methods. At the receiving end of the system, the signals were sorted
out and recovered with filters and demodulators.
Original aerospace telemetry systems used an
FDMA system to accommodate multiple sensor data on a single radio channel.
Early satellite systems shared individual 36-MHz bandwidth transponders in the
4-GHz to 6-GHz range with multiple voice, video, or data signals via FDMA.
Today, all of these applications use TDMA digital techniques.
TDMA is a digital technique that divides a
single channel or band into time slots. Each time slot is used to transmit one
byte or another digital segment of each signal in sequential serial data
format. This technique works well with slow voice data signals, but it’s also
useful for compressed video and other high-speed data.
A good
example is the widely used T1 transmission system, which has been used for
years in the telecom industry. T1 lines carry up to 24 individual voice
telephone calls on a single line (Fig. 2). Each voice signal
usually covers 300 Hz to 3000 Hz and is digitized at an 8-kHz rate, which is
just a bit more than the minimal Nyquist rate of two times the
highest-frequency component needed to retain all the analog content.
2. This T1 digital telephony frame illustrates TDM and TDMA. Each time
slot is allocated to one user. The high data rate makes the user unaware of the
lack of simultaneity.
The digitized voice appears as individual serial
bytes that occur at a 64-kHz rate, and 24 of these bytes are interleaved,
producing one T1 frame of data. The frame occurs at a 1.536-MHz rate (24 by 64
kHz) for a total of 192 bits. A single synchronizing bit is added for timing
purposes for an overall data rate of 1.544 Mbits/s. At the receiving end, the
individual voice bytes are recovered at the 64-kHz rate and passed through a
digital-to-analog converter (DAC) that reproduces the analog voice.
The basic GSM (Global System of Mobile
Communications) cellular phone system is TDMA-based. It divides up the radio
spectrum into 200-kHz bands and then uses time division techniques to put eight
voice calls into one channel. Figure 3 shows one frame of a GSM TDMA signal.
The eight time slots can be voice signals or data such as texts or e-mails. The
frame is transmitted at a 270-kbit/s rate using Gaussian minimum shift keying
(GMSK), which is a form of frequency shift keying (FSK) modulation.
3. This GSM digital cellular method shows how up to eight users can
share a 200-kHz channel in different time slots within a frame of 1248 bits.
CDMA is another pure digital technique. It is
also known as spread spectrum because it takes the digitized version of an
analog signal and spreads it out over a wider bandwidth at a lower power level.
This method is called direct sequence spread spectrum (DSSS) as well (Fig.
4). The digitized and compressed voice signal in serial data form
is spread by processing it in an XOR circuit along with a chipping signal at a
much higher frequency. In the cdma IS-95 standard, a 1.2288-Mbit/s chipping
signal spreads the digitized compressed voice at 13 kbits/s.
4. Spread spectrum is the technique of CDMA.
The compressed and digitized voice signal is processed in an XOR logic circuit
along with a higher-frequency coded chipping signal. The result is that the
digital voice is spread over a much wider bandwidth that can be shared with
other users using different codes.
The chipping signal is derived from a
pseudorandom code generator that assigns a unique code to each user of the
channel. This code spreads the voice signal over a bandwidth of 1.25 MHz. The
resulting signal is at a low power level and appears more like noise. Many such
signals can occupy the same channel simultaneously. For example, using 64
unique chipping codes allows up to 64 users to occupy the same 1.25-MHz channel
at the same time. At the receiver, a correlating circuit finds and identifies a
specific caller’s code and recovers it.
The third generation (3G) cell-phone
technology called wideband CDMA (WCDMA) uses a similar method with compressed
voice and 3.84-Mbit/s chipping codes in a 5-MHz channel to allow multiple users
to share the same band.
OFDMA is the access technique used in
Long-Term Evolution (LTE) cellular systems to accommodate multiple users in a
given bandwidth. Orthogonal frequency division multiplexing (OFDM) is a
modulation method that divides a channel into multiple narrow orthogonal bands
that are spaced so they don’t interfere with one another. Each band is divided
into hundreds or even thousands of 15-kHz wide subcarriers.
The data to be transmitted is divided into
many lower-speed bit streams and modulated onto the subcarriers. Time slots
within each subchannel data stream are used to package the data to be
transmitted (Fig.
5). This technique is very spectrally efficient, so it provides
very high data rates. It also is less affected by multipath propagation
effects.
5. OFDMA assigns a group of subcarriers to each user. The subcarriers
are part of the large number of subcarriers used to implement OFDM for LTE. The
data may be voice, video, or something else, and it’s assembled into time
segments that are then transmitted over some of the assigned subcarriers.
To implement OFDMA, each user is assigned a
group of subchannels and related time slots. The smallest group of subchannels
assigned is 12 and called a resource block (RB). The system assigns the number
of RBs to each user as needed.
SDMA uses physical separation methods that
permit the sharing of wireless channels. For instance, a single channel may be
used simultaneously if the users are spaced far enough from one another to
avoid interference. Known as frequency reuse, the method is widely used in
cellular radio systems. Cell sites are spaced from one another to minimize
interference.
In addition to spacing, directional antennas
are used to avoid interference. Most cell sites use three antennas to create
120° sectors that allow frequency sharing (Fig. 6a). New technologies
like smart antennas or adaptive arrays use dynamic beamforming to shrink
signals into narrow beams that can be focused on specific users, excluding all
others (Fig.
6b)
6. SDMA separates users on shared frequencies by isolating them with
directional antennas. Most cell sites have three antenna arrays to separate
their coverage into isolated 120° sectors (a). Adaptive arrays use beamforming
to pinpoint desired users while ignoring any others on the same frequency (b).
One unique variation of SDMA, polarization
division multiple access (PDMA), separates signals by using different
polarizations of the antennas. Two different signals then can use the same
frequency, one transmitting a vertically polarized signal and the other
transmitting a horizontally polarized signal.
The signals won’t interfere with one another
even if they’re on the same frequency because they’re orthogonal and the antennas
won’t respond to the oppositely polarized signal. Separate vertical and
horizontal receiver antennas are used to recover the two orthogonal signals.
This technique is widely used in satellite systems.
Polarization is also used for multiplexing in
fiber optic systems. The new 100-Gbit/s systems use dual polarization
quadrature phase shift keying (DP-QPSK) to achieve high speeds on a single
fiber. The high-speed data is divided into two slower data streams, one using
vertical light polarization and the other horizontal light polarization.
Polarization filters separate the two signals at the transmitter and receiver
and merge them back into the high-speed stream.
A unique and widely used method of multiple
access is carrier sense multiple access with collision detection (CSMA-CD).
This is the classical access method used in Ethernet local-area networks
(LANs). It allows multiple users of the network to access the single cable for
transmission. All network nodes listen continuously. When they want to send
data, they listen first and then transmit if no other signals are on the line.
For instance, the transmission will be one packet or frame. Then the process
repeats. If two or more transmissions occur simultaneously, a collision occurs.
The network interface circuitry can detect a collision, and then the nodes will
wait a random time before retransmitting.
A variation of this method is called carrier
sense multiple access with collision avoidance (CSMA-CA). This method is
similar to CSMA-CD. However, a special scheduling algorithm is used to
determine the appropriate time to transmit over the shared channel. While the
CSMA-CD technique is most used in wired networks, CSMA-CA is the preferred
method in wireless networks.
Established in 2000, the Soukacatv.com main products are modulators both
in analog and digital ones, amplifier and combiner. We are the very first one
in manufacturing the headend system in China. Our 16 in 1 and 24 in 1 now are
the most popular products all over the world.
For
more, please access to https://www.soukacatv.com.
CONTACT US
Company:Dingshengwei Electronics Co., Ltd
Address : BldgA,the first industry park of Guanlong,Xili
Town,Nanshan,Shenzhen,Guangdong,China
Tel : +86 0755 26909863
Fax : +86 0755 26984949
Mobile : 13410066011
Email : ken@soukacatv.com
Source:
electronicdesign
没有评论:
发表评论