Monday, 23 December 2013

Wireless Data Networks

Current wireless networks use standards developed by the IEEE:
LAN/MAN Standards Committee (aka IEEE 802).
802.11: Wireless Local-Area Networks (Wi-Fi)
802.11b: 11Mbps (6.5Mbps typ) @ 2.4GHz, 100m
802.11a: 54Mbps (25Mbps typ) @ 5GHz, 50m
802.11g: 54Mbps (11Mbps typ) @ 2.4GHz, 100m
802.11n: 540Mbps (200Mbps typ) @ 2.4GHz or 5GHz, 250m
802.15: Wireless Personal-Area Networks
802.15.1: (Bluetooth) 1-3Mbps @ 2.4GHz, 1:10:100m
802.15.3: (UWB) 100-500Mbps @ 3.1-10.6GHz, 1-3m
802.15.4: (ZigBee) 40kbps
@ 915MHz, 250kbps @2.4GHz, 10-75m
802.16: Broadband Wireless Access (WiMAX)
up to 70Mbps and up to 100km
(but not at the same time!)
@ 1-10GHz, or @ 10-66GHz
12 channels in 5GHz band
20MHz bandwidth (16.6MHz occupied)
Orthogonal Frequency Divi
sion Multiplexing (OFDM)
52 subcarriers (48 data, 4 pilot) 
802.11a Transmitter:
802.11a Receiver:

802.11a Packet Structure:
802.11a Virtual CSMA/CA :
Full Collision Detect would require full duplex radio ($$$)
Transmitter sends Request-To-Send packet specifying
source, destination and duration (cts+pkt+ack)
Receiver sends Clear-To-Send packet with duration (pkt+ack)
Everyone receiving RTS or CTS will mark channel as busy for
given duration
Transmitter sends Data packet, receiver checks CRC and
replies with ACK packet.

Wirless Networks

Sharing the channel: TDMA

Time Division Multiple Access
: Share same carrier frequency
by dividing signal into different time slots (used in GSM cell
phone networks)
Sharing the channel: FDMA, SDMA
Frequency Division Multiple Access: Divide
frequency channel into subbands
Different users assigned to different subbands
Each subband has its own carrier
Use different frequencies for transmit and receive
(frequency-division duplexing). Eg, using FDD, adjacent
cell towers don’t “hear” each other.
TDMA/FDMA/FDD used in GSM cell phone networks
Space Division Multiple Access: adjust antenna
radiation pattern (eg, using phased arrays)
depending on location of the user
Focus transmitter power in required direction
On receive, eliminate noise from other sources
Sharing the channel: CDMA
Two vectors are orthogonal if their dot products are 0.
Here’s a set of 4 mutually orthogonal vectors:
V1: (1, 1, 1, 1)
V2: (1, 1, -1, -1)
V3: (1, -1, 1, -1)
V4: (1, -1, -1, 1)
Assign each transmitter a particular one of the orthogonal
vectors (Vi) to use to encode its transmissions (called the
“chip code”). With vectors shown above we can support 4
If message bit is 0, transmit –Vi
If message bit is 1, transmit Vi
Channel will sum the transmitted values:
send 00 using V1: -1 -1 -1 -1 -1 -1 -1 -1
send 01 using V2: -1 -1 1 1 1 1 -1 -1
send 11 using V3: 1 -1 1 -1 1 -1 1 -1
send 10 using V4: 1 -1 -1 1 -1 1 1 -1
channel: 0 -4 0 0 0 0 0 -4
CDMA Receiver
At receiver take groups of len(V) bits and form
dot product with Vi for desired channel.
If result is negative, message bit is 0
If result is positive, message bit is 1
channel: 0 -4 0 0 0 0 0 -4
receive using V1: 1 1 1 1 1 1 1 1
dot product:
-4 -4
message bits:
0 0
receive using V2: 1 1 -1 -1 1 1 -1 -1
dot product:
-4 4
message bits:
0 1
receive using V4: 1 -1 -1 1 1 -1 -1 1
dot product:
4 -4
message bits:
1 0
Asynchronous CDMA:
Use N orthogonal vectors to multiplex N transmitters (e.g.,
use a NxN Walsh Matrix)
Scheme described above works for
synchronous CDMA
all symbols are transmitted starting at same moment. For
example this works fine for a cell tower transmitting to
mobile phones.
But hard to synchronize mobile phone transmissions, so use
asynchronous CDMA
Can’t create transmissions that are truly orthogonal if they
start at different times
Approximate orthogonality with longer uncorrelated pseudo-
random sequences (called pseudo-noise or PN). “pseudo” implies
that sequence can be reconstructed at receiver given a known
starting point.
Assuming equal signal strengths from each transmitter at
receiver, if we decode bits
using a particular PN sequence
synchronized with desired transm
itter, we’ll get desired signal
plus some uncorrelated noise from other transmitters.
Sharing the channel: CSMA
Carrier Sense Multiple Access
“Carrier Sense” – transmitter listens to ensure no other carrier
is present on channel before transmitting
“Multiple Access” – multiple transmitters share the channel,
transmissions received by everyone (not quite true for wireless)
If two transmitters start at the same time (both having
detected no other carrier), their
transmissions collide and entire
packet is lost (looks like other
errors that cause packet to be
discarded). Recover via retransmission.
CSMA with collision avoidance (CSMA/CA)
Transmitter informs others of intent to transmit (costs
bandwidth); collisions still
possible as in pure CSMA
Used by 802.11, 802.15
CSMA with collision detection (CSMA/CD)
Transmitter detects collision, stops, and retries after random
Used by original Ethernet, doesn’
t work with radio where range
effects cause some receivers not to hear certain transmitters
Transmitting Information: OFDM
Orthogonal Frequency Division Multiplexing:
Bit stream split into lower-bandwidth parallel bit streams
each transmitted on a separate channel
Symbol times long relative to propagation time
Add guard interval to reduce in
ter-symbol interference (multi-
path echoes die away during guard interval)
Channels are orthogonal
No cross-talk
Don’t need filters for each subchannel
Don’t need inter-carrier guard bands
Robust, makes good use of channel capacity
ADSL,VDSL (telephone line data transmission)
802.11a, 802.11g, 802.16, 802.15.3 (ultra wide band)
Terrestrial Digital TV
Power line networking
Transmitting Information: Spread Spectrum

Direct Sequence Spread Spectrum (DSSS)
Used in asynchronous CDMA where each “chip” from
pseudo-random “chip vector” is used to change phase of
Chip rate is many times higher than message rate (i.e.,
we transmit many chips per message symbol), so message
is spread out over large spectrum
Highest freq is twice the chip rate
Longer PN sequences and higher chip rate increase
rejection of uncorrelated transmissions (“process gain”)
Used in 802.11b, CDMA phones
Frequency-Hopping Spread Spectrum (FHSS)
Like DSSS except adjust carrier frequency instead of
Adapt sequence to avoid crowded frequencies (Bluetooth)

Saturday, 21 December 2013

Universal Mobile Telecommunications System (UMTS)

"third-generation (3G):

UMTS (Universal Mobile Telecommunications System) is a so-called "third-generation (3G)," broadband , packet -based transmission of text, digitized voice, video, and multimedia at data rates up to and possibly higher than 2 megabits per second ( Mbps ), offering a consistent set of services to mobile computer and phone users no matter where they are located in the world. Based on the Global System for Mobile ( GSM ) communication standard, UMTS, endorsed by major standards bodies and manufacturers, is the planned standard for mobile users around the world by 2002. Once UMTS is fully implemented, computer and phone users can be constantly attached to the Internet as they travel and, as they roaming service , have the same set of capabilities no matter where they travel to. Users will have access through a combination of terrestrial wireless and satellite transmissions. Until UMTS is fully implemented, users can have multi-mode devices that switch to the currently available technology (such as GSM 900 and 1800) where UMTS is not yet available.

Today's cellular telephone systems are mainly circuit-switched , with connections always dependent on circuit availability. packet-switched connection, using the Internet Protocol ( IP ), means that a virtual connection is always available to any other end point in the network. It will also make it possible to provide new services, such as alternative billing methods (pay-per-bit, pay-per-session, flat rate, asymmetric bandwidth, and others). The higher bandwidth of UMTS also promises new services, such as video conferencing. UMTS promises to realize the Virtual Home Environment ( VHE ) in which a roaming user can have the same services to which the user is accustomed when at home or in the office, through a combination of transparent terrestrial and satellite connections.
Trials of UMTS technology, using advanced mobile phone/computing device prototypes, are currently being conducted by Nortel Networks and BT (British Telecommunications).

The electromagnetic radiation spectrum for UMTS has been identified as frequency bands 1885-2025 MHz for future IMT-2000 systems, and 1980-2010 MHz and 2170-2200 MHz for the satellite portion of UMTS systems.

Friday, 20 December 2013

Mobile WiMAX

Mobile WiMAX, or Institute of Electronic and Electrical Engineer’s (IEEE) 802.16e-2005, emerged as a possible various to cellular technology for wide-area wireless networks. supported Orthogonal Frequency Division Multiple Access (OFDMA) associated approved by the International Telecommunication Union (ITU) as an IMT-2000 (3G technology) beneath the name OFDMA Time Division Duplex (TDD) Wireless Metropolitan space Network (WMAN), mobile WiMAX gained its greatest traction in developing countries as a set wireless various to wireline preparation.

WiMAX, or Worldwide ability for Microwave Access, was a reputation created by the WiMAX Forum, that was shaped in Gregorian calendar month 2001 to market conformity and ability of the IEEE 802.16 technology commonplace. The forum describes WiMAX as a standards-based technology facultative the delivery of walk wireless broadband access as another to cable and line. WiMAX provides wireless transmission of information employing a sort of transmission modes, from point-to-multipoint links to transportable and absolutely mobile net access. It ought to be noted that almost all usually, references to WiMAX deployments and scheme announcements embrace the mounted, transportable and mobile WiMAX technologies.

In the us, Clearwire, Sprint Nextel et al. (Intel, Google, Comcast, Time Warner Cable, and Bright House Networks) created a venture to deploy a nationwide WiMAX network. In Gregorian calendar month 2012, this network was on the market in eighty cities across the U.S. and coated over a hundred thirty million individuals. Clearwire, however, has started deploying LTE, and indicates it'll have thirty one cities coated by the primary 1/2 2013. WiMAX has gained the best traction in developing countries as another to wireline preparation.

Mobile WiMAX employs several of an equivalent mechanisms as HSPA to maximise output and spectral potency, together with high-order modulation, economical writing, accommodative modulation and writing additionally as Hybrid Automatic Repeat Request (HARQ).The principal distinction from HSPA is IEEE 802.16e-2005’s use of OFDMA. In five to ten MHz radio channels, there's no proof indicating that WiMAX can have any performance advantage compared with HSPA+. With relevancy spectral potency, WiMAX is akin to HSPA+. As for knowledge performance, HSPA+ in unharness 8—with a peak rate of forty two Mbps—essentially matches mobile WiMAX in ten MHz in TDD 3:1 DL:UL mistreatment 2X2 MIMO with a peak rate of forty six Mbps.

Relative to LTE, WiMAX has the subsequent technical disadvantages: five time unit frames rather than one time unit frames, Chase combining rather than progressive redundancy, coarser roughness for modulation and writing schemes and vertical writing rather than horizontal writing. One preparation thought is that TDD needs network synchronization. it's impractical for one cell web site} to be sending associated an adjacent cell site to be receiving at an equivalent time. totally different operators within the same band should either coordinate their networks or have guard bands to confirm that they don’t interfere with one another.

Although IEEE 802.16e exploits important radio innovations kind of like HSPA+ and LTE, it faces challenges like economies of scale and technology maturity. only a few operators these days have access to spectrum for WiMAX that will allow them to supply widespread coverage. this suggests that roaming with WiMAX is severely restricted.

In relevancy economies of scale, GSM-HSPA subscribers range within the billions. Even over succeeding 5 years, the amount of WiMAX subscribers is probably going to be quite low. Infonetics analysis comes 132 million subscribers by 2016.

One specific space during which WiMAX contains a technical disadvantage is cell size. In fact, 3G systems have a big link budget advantage over mobile WiMAX due to soft-handoff diversity gain associated an FDD duplexing advantage over TDD. Arthur D. very little reports that the radii of typical HSPA cells are going to be 2 to fourfold bigger than typical mobile WiMAX cells for high-throughput operation. One seller estimates that for an equivalent power output, frequency, and capability, mobile WiMAX needs one.7 times additional cell sites than HSPA. only if several universe deployments of HSPA can occur at frequencies like 850 MHz, and LTE at 700 MHz, WiMAX deployments at two.5 Gc ar at a big disadvantage.

Current mobile WiMAX networks use 2X2 MIMO or 4X2 MIMO, TDD, and ten MHz radio channels during a profile outlined by the WiMAX Forum called WiMAX Wave two or, additional formally, as WiMAX System Profile one.0. on the far side unharness one.0, the WiMAX Forum outlined a profile known as WiMAX unharness one.5. This profile includes numerous refinements meant to enhance potency and performance and will be on the market for preparation during a similar timeframe as LTE.

Release 1.5 enhancements embrace Medium Access management (MAC) overhead reductions for VoIP (persistent scheduling), relinquishment optimizations, load equalisation, location-based services support, Frequency Division Duplex (FDD) operation, sixty four QAM within the transmission, downlink accommodative modulation and writing, closed-loop MIMO (FDD mode only), and transmission MIMO. There aren't any current unharness one.5 preparation plans.

A succeeding version, Mobile WiMAX two.0, has been designed to handle the performance necessities of ITU IMT-Advanced Project and is commonplaceized during a new IEEE standard, IEEE 802.16m. it's unsure and unclear whether or not 802.16m can ever be commercial.

4G Americas considers WiMAX to be a distinct segment technology that will be acceptable in some circumstances and sure as shooting operators as an area of their network methods. At best, the guarantees of mobile WiMAX ar appealing however stay unverified within the universe. With such a large amount of WiMAX operators selecting LTE, the top of WiMAX market chance seems over already.