Good stuff.  In the order presented, the text makes some statements about RX 
threshold damping.

It is a powerful tool for a higher
modulation radio operating in a noisy environment, as it allows the radio to
block out and ignore signals received below the preset RF Rx Threshold.

 By creating an artificial receiver threshold below which no RF signals are
processed, the Receiver Threshold Dampening allows for the rejection of
distance interferences and reduces co-location interference at the expense
of a reduced coverage radius.

The text above immediately follows the excellent section on C/I.  Presenting in 
this order I felt the text might somehow imply that by setting the threshold 
higher than the interfering signals, that the receiver can ignore the 
interference (it says this in so many words).  If we're talking about the 
Carrier-to-Interference required above the surrounding interference it's giving 
you the wrong impression.  That would be incorrect, and since it immediately 
followed the section on C/I I thought I could improve a bit here.  You still 
need every inch of the required C/I above the interference.  All that is being 
ignored is the receiver's energy detection (and whatever impact it may cause in 
the MAC's channel access algorithm) from reacting to receive energy below the 
threshold.  The interference energy is still there, and additive with desired 
received signal.  Another way of looking at this is that you need the same 
margin above the receiver noise threshold as you need above the interference 
(you still need both SNR and C/I).

In my book this is not interference rejection at all.  You need the same amount 
of required SNR above sensitivity and C/I above interference, but the technique 
can be useful in masking far-away weaker signals from screwing up your channel 
access if you were using something like CSMA.

  ----- Original Message ----- 
  From: Charles Wu 
  To: 'WISPA General List' 
  Sent: Thursday, December 28, 2006 3:47 PM
  Subject: RE: [WISPA] Alvarion Comnet Radios have arrived - 
regardinginterference - Part 1

  I go to see Mickey Mouse for a few days and look where this thread has

  So, my 2 cents...

  One of the largest concerns in the license-exempt world is the question of a
  system's interference robustness.  However, before we can get into further
  detail on the pros and cons of Alvarion VL vs Canopy, CSMA/CA vs GPS, etc --
  it is necessary to realize that interference as a term is extremely broad
  and vague, and can mean just about anything to anyone.  Heck, all radios in
  the market have some sort of "interference robustness / avoidance
  capability" -- the trick to understanding a system's capabilities is knowing
  what TYPE of interference the system can actually handle.  Read on...I'll
  talk more about each particular platform when I get some time to write Part
  2 =)


  In the wireless world, interference, by definition, is a situation where
  unwanted radio signals operate in the same frequency channels or bands -
  i.e. they mutually "interfere," disrupt or add to the overall noise level in
  the intended transmission.

  Interference can be divided into two forms, based on whether it comes from
  your own network(s) or from an outside source.  If the interfering RF
  signals emanate from a network under your control, whether it is on the same
  tower or several miles away, it is termed "self-interference."  If the
  opposing signals come from a network, device or other source that is not
  under your control, it is termed "outside interference."  Thus, the
  definition of what type of interference is being combated is not based on
  technology, but ownership.

  In licensed bands, where spectrum is relatively scarce (due to high costs)
  self-interference alone must be taken into account; however given a more or
  less known operating environment (the radio spectrum will only have signals
  transmitting that are under control by a single entity) proper product
  design and network deployment can reduce these interferes to a level where
  they do not impact network performance.

  Self-interference is not a phenomenon that is confined to licensed band
  operations; license-exempt bands must address the same issues.  The
  techniques and design elements of a given product that serve to reduce and
  tame self-interference in licensed band operations can be applied directly
  to license-exempt systems. 


  In the license-exempt bands, not only must self-interference be accounted
  for, but, given the nature of the regulations governing these bands,
  external interference must be designed for as well.  This can be extremely
  challenging, as there is no way of knowing in advance where these outside
  signals may be or will be sourced from, or even how strong the interfering
  transmissions will be relative to the desired transmission.  This aspect of
  the license-exempt bands represents the possible "downside" of
  license-exempt network operation.

  Yet as potentially damaging and unpredictable as external interference can
  be in license-exempt networks, a properly designed and implemented broadband
  wireless system can make a significant difference in the performance of a
  network under siege from unwanted external radio transmissions.


  1. Modulation & the C/I Ratio

  At the most fundamental level, an interfering RF source disrupts the digital
  transmission by making it too difficult for the receiving station to
  "decode" the signal.  How much noise or interference a digital RF
  transmission can tolerate depends on the modulation used.

  Fundamentally, modulation is the method whereby zeros and ones are
  communicated by varying one of three aspects of radio signal.  The three
  portions of an RF signal that can be changed or modulated are phase,
  frequency and amplitude.  Shirting the properties of any of these parameters
  can be used to communicate different "states."  These states, in turn, are
  translated to zeros and ones for binary communications.

  For example, with frequency modulation, if the sine wave is at frequency
  one, it is decoded as a zero.  If the sine wave is shifted slightly to
  frequency two, this is decoded as a one.  This type of modulation is
  referred to as Binary Frequency Shift Keying (BFSK).  In this example, a
  system must only be able to tell the difference between one of two states or
  phases.  More complex modulations, such as 16 QAM (Quadrature Amplitude
  Modulation), attempt to differentiate among 16 different possible states of
  an incoming signal.

  The advantage to higher order modulation schemes, like 16QAM, is that
  compared to BPSK, 16QAM conveys more information per bandwidth (more
  bits/Hz).  The disadvantage of 16QAM lies in the fact that, in order to
  distinguish among the 16 different states, the signal must be very clean and
  very strong relative to background noise and/or interference.

  The ability of a receiving station to decode an incoming signal at the most
  basic physical layer is dependent on a factor called the "carrier to
  interference ratio," or C/I.  This term means exactly what it says: how
  strong the desired signal (the carrier) is relative to the unwanted signals
  (the interference).  C/I ratios are based primarily on the modulation used,
  with more complex modulations requiring higher C/I numbers than more robust
  modulations, such as BFSK.


  <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn1> [1]


  (20 MHz Channel)

  Rx Sensitivity

  C/I (dB)


  BPSK ½

  6 Mb

  -86 dBm

  6 dB

  BPSK ¾

  9 Mb

  -85 dBm

  10 dB

  QPSK ½

  12 Mb

  -85 dBm

  12 dB

  QPSK ¾

  18 Mb

  -82 dBm

  15 dB

  16 QAM ½

  24 Mb

  -80 dBm

  17 dB

  16 QAM ¾

  36 Mb

  -76 dBm

  21 dB

  64 QAM 2/3

  48 Mb

  -70 dBm

  29 dB

  64 QAM ¾

  54 Mb

  -66 dBm

  31 dB



  [1] RedLine Communications 5.8 GHz AN-50 System Specifications


  Due to the large amount of available license-exempt spectrum (a 500+ Mhz
  chunk in 5 GHz compared to 6 Mhz slivers in 3.5 GHz) one can easily justify
  sacrificing bandwidth efficiency in favor of more "interference resistant"
  lower-order modulation products.  For example, the Motorola Canopy product
  uses BFSK for modulation, and is able to operate properly with an error rate
  of 1E-4 at a 3 dB C/I; i.e., the wanted signal need be only 3 dB higher in
  power than the unwanted interferes.  A system operating with 16 QAM at these
  levels would require a C/I ratio closer to 20 dB.


  Putting this into perspective, with every 3 dB of additional signal
  strength, the power of a signal is doubled.  This means that a system with a
  3 dB C/I ratio can tolerate an interfering signal that is many times more
  powerful than a 16QAM system and still operate at the specified error rate.
  Whether the interference is from another cell site on the network or another
  network completely, systems employing lower-order modulations systems (like
  BFSK) will tolerate substantially higher levels of interference before the
  communication stream becomes impacted.  All other PHY layer techniques are
  designed to improve this most fundamental measurement of network robustness
  and operational effectiveness by sustaining the necessary C/I level.


  2. Another Method: Receiver Threshold Dampening


  In some instance, in order to avoid a "race to the bottom," certain
  license-exempt systems retain high modulation bandwidth efficiencies in
  interference heavy environments with a method known as receiver threshold
  dampening.  Trango Broadband uses a RF Rx Threshold command to specify the
  receiver sensitivity of the radio.  It is a powerful tool for a higher
  modulation radio operating in a noisy environment, as it allows the radio to
  block out and ignore signals received below the preset RF Rx Threshold.


  By creating an artificial receiver threshold below which no RF signals are
  processed, the Receiver Threshold Dampening allows for the rejection of
  distance interferences and reduces co-location interference at the expense
  of a reduced coverage radius.




  TDD Synchronization


  License-Exempt systems that use Time Division Duplexing (TDD) for separating
  upstream and downstream communications are ideally suited for dynamically
  changing asymmetric traffic, like data.  The ability to adjust the amount of
  bandwidth dedicated for upstream and downstream communications without
  changing hardware is a powerful feature.


  TDD systems operate by transmitting downstream (from the AP to the SU) for a
  period of time - 1 ms for example.  Following a short guard time, the SMs
  then transmit on the same frequency in the upstream.  For a cell site with
  more than one radio operating in TDD mode, it is important that all the
  sectors of the cell transmit and receive at precisely the same time.
  Otherwise, if sector 1 is transmitting when sector 2 is receiving, sector 2'
  incoming transmission can be interfered with because the sector 1 signal is
  so close that it is strong enough to "flood" or overwhelm the "front end" in
  sector 2.


  When deploying a TDD system in a cellular topology, it is desirable to be
  able to use the same frequency in each cell site even though those cell
  sites are possibly several miles away.  This means that sector 1 from AP A
  may interfere with sector 1 of AP B.  The frequency planning diagram below
  shows how such signals might interfere.  In this case, inter-cellular
  synchronization is required, making sure that all the sectors in all the
  cell sites are properly timed and synchronized in terms of downstream and
  upstream communications


  Delivering tight synchronization across potentially hundreds of square miles
  can be a challenge.  Some systems utilize TDD synchronization via GPS to
  solve this issue.  These precise satellite signals are used for timing and
  ultimately, transmit/receive synchronization, thus tying all sectors within
  the system to the same "clock."


  Another Method: Antenna F/B Ratio & ATPC


  When a BWA signal is followed from end to end, it leaves the radio and
  travels first through a transmitting antenna, over the air to a receiving
  antenna, and into the radio.  The antenna, an important component in the RF
  chain, can also have an impact on how well the network tolerates
  interference, both internal and external.


  Antenna performance is specified in a variety of ways, but for purposes of
  this discussion, we focus on the front-to-back ratio.  The front-to-back
  ratio of an antenna indicates how much of an incoming signal will be
  absorbed coming into the front of the antenna as compared to how much of a
  signal arriving at the back of the antenna is absorbed.


  When deploying networks in a cellular topology, the performance of the
  antenna in rejecting unwanted signals from behind is an important feature.
  In some cases, metal RF shields can be mounted on an antenna, augmenting and
  further increasing an antenna's F/B ratio.


  Automatic Transmit Power Contrrol (ATPC) is a feature that allows the system
  to self-optimize the transmit power and provide for the best overall link
  performance.  The ATPC function automatically will adjust the output power
  level of remote-end systems to match a pre-specified signal strength value.
  When ATPC is enabled, the system will attempt to establish the wireless link
  and exchange performance information.  Once the wireless link is
  established, the master-end system will dynamically adjust the remote-end
  systems transmit power to maintain optimum link characteristics while
  minimizing power output.  In short, ATPC optimizes the transmission power
  for best operation, while minimizing excess power and interference with
  other devices.




  Frame/Slot Size


  A typical MAC frame for a TDD system is shown below.  As can be seen, the
  upstream and downstream portions of the frame are divided into slots, each
  slot carrying what can be termed a "radio data packet," or RDP.  The
  original data, an IP packet datagram, for example, is segmented into packets
  that fit into a RDP.


  Despite all the best system deployment designs, there will be instances
  where interference will overcome these measures and corrupt a MAC frame or a
  portion of a MAC frame.  When this happens, the corrupted data must be sent
  again.  If the MAC frame is designed for large RDPs on the order of several
  hundred bytes, the entire slot must be re-transmitted even if only a small
  amount of this packet is damaged.


  The impact on network throughput as a result can be large, with a few bytes
  in error causing hundreds of bytes to be re-sent.  By using a smaller RDP
  size, the re-transmission can be contained to only those bytes that were
  damaged, thus avoiding the re-send of large chunks of valid data.  However,
  as RDP size decreases, the slot header which is fixed becomes a more
  significant portion of the packet data, hence increasing the MAC layer



  Automatic Retransmission Request


  In wireless broadband systems, small amounts of interference can have large
  impacts on end-to-end network performance.  This is tied to the way TCP/IP
  networks were designed to operate in the wired world.


  TCP/IP was designed to operate over wire, where interference was assumed to
  be negligible.  The protocol design calls for positive acknowledgement sent
  from the receiving station to the sending station for every IP packet sent
  out.  If the sending station does not receive the TCP ACK in a certain
  amount of time, it is assumed that the cause was congestion of the network -
  not an error resulting from transmission impediments.  When encountering
  congestion, TCP responds by dramatically slowing down the transmission and
  then increasing transmission speed slowly.


  In a BWA network, a lost or corrupted packet can occur from interference
  <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn1> [1].
  However, the TCP protocol has no knowledge or ability to account for higher
  error rates and responds by slowing down the end-to-end data rates.  This is
  a phenomenon that can multiply a small amount of RF interference into
  significant network degradation.
  <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn2> [2]


  In most license-exempt broadband wireless systems, this is not a factor.
  Automatic Retransmission request (ARQ) is a feature that addresses this
  issue.  ARQ inspects the RDPs that come in and looks for errors.  If an
  error is detected, the system will send a request to the sending entity to
  re-send the RDP.  All of this is accomplished two layers below TCP in the
  protocol stack.  The net effect is that as far as TCP is concerned, it never
  receives a packet of data with an error as a result of the wireless portion
  of the network, thus preventing TCP from invoking the slow start algorithm
  and keeping the end-to-end data rates at the peak or just slightly below
  peak operational rates.
  <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn3> [3]


  Centralized Transmission Control


  The popularity and cost-economics of the IEEE 802.11 standards make it a
  potentially attractive BWA solution.  However, it is worth noting that the
  IEEE 802.11 MAC operates in what is known as a distributed control manner
  (CSMA/CA).  In this "contention-based" scenario, each SU has the ability to
  send a packet at its own discretion.  Typically in this scenario the SU will
  "listen" and if it does not hear any transmission, it will assume that the
  channel is clear and will send its data.


  In high density or high traffic deployments, the problem arises due to the
  fact that (1) license-exempt wireless communications are typically half
  duplex and (2) the sending SU typically cannot hear other SUs in the system
  "hidden node" problem.  In this instance, two or more SUs may send a packet
  at the same time, corrupting both and causing a retransmission.  Unlike
  contention-based Ethernet networks (CSMA/CD), the half-duplex nature of
  license-exempt wireless communications precludes the sending of an "error
  notice" until initial transmission is complete, further delaying the
  correction.  In addition to this "self-induced" interference, external
  sources can also block SUs from hearing each other to the same effect.



  [1] It is also worth noting that ARQ is also applicable in licensed bands,
  as atmospheric effects, such as multipath, fading and ducting effects, are
  also a cause for lost or corrupted packets within BWA network operations

  [2] Reference Layer-2 Frame Loss Concealment Discussion in our 3.5 GHz
  Wireless DOCSIS Review for a more detailed analysis of this phenomenon

  [3] It is worth noting that not all ARQ implementations are created equally.
  For example, 802.11 implements a form of ARQ known as the stop and wait
  protocol.  Essentially, the transmitter sends the packet, and then waits for
  an ACK from the receiver before it tries to send the next packet.  If the
  transmitting station is the AP, this delay will impact all of the SUs
  because they will have to wait to send any transmission destined for the AP.
  If the stop and wait ARQ is combined with the RTS/CTS protocol, even each
  ACK will have to be preceded by an RTS and followed by a CTS, thus slowing
  down the network significantly.


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