Wireless Network

1G ：
Analog / Vehicle use / unsafe （Eavedropping/cloning）
except NMT（ The band is 450/900, all No roaming ）

2G：
Voice and low bit rate data services / digital transmission / More secure / More popular

e.g. GSM Global mobile communication system (Global System for Mobile Communications)

TDMA: different time slots, same frequency
FDMA: different frequency, same time
CDMA: different codes, same frequency and time
OFDMA: different Orthogonal Frequency, Compared to traditional code schema, More capacity (allow more users to working on same frequency band and time)

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GSM(Global System for Mobile Communications)

features:

• TDMA 8 channels

  (TDMA ： Time division multiple access / Only one device occupies / Can support 4Kb Voice coder )

• FDD (Frequency Division Duplex) –different frequencies for uplink and downlink

  ( Duplex is realized at different frequencies through upload and download )

GSM Four variants of :

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e.g. GSM900

• Use 890–915 MHz(uplink)

• Use 935–960 MHz(downlink）

• The separate bandwidth of uplink and downlink is 25 MHZ, The duplex interval is 45 MHz

• Provide 124 A wireless channel （ Channel number 1-124）

  Each channel occupies 200 kHz, 100 kHz Protection bandwidth Placed at both ends of the frequency band

The duplex interval is : ** Uplink start The frequency and Downlink start ** Frequency interval

25 Plus duplex spacing ( Is the starting frequency distance between the two frequency bands , So just ＋ once 25)

GSM network architecture

• BTS: Base Transceiver( Transceiver ) Station The base station

• BSC: Base Station Controller Base station control

• BSS: Base Station Subsystem Base station subsystem

• MSC: Mobile Switching Center Mobile switching center

  In different contexts ,MSC There are different names , This also reflects their complex role in the network . Although all these terms can refer to the same MSC, But their functions are different at different times .

  • Gateway mobile switching center (GMSC)

    from **HLR** Query the visitor information and send it back to VLR, And select the route according to this information

  • Visit the mobile switching center (VMSC)

    Is where the customer is currently located MSC. With this MSC The associated VLR Will have the data of the contracted user .

  • Anchor attached mobile switching center

    Anchor attachment MSC（anchor MSC） The switch is initiated MSC.

  • Target mobile switching center

    The goal is MSC（target MSC） Is the one you're going to switch to MSC.

• HLR: Home Location Register Home register

  The home location register and the mobile switching center communicate with each other .HLR Store all user information （ Including the current roaming location MSC and VLR）

• VLR: Visitors Location Register Visitor location register

• AuC: Authentication Center authentication center

• GMSC: Gateway Mobile Switching Center

  ​ Mobile switching center

  from **HLR** Query to get called MS Current roaming code , And select the route according to this information

  GMSC It can be arbitrary MSC, It can also be set separately . When set separately , Don't deal with MS The call of . therefore There is no need to set VLR, Not with BSC Connected to a

• PSTN: Public Switched Telephone Network

  ​ Public switched telephone network

  PSTN, Public switched telephone network , It is a circuit switching network based on analog technology . Among many Wan Interconnection Technologies , adopt PSTN The communication cost required for interconnection is the lowest , But its data transmission quality and transmission speed are also the worst , meanwhile PSTN The utilization rate of network resources is also relatively low .

• EIR: Equipment Identity Register Device identification register

  Mobile phone users initiate calls , MSC and VLR Request from mobile phone IMEI( Cell phone serial number ), And send it to EIR, EIR Will receive IMEI And white 、 black ( steal / There is a fault without model certification )、 Compare the three tables , Send the results to MSC/VLR

• SMS-C: Short Message Service Center Embedded in MSC in

• OMC: Operation and Maintenance Center Each functional entity in the operation and maintenance system

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• among **GMSC** Mainly the function of translation , take PSTN The fixed network signal is translated into MSC Signals that mobile networks can understand ;MSC after GMSC to PSTN It's the same thing .

• GMSC You can also HLR communicate , But through AuC Certification by

A call / Positioning users :

• The switching device will immediately determine whether the user is calling from local , If the user is visiting from other places , So local VLR Will go to MSC Send query signal , Find relevant information about this user .（ At this time, I don't know who the caller is ）
• MSC And HLR signal communication , Send relevant information back VLR in
• VLR Send the relevant routing information to MSC, In this way, the mobile switching center can find the correct route of the user's location , Finally, the whole call connection is established .

Location of mobile terminal (Locating a Mobile terminal):

• MT start-up location updating procedure
• new MSC/VLR notice **HLR**
• HLR Delete the old MSC/VLR Information
• HLR Confirm and update new MSC/VLR
• Update and update the location area MT Confirm .

How do mobile terminals receive incoming calls (Mobile Terminating Call)：

• PSTN Incoming from **Fixline** The call of , Pack it first , then PSTN Check whether the fixed network can connect to the user
• GMSC Translate signals ( Will come from PSTN Of data frame Change to be able to be mobile network The form of recognition ), Create with HLR Two way connection of ,HLR Know the latest network connection status . GMSC Communicating data to MSC Before , According to the specific needs of the service , Decide whether to pass HLR Authentication
• MSC Received a better mobile signal , And VLR Create connection , Start mobile roaming
• BSC Start looking for the location of the connection object , Find the one with the strongest signal BTS Connect

Roaming is called (Roaming incoming call):

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In the figure Roaming leg paid by recipient: Roaming segment paid by the recipient

GMSC Through HLR When doing validation , According to the information stored in it from VLR The record of , Know the terminal location , Direct to visited network and ** Not in the home network Waste in traffic Time **

Roaming out Roaming outgoing call

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mobile network Medium traffic need GMSC Packaging function , To send further information to PSTN. Sometimes MSC and GMSC Can be configured in the same physical device （ In Physics , Both can use two different nodes , You can also use a node ）

GSM Mobility management :

1. Authentication（ Authentication ）

• At terminal SIM card and MSC There is a pair of... In the database secret key
• Outside , key ki in MSC And a string of random serial numbers , after A3 Algorithm , Output a value SRESMSC
• Inside the mobile terminal ,key ks in SIM and The same string of random serial numbers , after A3 Algorithm , Output a value SRESMT
• Compare SRESMSC and SRESMT, If equal, the verification is successful .
  （ advantage ： Security ）

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2. Radio Access（ Radio interview ）

• Three sectors
• Each sector contains 2 An antenna
• The antenna has a low noise amplifier
• In some cases ,BTS It can be connected with by microwave BSC Connect （ When the cable connection is not available ）

3. Diversity( diversity ): That is, multiple channels can reach , A path is blocked , The other one is still normal ; By algorithm , Two weak signals can be superimposed

Different antennas have different multipath rays for different signal sources , Therefore, the interference generated by different antennas will be different .
（ Choose the one with strong signal ,diversity high ）

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4. Frequency reuse（ Frequency multiplexing ）

• as long as cells If the distance between them is far enough, you can use the same frequency
• Whether it can be received only depends on the strength of the signal
• One cell Yes 6 individual neighbors
• The following figure , One cluster Yes 7 A community ,7 Two different frequencies

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In a wireless communication system :

• At the end is the antenna , Responsible for wireless signal transmission .
• The next level ahead is the community , It is called sector in mobile communication . A sector will have 1 Sub or many sub transmitting antennas .
• The next level is the base station , A base station will have 1 To 4 A community .
• At the top is the port , Be responsible for the connection between the client and the server .

Cluster more ？

• Less interference less interference
• Smaller capacity lower capacity（ Because the bandwidth is certain , Block more , Relatively small capacity ）

Sectoring（ Fan ）
It is generally divided into 3 Distribution method of shares

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• Partitioning can make the area covered by each part smaller , Therefore, the quality of communication may be higher , Less interference .

Radio Resource Management（ Management of wireless power supply ）

1. power control

• Reduce interference , Save energy

• terms of settlement ： Signals from other zones can pass through power Filter out the size of

• CDMA Medium cell breathing( Just breathe ), That is, the cell boundary will shrink to a certain extent , So as to avoid signal interference outside the cell

  1.Downlink interference

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• Problem description ： For example GSM In the system , When adjacent BTS Same use frequency, On the edge of terminal Will receive Downlink interference（ Received different BTS Different signals ）

• Affected people ： terminal

• Question why ： Some BTS The signal is too strong

• terms of settlement ：Power control on BTS( Control signal strength , Form a virtual boundary )compensates for and minimises inter-cell interference

  2.Uplink interference

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• Problem description ： For example 3G in ,Node B1 The received is served by others Node B Of [UE](user equipment) The signal of , Interferes with the use of the same from its services code Of UE The signal of
• Affected people ：BTS(Node B)
• Question why ： Same use code, Some UE On the edge （ Try to increase the signal strength to communicate ）
• terms of settlement ：Power control on UE( control UE Signal strength , Reduce energy consumption . The latter in DL interference Not considered , because BTS The power supply is generally sufficient , and UE It may be shut down due to insufficient power supply ）

 3. SNR&SINR

• SNR>target : decrease power
• SNR<target : increase power

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• Three types of power control
  1. Open-loop PC: used as initial power setting
  2. Closed-loop PC: fast power control loop (1.5 KHz)
  3. Outer-loop PC: set target for closed-loop PC.

4.Cell breathing in CDMA

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a mobile might not get sufficient signal In gap （ because power control） And no signal

2. handover control（ Handover control ）

• When a mobile user crosses a cellular boundary , Keep wireless service
• Maintain the required QoS
• By connecting the mobile phone to the strongest base station or base station , Minimize the interference level of the whole system .
• Roaming between different networks
• Distribute load from hot areas ( Load balancing )

1. Hard Handover (GSM) / Hard switching 

• Signal first To break off , Again Connect
• MT(Mobile Terminal) Need to switch frequency

2. Soft Handover (W-CDMA)/ Soft handoff 

• Signal first Connect , Again To break off

• Every cell All use the same frequency

Advantages of soft switching ：

• Less the“ping-pong”effect Reduce the ping-pong effect
• Smoother transmission Smooth transmission process
• No hysteresis margin Don't delay
• Macrodiversity gain Diversity gain
• Reduced overall uplink interference
• Reduce uplink and downlink Interference at the upper and lower levels

Disadvantages of soft handoff ：

• More complexity More complicated
• Additional network resources are consumed in the downlink direction (code resource and power resource) The energy loss on the lower chain increases

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The challenge of wireless network communication

• Limited resources : Limited radio spectrum
• Support mobility
• Maintain quality of service over unreliable wireless links (QoS)
• Connection and coverage
• Security
• Wireless channel “ to open up ”
• authentication
• Battery life of mobile terminal
• Provide integrated services through a single network

Thank you, sister Dear Slim. And my sister / Long Hermi_Mire Share your notes ！！！ This article is their rearrangement , thank ！！