Shulou(Shulou.com)06/01 Report--

Recently in the attention of the Internet of Things NB-IOT technology, wireless communication this part has some basic concepts to understand.

This paper summarizes some basic concepts and external characteristics of wireless communication (GSM/WCDMA/LTE).

GSM Communication Fundamentals

1. frequency division multiple access

GSM allocated frequency bands GSM900 and GSM1800 .

GSM900 --- 890M - 915M (uplink band) 935M - 960M (downlink band) 25M bandwidth each

GSM1800 --- 1710M - 1785M (uplink band) 1805M-1880M (downlink band) 75M bandwidth each

Carrier frequency bandwidth 200kHz . GSM900 upper and lower frequency points 25*5-1=124. GSM1800 downlink frequency points 75*5-1374.

GSM900 carrier frequency point f(n)=890+0.2n(MHz)

GSM1800 carrier frequency point f(n)=935+0.2n(MHz)

Each carrier frequency point can transmit the signal carrier through its own center frequency.

For a base station (a directional antenna area), when N mobile phones are connected at the same time, different mobile phones use different carrier frequencies when communicating.

2. Time division multiplexing.

A carrier frequency point (200kHz bandwidth) constitutes an actual physical channel.

Although MS and BTS have agreed on physical channel (carrier frequency) for communication, they are not exclusive. The physical channel is divided into N(8) time slots. So each cell phone actually occupies only 1/8 of the transmission bandwidth.

Why are there 8 slots? It's probably the bandwidth allocation--200kHz bandwidth, based on Nyquist bandwidth--the maximum theoretical transmission rate is 2XB, which is 400Kbps . It is about 270Kbps. The modulation rate of each voice channel on TCH(Traffic Channel) is 33.8 kbps. 33.8 x8 =270.833. Just enough for 8 slots.

Nyquist bandwidth Cmax=2×B×log 2 L L is the symbol value carrying data. L depends on the minimum number of bits that can be transmitted at a time by FM. A symbol can represent 0 or 1, or 00, 01, 10, 11. The baud rate is different from the bit rate itself.

Another Shannon theorem is Cmax= B* log2 (1+S/N) . S/N is the signal to noise ratio. ah, this... It seems to conflict with Nyquist bandwidth, Shannon's law is the maximum capacity, and it seems unclear how to achieve it..

After frequency division and time division multiplexing. The number of terminals that can be simultaneously connected can be roughly regarded as the number of carrier frequencies allocated by the cell X 8 , of course, there are more detailed algorithms. The main problem is channel allocation. Not all physical channels are TCHs (Traffic Channels) used for actual speech transmission. There are also control channels.

3. frequency hopping

Base station and terminal jump to the designated carrier frequency in the next time slot according to the agreed rule.

4. interleaving

Scatter successive bits of a codeword into multiple frames to reduce correlation. Avoid large signal loss caused by channel transmission problems.

5. Power reduction techniques

DRX DiscontinuousReception Handsets are divided into paging groups, and the handsets in the paging group only turn on reception at the appointed time period to detect whether there is a message of their own in the broadcast channel.

DTX Discontinuous Transmission During conversations, only 40% of them are voice transmissions. (statistics) transmitted only when speaking.

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GSM Channel Division

This is the logical channel. And specific physical channels are not exactly the same. Especially the control channel.

Roughly divided into TCH traffic channels and CCH control channels

Frequency Correction Channel (FCCH): Initial synchronization to transmit frequency correction signals to mobile stations, so that mobile stations can tune to the corresponding frequency.

Synchronization Channel (SCH) : provides all the messages that MS needs to synchronize, TDMA frame number (22 bits) and base station identification code (BSIC) number (6 bits) of this cell.

Broadcast Control Channel BCCH : Broadcast cell information such as location area identification number (LAI) , cell identification number (CGI), cell frequency list, neighbor cell description, etc.

When the mobile phone is turned on, the channels on which it is synchronized in sequence are: FCCH, SCH and BCCH.

CCCH common control channel, including PCH (paging channel downlink) AGCH (allowed access channel downlink) RACH (random access channel uplink)

DCCH dedicated control channel, divided into

Independent Dedicated Control Channel (SDCCH): Signaling for mobile station and base station connectivity and channel assignment; also carries call forwarding and short message information.

The companion channel (ACCH) provides periodic information transmission (not detailed)

Roughly understood scenarios:

1. After the mobile phone is turned on, it must synchronize FCCH, SCH and BCCH before it can know its cell environment and how to communicate with the base station.

2. Report to cell base station via RACH channel. Periodic communication messages via DCCH and broadcast channels.

3. When there is a call notification, the base station pages via PCH/AGCH.

4. Voice transmission over TCH channels.

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About SIM Card

SIM card is composed of MCU +RAM+ ROM.

SIM card has PIN code and PUK code. Pin can be used to lock SIM card, PUK code is unlocked.

SIM card internal ROM storage content:

IMSI (International Mobile Subscriber Identity) is the key one. Ki is authentication encryption related data.

China is still mainly a separate SIM card, card machine separation. Convenient for users to change machines.

If the Internet of Things nb-iot, it is estimated that a large number of eSIM promotion. That is, directly a chip, soldering on the PCB can be.

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GSM radio receive signal path from RF to baseband signal typical process.

The mixer converts RF signal from high frequency to intermediate frequency for amplification. It will not affect the demodulation of the signal it carries.

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GSM network architecture

WCDMA Network and Interface Diagram

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GSM numbering scheme

MSISDN (Mobile Station International ISDN Number)

Simple point is usually used mobile phone number, plus the country code (86) in front

For example, mobile phone number 86 13578720572

CC = 86 (country code)

NDC = 135 (mobile access code)

SN =78720572 User Code.

IMSI, International Mobile Subscriber Identity

IMEI (International Mobile Equipment Identity)

SIM card independent, serial number of mobile device. A unique identifier for a mobile device like a cell phone or tablet.

Is the bit in the picture above a binary bit? A decimal digit. You can check the IMEI number in your phone. Settings--> About Mobile. Or dial "*#06#"

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Base station information (GSM)

CGI Global Cell Identifier

Cell information is used in cell phone location.

The mobile app queries the current location through the map server by obtaining cell base station information (MCC LAC CI ) + BSSS (Base Station Signal Strength).

in addition,

CDMA base stations and 4G base stations also have associated coding parameters:

CDMA Base Station:

SID (system) , NID (network) , BID (base station)

SID is the system identification code, each prefecture-level city only one sid, is unique.

NID is the network identification code, managed by each local network, that is, allocated by prefecture-level branches. Each prefecture-level city may have 1 to 3 nids.

BID Base Station ID

Cell Global Identity ECGI for LTE

ECGI =(PLMN ID) + (Cell Identity)

PLMN(Public Land Mobile Network) consists of MCC(Mobile Country Code)+MNC(Mobile Network Code).

Cell Identity consists of EnID +Cell ID. It contains 28 bits of information, with the first 20 bits representing EnID and the last 8 bits representing Cell ID.

MCC: Mobile Country Code, the resources of MCC are uniformly allocated and managed by ITU, uniquely identifying the country to which the mobile user belongs, with a total of 3 digits, 460 for China;

MNC: Mobile Network Code, 2 digits in total, 00 for China Mobile TD system, 01 for China Unicom GSM system, 02 for China Mobile GSM system, 03 for China Telecom CDMA system

LTE base station positioning can be performed through MNC EnodeID and Cell ID.

Articles for individuals (non-communications professionals) to understand the basics of wireless communications.

vivekchen@qq.com

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