An insight to digital communication systems and associated phenomenon

TDMA – Time Division Multiple Access

FDMA – Frequency Division Multiple Access

CDMA – Code Division Multiple Access

Rx: 869-894MHz Tx: 824-849MHz 832 Channels spaced 30kHz apart

(3 users/channel) DQPSK modulation scheme 48.6kbps bit rate Interim Standard (IS) – 54 Digital AMPS (Advanced Mobile Phone

System) Uses Time Division Duplexing (TDD)

usually

The incoming data from each source are briefly buffered and scanned to to form a composite digital data stream mc ( t ) .

Preston University Kashif Shahzad 07-5

Buffer

Buffer

Buffer

1 2 N

U1U1

U2U2

UNUN

m1 ( t )

m2 ( t )

mN ( t )

mc ( t )

Scan operation

preamble

1 2 N

preamble

...

information

Each slot may be empty or occupied.+ has preamble & guard bits

Frame Frame

Time slot

Flexible bit rate No frequency guard band required No need for precise narrowband filters Easy for mobile or base stations to initiate

and execute hands off Extended battery life TDMA installations offer savings in base

station equipment, space and maintenance

The most cost-effective technology for upgrading a current analog system to digital

Requires network-wide timing synchronization

Requires signal processing fro matched filtering and correlation detection

Demands high peak power on uplink in transient mode

Multipath distortion

Rx: 869-894MHz Tx: 824-849MHz 832 Channels spaced 30kHz apart

(3 users/channel) DQPSK modulation scheme 48.6kbps bit rate Used in analog cellular phone systems

(i.e. AMPS) Uses Frequency Division Duplexing (FDD) ISI (Intersymbol Interference) is low

If channel is not in use, it sits idle Channel bandwidth is relatively narrow (30kHz) Simple algorithmically, and from a hardware

standpoint Fairly efficient when the number of stations is

small and the traffic is uniformly constant Capacity increase can be obtained by reducing

the information bit rate and using efficient digital code

No need for network timing No restriction regarding the type of baseband

or type of modulation

The presence of guard bands Requires right RF filtering to minimize

adjacent channel interference Maximum bit rate per channel is fixed Small inhibiting flexibility in bit rate

capability Does not differ significantly from

analog system

Rx: 869-894MHz Tx: 824-849MHz 20 Channels spaced 1250kHz apart

(798 users/channel) QPSK/(Offset) OQPSK modulation scheme 1.2288Mbps bit rate IS-95 standard Operates at both 800 and 1900 MHz

frequency bands

Spread Spectrum Multiple Access Technologies

Many users of CDMA use the same frequency, TDD or FDD may be used

Multipath fading may be substantially reduced because of large signal bandwidth

No absolute limit on the number of users Easy addition of more users Impossible for hackers to decipher the code

sent Better signal quality No sense of handoff when changing cells

As the number of users increases, the overall quality of service decreases

Self-jammingNear- Far- problem arises

A communications channel is classified as one of three types:(depending on the direction of transfer) Simplex Full-Duplex Half-Duplex

Simplex: a simplex mechanism can only transfer data in a single direction It is analogous to broadcast radio or television Figure 9.8a illustrates simplex communication

Full-Duplex: allows transmission in two directions simultaneously It is analogous to a voice telephone conversation

▪ in which a participant can speak even if they are able to hear background music at the other end

Figure 9.8b illustrates the concept1616

1717

RS-232 is a popular communications interface for connecting modems and data acquisition devices (i.e. GPS receivers, electronic balances, data loggers, ...) to computers.

RS-232 can be plugged straight into the computer’s serial port (know as COM or Comm port).

Architecturally RS-232 is a bi-directional point to point link.

(serial port - PC side)

Two independent channels are established for two-way (full-duplex) communications.

RS-232 can also carry additional signals used for flow control (RTS, CTS) and modem control (DCD, DTR, DSR, RI).

Common 25 pin D-shell connector pinout used for asynchronous data communications.

PinPin SignalSignal1 PGND Protective Ground2 TXD Transmit Data3 RXD Receive Data4 RTS Ready To Send5 CTS Clear To Send6 DSR Data Set Ready7 SG Signal Ground8 CD Carrier Detect20 DTR Data Terminal Ready22 RI Ring Indicator

(serial port - PC side)

How fast can RS-232 be? The maximum speed, according to the standard, is 20kbit/s.

However, modern equipment can operate much faster than this. (i.e. Lynx can reach 115200 baud.)

The length of the cable also plays a part inmaximum speed.The longer the cable and the slower the speed at which you can obtain accurate results.A large wire capacitance and inductance limits the maximum length of the cable and/or the maximum speed; Moreover higher is the capacitance of the cable higer is the interference between two adjacent signal wire.50 feet (15m) @ max baudrate is commonly quoted as the maximum distance.It is not specified in EIA standard but it’s recommended respect these values.

One byte of async data has: Start Bit = 1 (always) Data Bits = 8 (or 7)

Stop bits = 1 (or 2) Parity = NONE (or EVEN or ODD)

+ 25

- 25

Long distance transmission is typically done over a network of switched nodes

Nodes not concerned with content of data

End devices are stations Computer, terminal, phone, etc.

A collection of nodes and connections is a communications network

Data routed by being switched from node to node

Nodes may connect to other nodes only, or to stations and other nodes

Node to node links usually multiplexed Network is usually partially connected

Some redundant connections are desirable for reliability

Two different switching technologies Circuit switching Packet switching

Dedicated communication path between two stations

Three phases Establish Transfer Disconnect

Must have switching capacity and channel capacity to establish connection

Must have intelligence to work out routing

Inefficient Channel capacity dedicated for duration

of connection If no data, capacity wasted

Set up (connection) takes timeOnce connected, transfer is

transparentDeveloped for voice traffic (phone)

Circuit switching designed for voice Resources dedicated to a particular call Much of the time a data connection is

idle Data rate is fixed

▪ Both ends must operate at the same rate

Data transmitted in small packets Longer messages split into series of packets Each packet contains a portion of user data

plus some control info Control info

Routing (addressing) info Packets are received, stored briefly

(buffered) and past on to the next node Store and forward