Module 14

Applications of Digital Communication Systems

2

Module Intro

•  The last 15 years have been an on-going revolution in the communications field. Television and cell phones have both made the jump to all digital. Radio has also gone digital with satellite service, but terrestrial radio remains the only analog hold out (for now).

•  In this final module we look at several digital applications. We will focus on 3 areas of high development activity. First, we’ll discuss the new digital TV definition. Then, we look at a snapshot of 2nd,3rd, and 4th generation cellular phones. And, we conclude with WiFi and Bluetooth (IEEE 802.11).

3

•  Digital TV Broadcasting •  ATSC Digital Standard •  HD and SD Formats

•  Digital Cellular Systems •  GSM/CDMA2000 •  1G,2G,3G,4G •  Foliage Penetration

•  Other Wireless Applications •  Bluetooth •  WiFi

Module Outline

Lathi: Sections 6.8, 12.3, 12.8

4

•  2009 – TV broadcasting in the US switched from analog (NTSC) to digital (ATSC- Advanced Television Systems Committee).

•  6 MHz - Bandwidth allowances for TV broadcasting did not change. So,

channels are still allocated 6MHz of broadcast bandwidth. With MPEG-2 video compression, a 19.39 Mbit/s stream can fit into this bandwidth.

•  19.39 Mbit/s - A broadcaster can choose to send a single program in full HD

with 19.39 Mbps.

•  Sub-channels - A broadcaster can also choose to divide the channel into several different streams (perhaps four streams of 4.85 Mbps each). These streams are called sub-channels, and this type of broadcasting is called multicasting. For example, if the digital TV channel is Channel 53, then 53.1, 53.2 and 53.3 could be three sub-channels on that channel. Each sub-channel can carry a different program. This feature can be varied. For the premier of a movie the broadcaster can stream only the 1 HD channel or convert to HD during prime time while allocating 4 SD channels during the day.

HDTV

Digital TV

5

480i - The picture is 704x480 pixels, sent at 60 interlaced frames per second (30 complete frames per second). 480p - The picture is 704x480 pixels, sent at 60 complete frames per second. 720p - The picture is 1280x720 pixels, sent at 60 complete frames per second. 1080i - The picture is 1920x1080 pixels, sent at 60 interlaced frames per second (30 complete frames per second). 1080p - The picture is 1920x1080 pixels, sent at 60 complete frames per second.

i=interlaced (half the picture image updates every 60th of a second)

Standard Definition

DTV

HDTV

Formats

p=progressive (the entire picture image updates every 60th of a second)

Digital TV

Digital TV MPEG-2 Compression

I-frames – “Intraframe-coded” - This frame does not require information from previous frames to reconstruct the image.

P-frames – “Prediction”

- Information from the previous frame is used to predict the new image content. Parts of the image that have changed since the last frame may be sent via intraframe-coding. This means those parts of the P-frame are not predicted from previous frames. - These frames allow the data stream to be greatly decreased in size.

B-frames – Contains information about future frames.

- B-frames allow for higher performance but also increase the latency between reception of a video stream and display.

6

8VSB is the modulation method used for broadcast in the ATSC digital television standard. ATSC and 8VSB modulation is used primarily in North America; in contrast, the DVB-T standard uses COFDM. A modulation method specifies how the radio signal fluctuates to convey information. ATSC and DVB-T specify the modulation used for over-the-air digital television; by comparison, QAM is the modulation method used for cable. The specifications for a cable-ready television, then, might state that it supports 8VSB (for broadcast TV) and QAM (for cable TV). 8VSB is an 8-level vestigial sideband modulation. 8VSB is capable of transmitting three bits (23=8) per symbol by amplitude modulating a sinusoidal carrier to an intermediate frequency. The resulting signal is then band-pass filtered with a Nyquist filter to remove redundancies in the side lobes, and then again modulated to the broadcast frequency.

Source: http://en.wikipedia.org/wiki/8VSB

Digital TV in North America uses the same 6MHz channel allocations as the former Analog TV scheme.

8VSB

Digital TV

7

Generations of Mobile Phone Networks

2G (1992) – Differs from 1G in that both signaling and speech channels are digital.

3G (2002) – Mobile Broadband. Based on CDMA techniques. Services include voice telephone, mobile internet, video calls, and mobile TV.

4G – Mobile phone generations have appeared every 10th year since 1G. Devices currently marketed as 4G, however, are more like advanced versions of 3G

(aka 4G LTE). 4G device standard does not include telephones but data rates are expected to be up to 1Gbit/s.

1G (1982) – Original, analog cellular systems (AMPS).

Digital Cell Systems

8

TDMA characteristics (GSM) •  Shares single carrier frequency with multiple users •  Non-continuous transmission makes handoff simpler •  Slots can be assigned on demand in dynamic TDMA •  Less stringent power control than CDMA due to reduced intra cell interference •  Higher synchronization overhead than CDMA •  Advanced equalization may be necessary for high data rates if the channel is "frequency selective" and creates Intersymbol interference •  Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA. •  Frequency/slot allocation complexity •  Pulsating power envelop: Interference with other devices

GSM – Global System for Mobile communications

2G – GSM is primarily a Second Generation communication network. It uses TDMA to allow multiple access to its users. 80% - of the global mobile market uses GSM. (1.5 billion people across more than 212 countries)

Digital Cell Systems

9

Characteristics (CDMA2000) •  User signals have 1.25MHz uplink and

downlink bandwidth.

•  Adaptive modulations including: QPSK (Quad Phase Shift Keying or 4-PSK, 8-PSK, and 16-QAM.

•  US Frequency Allocations:

CDMA2000/cdmaOne 2G – Verizon Wireless and Sprint Nextel offer CDMA based devices. (T-Mobile has always been GSM while AT&T made the switch to GSM back when it was still Cingular.)

4G LTE – Departs from CDMA to use a GSM based method.

Digital Cell Systems

3G – Smartphones continue to be CDMA. Mobility

110M

100M

50M

30M

Num. US Customers

http://en.wikipedia.org/wiki/List_of_mobile_network _operators_of_the_Americas#United_States

10

11

Tower

Digital Cell Systems

588.0284.033.1 fGHzf dfL =•  Full summer bloom. •  Volume 100% full of trees (no gaps). •  Should be max attenuation under dry conditions.

Foliage Penetration

•  A challenge quickly identified in the cellular application is that of foliage penetration.

•  RF energy is scattered by tree trunks and absorbed by leaves. •  The solution to the loss problem is to put the cell antenna on a tower. •  Shown here is a mathematical model for the loss observed by trees.

12

•  Red line is expected signal strength, if no trees are present. •  Dashed black line is the design spec: -80dBm. (About 3 “Bars”) •  Blue line is expected signal strength, with 30’ tall trees raising antenna height from 30’ to 500’.

Digital Cell Systems

[1]  Weissberger, M. A., "An Initial Critical Summary of Models for Predicting the Attenuation of Radio Waves by Trees," ESD-TR-81-101, Electromagnetic Compatibility Analysis Center, Annapolis, MD, July 1982.

Loss Model

- Differs from WiFi (IEEE 802.11) in that it is designed for short distance comm links. (like a wireless mouse or printer) - Employs Frequency Hopping Spread Spectrum (FHSS) (In fact, Bluetooth is 802.11 with a frequency hopping option. This option was dropped from the 802.11 definition after the release of the 802.11a variant.) - Operates in the 2.4 - 2.4835 GHz band, which is a license-free frequency band set aside for Industrial, Scientific, and Medical (ISM) purposes. - Uses 79 subchannels with 1MHz bandwidths. - Frequency hopping is slow - at a rate of 1600Hz. - Modulation is binary GFSK (Gaussian Frequency Shift Keying); also called GFSK-2. - Throughput is 723.1 kbit/s - These details are for Version 1. A second Bluetooth version has also been released. - Version 2 throughput rate is 2Mbit/s.

Bluetooth

Wireless Applications

13

IEEE 802.11 (WiFi)

802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, 802.11g and 802.11n versions to provide wireless connectivity in Wireless Local Area Networks (WLAN). It was originally released in 1997. Frequency bands of use can be either 2.4GHz or 5.0GHz. 802.11b ruled as the default WLAN method for a while. It dropped the FHSS option of the original definition and switched to CDMA. The 802.11a standard uses the same core protocol as the original, operates at 5 GHz and uses a 52-subcarrier orthogonal frequency-division multiplexing (OFDM) with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable throughput in the mid-20 Mbit/s. 802.11g is meant to be backward compatible with 802.11b. It thus has both CDMA (802.11b) and OFDM (802.11a) mechanisms. 802.11n uses MIMO (multiple input multiple output) to increase data rates from 54 to 600 Mbit/s.

Wireless Applications

14

15

•  Digital TV has become a standard for television broadcasting in North America. A 19.39Mbit/s data stream is compressed using MPEG-2 and line coded with an 8 level technique. This digital signal is then modulated to a 6MHz bandwidth using Vestigial Sideband modulation. The same UHF/VHF frequency allocations are still used for the new digital signals.

•  Cell phone technology is a fast paced world of development. Two primary technology camps, GSM and CDMA, continue to compete for dominance. Since introduction in 1982, cell phones have undergone generational changes. We currently find ourselves at the dawn of the 4th Generation.

•  We make use of WiFi and Bluetooth devices every day. Bluetooth is a derivative of the original 802.11 definition, which still makes use of frequency hopping. It’s purpose is extremely short distance connectivity. WiFi on the other hand is our bread and butter connection between our computer and the coffee shop’s wireless internet router. WiFi is officially called “IEEE Standard 802.11”. We’ve encountered 802.11a, b, and g variants. The latest and greatest 802.11n makes use of MIMO to give us even faster data transfer rates.

Summary