Some of the Existing Systems

Wired Communication – Telephone Company Dial-up – 56kbps DSL – Digital Subscriber Line – ADSL: Asymmetric DSL, different upload and download bandwidth – Available bandwidth is about 1.1MHz, divided into 256 channels, one for voice, some unused or for control, the rest divided among upstream and downstream data. My DSL at Pittsburgh was 100kbps upstream and 768kbps downstream – How ADSL is set up. Fig The ADSL modem is 250 QAM modems operating at different frequencies. The actual QAM depends on the noise.

Wired Communications – The Cable TV Company Cable frequency allocation. Fig – Downstream channel bandwidth is 6MHz, and may use QAM-64. – Upstream channel is worse so use QPSK. – Upstream – stations contend for access (MAC layer issue, will be discusses later) – Downstream – no contention, from the head end to user – Shared medium, so some security is needed

Wired communication – Optical Backbone SONET – Synchronous Optical Network – OC-1: 51.84Mbps – OC-3: 3*51.84Mbps – OC-9: 9*51.84Mbps – … Used for backbone switching SONET Ring. Adder. Dropper. WDM (Wavelength Division Multiplexing)

Cellular Phone Networks User – base station – Telephone network FDMA – Frequency division multiplexing TDMA – Time division multiplexing CDMA – Code division multiplexing

GSM – Global System for Mobile Communications Second generation cell phone system (digital, first generation analog). GSM-900 and GSM-1800 are most widely used – GSM-900 uses MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and MHz for the other direction (downlink). Mobile StationBase Transceiver Station FDMA + TDMA Each user transmitting on a frequency and receiving on another frequency. 124 pairs of 200 KHz channels. Each channel divided into 8 time slots for 8 users. Each user is has a chance to transmit every ms. Each time he can send 114 data bits – 24.7kbps.

CDMA The 3G network uses CDMA. A good analogy in the book – A group of people in a room. – TDMA means they talk in turn. – FDMA means that those who wants to talk sit in different corners and can’t hear other pair. – CDMA means each pair talks in a different language and other people’s voice is noise to them.

CDMA The whole bandwidth is used by every user. Meaning that they can send out symbols really fast. The trick is to make what A sent appear as 0 to B. – Because we have a fast symbol rate, for each data bit, we send out, say, 8 bits, call the “small bits” chips. – Given a bit, if 1, send out, say, -1,-1,-1,1,1,-1,1,1, and if 0, 1,1,1,-1,-1,1,-1,-1 – This is called the chip sequence. – The key is that each station has a unique chip sequence (language), and different languages are orthogonal. – Fig

Wireless LAN Physical Layer b,g, n in the 2.4G band and a in the 5G band. People now consider as the notion of MAC layer protocol, while a, b, g, or n, are about physical layer b. 1, 2, 5.5, 11Mbps. – 1Mbps: BPSK modulation. 1 bit into 11 chips with Barker sequence −1 −1 −1 +1 −1 −1 +1 −1. Why spread spectrum? Required by FCC but was later removed – 2Mbps: QPSK. – 5.5M and 11M: use some bits to select chip sequence and use two bits for QPSK a. Up to 54Mbps. OFDM g. Up to 54Mbps. OFDM n. Up to 300Mbps. OFDM on MIMO.

Multipath Fading In wireless communications, in addition to the bandwidth limit and additive noise, you also have multipath fading! The faster your symbol rate is, the more badly you will be affected by multipath fading.

OFDM (Orthogonal Frequency Division Multiplexing) OFDM: given a wideband channel, divide it into many sub channels. Each sub-channel can be modulated/demodu lated independently. A clever trick using guard symbols removes the multi- path effect, at the cost of some overhead. In implementation, use IFFT and FFT. Figure

MIMO Used in n. t transmit antennas and r receive antennas. With the knowledge of channel matrix, by pre-processing the data, equivalent to min{t,r} channels.