What makes CDMA work for my smartphone?

Question 1

Data Applications

Answer 1

• texting, emailing, browsing the web
• measured in bits per second (bps)
• flies through a cellular network and the Internet
  
  • cellular network consists of the radio air-interface and the core network

Question 2

Cellular Architecture

Answer 2

• entire space of deployment is divded into smaller regions called cells, often represented by hexagons
  
  • thus, the name cellular networks and cell phones
• one base station (BS) in each cell. connected on the one side to switches in the core network
  
  • has three directional antennas, each of which covers a 120-degree sector
• mobile stations (MS), could be a smart phone, tablet, or any device that can transmit and receive frequencies

Question 3

Why do we divide the space into smaller regions?

Answer 3

Because the wireless spectrum is scarce and the radio signals weaken over space

Question 4

How can the users in the same cell share the same frequency band?

Answer 4

There are two main approaches: orthogonal and non-orthogonal allocation

Question 5

Orthogonal Allocation

Answer 5

• each user is given a small band of frequency in Frequency-Division Multiple Access (FDMA)
• or a timeslot in Time-Division Multiple Access (TDMA)

Question 6

Non-Orthogonal Allocation

Answer 6

• allows all users to trasnmit at the same time over the same frequency band, as in Code-Division Multiple Access (CDMA)

Question 7

Spread Spectrum

Answer 7

• transmitter multiplies the digital signals by a sequence of 1s and minus 1s, called the spreading code
• the receiver multiplies the received bits by the same spreading code to recover the original signals
• only one spreading code used to recover a family of spreading codes, called a family of orthogonal codes

Question 8

Uplink and Downlink

Answer 8

• Uplink: mobiles talk to the base station
• Downlink: base station talks to the mobiles

Question 9

Top Three Issues in Wireless Channels

Answer 9

• one user’s signal is every other user’s interference
• attentuation of signals over distance
• fading of signals along multiple paths

Question 10

Summary - Cellular Technology

Answer 10

• TDMA and FDMA are both orthogonal resource allocations
• in a cellular network, frequencies can be re-used in non-adjacent cells
• for wireless transmission, the relationship between attenuation (A) and distance (d):
  
  • attenuation is inversely proportional to somehwere between the square and fourth power of the distance, depending on the propagation environment
• CDMA is a non-orthogonal resource allocation

Question 11

Negative Externality

Answer 11

• your cup of tea is other people’s poison

Question 13

Near-far Problem

Answer 13

• a user standing right next to the BS can easily overwhelm another user far away at the edge of the cell

Question 15

Transmit Power Control

Answer 15

• receiver infers the channel quality and sends that back to the transmitter as feedback
• all the received signal strengths will be made equal

Question 17

Signal to Interference Ratio (SIR)

Answer 17

• the ratio between the received signal strength and the sum strength of all the intereference plus the receiver noise strength
• SIR_i = P_iG_ii/(ΣP_jG_ij + ni)
  
  • y = optimal SIR
  • P = constant mW
  • Gii = channel gain
  • Gij = channel loss

Question 19

What affects the SIR of a link?

Answer 19

• the direct channel gain of the link
• the transmit powers of other links
• the noise at the link’s receiver
  
  • the higher the link’s direct channel gain, the higher the SIR
  • the lower the transmit power of the other links, the less the interference on this link, which raises the SIR
  • and the own link’s reciever noise affects it’s SIR
  • but the noise on the other link’s receivers does not affect the SIR

Question 21

Distributed Power Control (DPC)

Answer 21

• each pair of transmitter and receiver does not need to know the transmit power or channel quality of any other pair
• at each timeslot, all it needs to know is the actual SIR it currently achieves at the receiver
• by taking the ratio between the fixed, target SIR and the variable, actual SIR value measured for this timeslot, and multiplying the current transmit power by that tratio, we get the transmit power for the next timeslot
• P₁(t + 1) = (Y₁/SIR₁(t)) * P₁(t)

Question 23

Suppose a link has a transmit power P(t), with a measured SIR of 2 and desired SIR of 3. According to DPC, what should the transmit power of the next iteration, P(t+1), be?

Answer 23

• 3/2 * P(t)
  
  • the transmit power updates to the ratio of the desired to measured SIR

Question 25

DPC Theory Modules

Answer 25

• optimization theory
• game theory

Question 27

Optimization Theory

Answer 27

• Objective: power minimization
• Constraints: achieve target SIRs for all users
• Variables: transmit powers
• Constants: channels, noise, target SIRs

Question 29

What describes a feasible optimization problem?

Answer 29

• The constraints can be simultaneously satisfied by some choice of variables
  
  • a feasible optimization problem need not have a unique solution: There could be multiple optima
  • it need not be linear or convex: The objective and/or constraints could be linear, non-linear, convex, non-convex, and so on
  • a feasible optimization problem simply guarantees that some choice of variables will work

Question 31

Game Theory

Answer 31

• power control is a compeition
• games are models for
  
  • competition
  • cooperation
• a game consists of:
  
  • a set of players
  • a strategy space for each player
  • a payoff (utility) or cost function for each player

Question 33

Prisoner’s Dilemma

Answer 33

• game analyzed in game theory that shows why two purely “rational” individuals might not cooperate, even if it appears that it is in their best interests to do so
• strategies
  
  • best response (disregarding everybody)
  • dominant (taking everybody’s strategies into account)

Question 35

Coordination Game

Answer 35

• a class of games with multiple pure strategy Nash equilibria in which players choose the same or corresponding strategies
• best strategies match
• no dominant strategies

Question 37

Nash Equilibrium

Answer 37

• a solution concept of a non-cooperative game involving two or more players, in which each player is assumed to know the equilibrium strategies of the other players, and no player has anything to gain by changing only their own strategy
• if each player has chosen a strategy and no player can benefit by changing strategies while the other players keep theirs unchanged, then the current set of strategy choices and the corresponding payoffs constitute a Nash equilibrium
• stated simply, Amy and Will are in Nash equilibrium if Amy is making the best decision she can, taking into account Will’s decision, and Will is making the best decision he can, taking into account Amy’s decision
• likewise, a group of players are in Nash equilibrium if each one is making the best decision that he or she can, taking into account the decisions of the others in the game

Question 39

Calculating Convergence in SIRs/Powers

Answer 39

Calculate SIRs

• SIR_i = P_iG_ii/(ΣP_jG_ij + n_i)
  
  • y = optimal SIR
  • P = constant mW
  • G_ii = channel gain
  • G_ij = channel loss

Calculate Power Levels

• P₁(1) = (y₁/SIR(0)) * P₁(0)

Calculate new SIRs

• subsitute power levels to original SIRs

Question 41

What makes CDMA work for my cell phone?

Summary

Answer 41

• Different users’ signal interfere with each other in the air
  
  • feasible SIR region with a Pareto-optimal boundary
• Interference coordination in CDMA uses DPC with feedback
  
  • solves an optimization problem in the form of LP
  • or modeled as a non-cooperative game