More on Cellular Telephony LUCID Summer Workshop July 28, 2004
Overview of Last Time We introduced the cellular concept. • Geographic area is broken into smaller cell. • Cell phone users in each cell communicate with base station, which has a high antenna tower. • As users move from one cell to the next, call is handed off from one base station to the next. • Frequency reuse is used to support a large number of users over a large coverage area using limited spectrum. • Local base stations are connected to and controlled by a mobile switching center (MSC).
Outline for Today • We will continue our discussion on AMPS, the analog first generation cellular system deployed in the U.S. • We will also look at how the second generation digital systems differ from AMPS. • Finally, we consider what components make up a cell phone and what a base station looks like.
Cellular Network MSC Public (Wired) Telephone Network MSC MSC MSC
Overview (Cont’d) We began looking at AMPS, U.S.’s first cellular system. • Analog system based on FDMA • 832 frequency channels available; 416 in each direction. • 42 of them are control channels (used for coordination between user terminals and base station). • Remaining are voice channels (that actually carry phone conversations).
Cellular Identifiers • In AMPS, a cell phone subscription is identified using three numbers: • Electronic Serial Number (ESN): unique 32 bit number programmed into the cell when it was manufactured. • Mobile Identification Number (MIN): 10 digit phone number. • System Identification Code (SID): Unique 5 bit code assigned by the FCC to each service provider.
What happens when you receive a call? • When you first power the phone, it listens for an SID on the control channel. • Recall: control channel is special frequency that the phone and base station use to talk to each other about things like call setup and channel changing, etc. • If phone cannot find any control channels, then it is out of range and it lights up the “No Service” light.
Call Reception • When it receives in SID, the phone compares it to the SID programmed in the phone. • If it matches, then phone is in the home system. • If it does not match, the phone is roaming. • Phone transmits a registration request to the base, which forward this request to the MSC.
Location Registry • MSC uses this registration request to update a large database (called the location registry) which keeps track of the latest location of the cell phone. • This helps network find a phone when a call comes in for it. • Also it informs the MSC if the cell phone user is valid (legitimate paying customer). • MSC also learns of phone subscription features, like caller-id, etc., from the MSC.
Call Reception • Assume a call comes in for the phone. • The MSC tries to find the phone by looking up the database. • MSC uses a frequency in the cell in which the phone was last in, and transmits an “incoming call” message over the control channel with the phone’s ESN and MIN numbers. • This message also tells the phone which frequency to switch to communicate with the base and complete the conversation.
Call Reception; Call Handoff • The phone and base station tower switch to these frequencies and the call is connected. • Now assume the phone user is moving around and moves to the edge of its serving cell. • Base station notes that the strength of the radio waves from this phone is diminishing. • Meanwhile, a nearby base station notes that the signal strength to this phone is increasing.
Call Handoff • All base stations constant monitor the signal strength on all voice channels (all 416) in order to pinpoint users who may be moving into their coverage area. • When the signal gets weak enough at the first base station and strong enough at the second base station, the base stations send a signal to the MSC. • The MSC determines the new frequency in the new cell that user should switch to.
Call Handoff (Cont’d) • The new frequency is conveyed to the phone. • The phone switches to the new frequency (seamlessly) and the new base station tunes into this frequency and starts receiving signals from the phone. • This way the phone gets handed-over to the new base station.
Roaming • When SID of the phone does not match the SID of the nearest base station, the phone knows its roaming. • The MSC of the system that the phone is roaming in contacts the MSC of the phone’s home system. • The home MSC verifies the phone (valid, paying user, etc.) to the local MSC. The local MSC then keeps track of the phone as it moves thru the local system. Each time updating the database at the home system.
Cell to Cell Call • Let’s say there is a phone in a cell that wishes to talk to another phone in that cell. • Assume that both the phones are in a cell of their home system (thereby, they both have the same home system). • These two phones must talk to each other via the base station. • Future cell phones systems (perhaps 4G) may allow phones to connect directly with each other (peer-to-peer connection).
Cell to Cell Call (Cont’d) • Now assume the two phones are in the same cell, but current cell is part of home system for only one of the phones. • Assume current cell is in Susquehanna County. • The other cell phone user is visiting from Florida, where its home system is. • Assume that the phone from Florida makes a phone call to the Susquehanna phone.
Cell to Cell Call (Cont’d) Florida Phone Registration Request Verify Phone Wired Network Registration Request Susquehanna MSC Susquehanna Phone Verify Phone Verified Verified Assign Frequency Location Registry Florida MSC Location Registry All of this happens in the matter of a few seconds!
Cell to Cell Call (Cont’d) Florida Phone Susquehanna Phone
Recall: Analog vs Digital • In analog cellular, the audio signal (conversation) is converted into a radio wave directly. The speech is embedded by varying the frequency of the radio wave (FM modulation). • In digital cellular, the audio speech signal is first converted to a sequence of 0s and 1s. The transmitted radio waves conveys this sequence of 0s and 1s over the air from the transmitter to the receiver.
What’s the benefit of going digital? • Digital cell phones use same radio technology as analog phones, but they use it in a different way. • Analog systems do not fully utilize the bandwidth between the phone and the cellular network. • For example, analog signals cannot be compressed and manipulated as easily as a true digital signal. • Digital signals are basically more efficient.
Quick Aside: Review Multiple Access Methods • There are three common technologies used by cell-phone networks for transmitting information: • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Code division multiple access (CDMA) • FDMA puts each call on a separate frequency band. • TDMA assigns each call a certain portion of time on a designated frequency band. • CDMA gives a unique code to each call and spreads it over the available frequencies.
TDMA: A Digital Cellular Method • The benefit of digital cellular systems over the FDMA analog systems can be seen by looking a simple gain offered by a digital TDMA system. • This gain is result of a property of human speech. • Human speech (generally sound) is caused by minute changes in the air pressure. • When a caller speaks on his/her phone, the microphone records these varying pressure changes into varying electronic signals.
Speech Signals • Microphone outputs an analog electrical signal. • For example, here is a graph showing the analog wave created by saying the word "hello“.
A to D Conversion • In digital systems, this analog signal is converted to a sequence of 0’s and 1’s. • One way this can be done is to sample the analog signal in fixed time intervals: Sampling instances
A to D Conversion (Cont’d) • At each sampling instant, the value of the analog signal (a number) is recorded and converted into binary format. • This produces a digital speech signal. • The receiver than performs the reverse process to get an (approximate) analog signal. • It can be shown that the received analog signal sounds like the original speech if the sampling is done at least 8000 times per second, i.e., once every 0.125 milliseconds (ms).
Sampling Rate • So, a cell phone needs to convert its speaker’s voice into a binary number only once every 0.125 ms. • In other words, if the cell phone transmits the digital signal once every 0.125 ms, it will be received by the base once every 0.125 ms and the base will be able to reconstruct an almost perfect replica of the speaker’s words.
Packing in More Users on a Frequency Channel • In analog cellular systems, we give a cell phone user exclusive access to a frequency channel. • If we first convert the analog signal to digital (as in digital systems), then a cell phone user only needs access to a frequency channel every 0.125 ms. • We can then use the frequency channel during the remaining time to support other cell phone users in the cell.
Result: TDMA • Thus a frequency channel is able to support multiple voice users by allowing round robin access. • This is just TDMA. • We see therefore that by converting analog speech to digital speech, we are able to use TDMA to improve the total capacity of the cellular system. • Another benefit of digital speech is that once a signal of 0s and 1s has been generated compression techniques can be used to reduce the number of binary digits the signal contains.
Digital Compression • Digital phones convert your voice into binary information (1s and 0s) and then compress it. • This process and subsequent compression allows between three and 10 digital cell-phone calls to occupy the space of a single analog call. • There are several compression techniques used to further pack in more phone conversations over the same frequency band.
Voice Activity Detection: A Method to Pack in More Users • In phone conversations, we actually speak only 40% of the time. • The remaining 60% is silence. • In analog system, the frequency given to a phone call sits idle, i.e., unused, during these silences. • In digital systems, during silences there is no “signal” to encode into 1’s and 0’s. Thus, there is no need to create a signal during silences.
Voice Activity Detection • Further, it is really easy to insert another phone conversation over the same frequency band during these silences. • Thus, for the same frequency band, the digital system can support more phone conversations than typical analog systems. This method is used in CDMA cellular systems. • Techniques like voice activity detection can be easily implemented in digital systems, using relatively inexpensive digital electronics.
Modulation Differences • Analog systems use FM modulation, i.e., frequency of transmitted radio wave changes as the amplitude of the analog signal changes. • Many digital cellular systems rely on digital modulation schemes, e.g, binary phase shift keying (BPSK) or frequency shift keying (FSK). • FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly between the two to send digital information.
IS-136: 2G TDMA in the U.S. • TDMA is the access method used by Interim Standard 136 (IS-136), 2G standard in the U.S. • Using TDMA, a frequency band that is 30 kHz wide is split time-wise into three time slots, each slot is 6.67 ms long. • Thus, each conversation gets the radio channel for one-third of the time. • Once again, this is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space.
North American TDMA System • Therefore, TDMA has three times the capacity of an analog system using the same number of channels. • TDMA systems operate in either the 800-MHz or 1900-MHz frequency bands. • By the time second generation cellular came about, the FCC had allocated another chunk of spectrum (in the 1900 MHz range) for mobile telephony.
GSM: The Other TDMA System • TDMA is also used by Global System for Mobile communications (GSM). • However, GSM implements TDMA in a somewhat different and incompatible way from IS-136. • GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia, and in the 1900-MHz band in the United States. • GSM is the international standard in Europe, Australia and much of Asia and Africa. • It was developed and deployed well before 2G (digital) systems were in the U.S.
SIM Cards • In covered areas, cell-phone users can buy one phone that will work anywhere where the standard is supported. • To connect to service providers in different countries, GSM users simply switch subscriber identification module (SIM) cards. • SIM cards are small removable disks that slip in and out of GSM cell phones. They store all connection data and identification numbers you need to access a particular wireless service provider.
IS-95: The 2G CDMA Standard • All users transmit in the same wide-band chunk of spectrum. • Each user's signal is spread over the entire bandwidth by a unique spreading code. • At the receiver, that same unique code is used to recover the signal. • CDMA systems need to put an accurate time-stamp on each piece of a signal; they reference the GPS system for this information.
IS-95 (Cont’d) • Between eight and 10 separate calls can be carried in the same channel space as one analog AMPS call. • CDMA technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800-MHz and 1900-MHz frequency bands. • cdma200, a 3G cellular standard, is an enhanced version of IS-95.
Improved Security • Another benefit of digital cellular is that it makes phone conversations much more secure. • Security features help ensure eavesdropping does not occur. Also reduces fraudulent use. • Encryption methods make it difficult for an undesirable user to hear someone else’s phone conversation. • Authentication methods make sure that both the cell phone and the cell subscription are legitimate.
Cellular versus PCS • Personal Communications Services (PCS) is a wireless phone service very similar to cellular phone service, but with an emphasis on personal service and extended mobility. • The term "PCS" is often used in place of "digital cellular," but true PCS means that other services like paging, caller ID and e-mail are bundled into mobile telephony service. • While cellular was originally created for use in cars, PCS was designed from the ground up for greater user mobility.
Cellular versus PCS (Cont’d) • PCS has smaller cells and therefore requires a larger number of antennas to cover a geographic area. • PCS phones use frequencies between 1.85 and 1.99 GHz (1850 MHz to 1990 MHz). • Technically, cellular systems in the United States operate in the 824-MHz to 894-MHz frequency bands; PCS operates in the 1850-MHz to 1990-MHz bands.
Dual Band versus Dual Mode • If you travel a lot, you will probably want to look for phones that offer dual band, dual mode or both. • Dual band - A phone that has dual-band capability can switch frequencies. This means that it can operate in both the 800-MHz and 1900-MHz bands. For example, a dual-band TDMA phone could use TDMA services in either an 800-MHz or a 1900-MHz system.
Dual Band versus Dual Mode (Cont’d) • Dual mode - In cell phones, "mode" refers to the type of transmission technology used. So, a phone that supported AMPS and TDMA could switch back and forth as needed. It's important that one of the modes is AMPS -- this gives you analog service if you are in an area that doesn't have digital support. • Dual band/Dual mode - The best of both worlds allows you to switch between frequency bands and transmission modes as needed.
Dual Band versus Dual Mode (Cont’d) • Changing bands or modes is done automatically by phones that support these options. • Usually the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at that frequency with that technology first. • If it supports dual bands, it will switch to 800 MHz if it cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the digital mode(s) first, then switch to analog.
Tri-Mode • Sometimes you can even find tri-mode phones. This term can be deceptive. • It may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well as analog. • It can also mean that it supports one digital technology in two bands and also offers analog support.
Tri-Mode (Cont’d) • A popular version of the tri-mode type of phone for people who do a lot of international traveling has GSM service. • Specifically, GSM is supported in the 900-MHz band for Europe and Asia and the 1900-MHz band for the United States, in addition to the analog service.