CS 6956 Wireless & Mobile Networks April 1 st 2015

Transcription

1 CS 6956 Wireless & Mobile Networks April 1 st 2015 The SIM Card Certain phones contain SIM lock and thus work only with the SIM card of a certain operator. However, this is not a GSM restriction introduced by mobile network operators to ensure that a subsidized phone is used only with SIM cars of their network. Contains IMSI (International Mobile Subscriber ID) and secret key K (for authenticating with the home network) There are tools to read parameters of the SIM card except sensitive parameters that are read protect. The sensitive parameters are accessed with a special unlock code that is not available to the end user. Can execute programs that the network operator installs on it (through the SAT SIM applications toolkit). Here is a picture of the SIM card components from Martin Sauter s book.

2 Universal Mobile Telecommunication System (UMTS) as much an evolution as a revolution Design consideration: Increase in processing power enables use of much more complex intensive air interface transmission methods that utilize the scarce bandwidth on the air interface more effectively (in comparison to GSM). 3GPP responsible for evolving the GSM, UMTS, and LTE standards Release 99- first release of UMTS Completely redesigned Radio Access Network (RAN) UMTS terrestrial Radio Access Network (UTRAN) R99 Up to 384 kbps BSC (in 2G) called Radio Network Controller BTS (in 2G) called NodeB MS (in 2G) called User Equipment (UE) Europe and Asia 12 blocks of 5 MHz each for uplink and downlink ( MHz; MHz) Different bands require additional circuitry in the UE. Security Enhancements authenticate the network, add integrity protection, better cipher algorithms Core network about the same as GPRS network Used ATM in interface from RNC to NodeB (instead of frame relay) Seamless voice handover from GSM to UMTS and vice versa Data session handover is not that seamless due to lower rate of GSM/GPRS UMTS uses CDMA Code Division Multiple Access (CDMA) Most of the material here is from the Wireless Networks book (2 nd edition) by William Stallings - Multiplexing technique used with spread spectrum - Start with data signal with rate D (call it bit data rate) - Break each bit into k chops according to a fixed pattern that is specific to each user -> user s code - New channel chip data rate of kd chips per second - Figure below shows three codes for three different users

3 Code Message "1101" Encoded User A User B User C Figure 7.10 CDMA Example Receiver R receives pattern, d = < d1, d2, d3, d4, d5, d6 >, it performs the following decoding function. S u (d) = d1 X c1 + d2 X c2 + + d6xc6 C A = < 1, -1, -1, 1, -1, 1>, let S A be decoded value at user A. when bit 1 is transmitted, d = < 1, -1, -1, 1, -1, 1 >, S A = 6 when bit 0 is transmitted, d = < -1, 1, 1, -1, 1, -1 >, S A = -6 If the value is not -6 or +6, either someone else (not user A) is sending information or the information is received in error. When user B sends a 1, d = < 1, 1, -1, -1, -1, 1 >, d received at A will result in S A = 0. Thus, unwanted signal from B does not show up at all. Let s A and s B be transmitted signals. Then, S A (s A + s B ) = S A (s A ) + S A (s B ) = S A (s A ), since the decoder ignores B when it is using A s code.

4 The codes of A and B that have the property that S A (C B ) = S B (C A ) = 0 are called orthogonal. This is a desirable characteristic of the code but there are not that many orthogonal codes. The more common case is that S X (C Y ) is small in absolute value when X is not equal to Y. Now, when using the codes for users A, B, and C, S B (C C ) = S C (C B ) = 2. The signal from B (or C) makes a small contribution to the decoded signal at C (or B). Using the decoder S u, the receiver can sort out transmission from u even when there are other users broadcasting in the same channel. If these competing signals are send with higher power, then the system breaks down. Power control is very important in CDMA. In practice, the CDMA receiver can filter out the contribution from unwanted users or they appear as low-level noise. However, if there are many users competing for the channel with the user the receiver is trying to listen to, or if the signal power of one or more competing signals is too high, the system breaks down. UMTS Frequency and Cell Planning - All cells in UMTS radio network can use the same frequency - All neighboring cells use the same frequency - If a new cell is installed to increase the bandwidth in an area that is already covered by other cells, the most important task in a UMTS network is to decrease the transmission power of the neighboring cell. - Near-far effect: The network can instruct each handset (1500 times per second) to adapt its transmission power. The users that are close by are instructed to transmit at a low power. This power control increases the operating time of the mobile device battery. - Actual transmission of a bit stream -> use QPSK, e.g., shown in one of the earlier classes. Soft handover CDMA allows a user to communicate with multiple base stations (Node-Bs) at the same time (not supported in GSM/ GPRS/LTE). The two figures below (from Martin Sauter s book) show the soft handover from the network s and from the device s viewpoint.

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7 UMTS also has logical channels common and dedicated channels Long term Evolution (LTE) [Primary from Sauter s book] CDMA (used in UMTS) does not scale very well, primarily due to inter-symbol interference. Use OFDM That transmits data over narrowband carriers of 180 khz each Data stream split into many slower data streams that are transmitted simultaneously Attainable data rate comparable to UMTS in the same bandwidth but the multipath effect is severely reduced If less than 5 MHz is available, LTE can easily adapt and the number of narrowband carriers is simply reduced LTE 1.25 MHz 20 MHz (20 MHz can result in beyond 100 Mbps under very good signal conditions) LTE must support MIMO resulting in even higher data rates